Abstract

To determine accurately the rates of late Pleistocene megafaunal loss, it is fundamentally important to have accurate taxonomic information for every species. In Australia, accurate taxonomic information is lacking for several Pleistocene groups, including the largest marsupial ever to live, DiprotodOwen, 1838. Diprotod taxonomy has been complicated by early nomenclatural problems and by the occurrence of two distinct size classes of individuals that do not reflect an ontogenetic series. Traditionally, the two size classes have been regarded as separate species. However, a taxonomic investigation of large samples (> 1000 teeth) of Diprotod material from several different fossil localities in Queensland, New South Wales, South Australia and Victoria suggests that there is little evidence for the discrimination of more than one morphospecies. Thus, Diprotod is here considered a monotypic genus and the single morphospecies, D. optatOwen, 1838 is considered to have been highly sexually dimorphic. By drawing analogy with extant sexually dimorphic megaherbivores and marsupials, the large form was probably male, and the small form was probably female. Diprotodon optat probably moved in small, gender-segregated herds, and exhibited a polygynous breeding strategy. As a single morphospecies, D. optat had a near-continental geographical distribution, similar to that of extant megaherbivores, possibly indicating its niche as a habitat generalist.

INTRODUCTION

The extinct Diprotod was the largest marsupial that ever lived, approaching 3 m in length, standing 1.8 m high at the shoulder, and weighed in excess of 2500 kg (Archer, 1984; Wroe et a, 2003). Essentially, Diprotod filled the role of the Pleistocene ‘big game’ megaherbivore (terrestrial herbivores > 1000 kg in body weight; Owen-Smith, 1988) in Australia, as the mammoth and mastodon did in the Pleistocene of Eurasia and America. Like other terrestrial megafauna, Diprotod also suffered extinction sometime during the late Pleistocene, although the timing and mechanisms involved in such faunal losses are unclear (Wroe & Field, 2006; Brook et a, 2007). Since its original description in 1838, several species have been erected. Those species include D. optatOwen, 1838, D. australOwen, 1844, D. annexta McCoy, 1861, D. minHuxley, 1862, D. longiceMcCoy, 1865, D. lodeKrefft, 1873a, D. bennettKrefft, 1873b (nec D.‘bennettOwen, 1877) and D. ‘bennettOwen, 1877 (nec D. bennettKrefft, 1873b) (Mahoney & Ride, 1975). Variations of such names have also been applied (incorrectly), including D. optat (Woods, 1960; Stephenson, 1967; Stirton, Woodburne & Plane, 1967; Woods, 1968; Bartholomai, 1976; Bartholomai & Woods, 1976; Guerin et a, 1981) and D. austra (Molnar & Kurz, 1997). However, the taxonomic validity of many of those Diprotod species is problematic at best. The natural reaction of pioneering palaeontologists was to erect new species for variants collected from wide geographical areas (Archer, 1984). Most Diprotod species were erected on the basis of single, isolated specimens (e.g. Fig. 1) collected from the Darling Downs (south-eastern Queensland), Wellington Caves and surrounds (central eastern New South Wales) and Lake Colac (southern Victoria).

Figure 1

Right dentary fragment, Diprotodon optatOwen, 1838 (=D. australOwen 1844; see Appendix 1), genus and species holotype (BM10796), collected from the Wellington Caves, central eastern New South Wales. A, lateral view. B, internal view.

Figure 1

Right dentary fragment, Diprotodon optatOwen, 1838 (=D. australOwen 1844; see Appendix 1), genus and species holotype (BM10796), collected from the Wellington Caves, central eastern New South Wales. A, lateral view. B, internal view.

Thus, Diprotod taxonomy has been complicated by early nomenclatural problems and, more significantly, by the occurrence of two size classes of individuals that do not represent an ontogenetic series (Murray, 1991). Based on skeletal morphometric differences, the small form Diprotod is approximately one-third the size of its larger congener (Murray, 1991). Murray (1991) treated the large form as D. optat, and the small form as D. min, not to facilitate taxonomic treatment of the genus, but rather to discuss the distribution and occurrence of both size classes. Molnar & Kurz's (1997) approach was more conservative and divided Diprotod by listing individual specimens simply as ‘Diprotod large form’ or ‘Diprotod small form’. Although both size classes occur sympatrically in some Pleistocene deposits (Stirling & Zietz, 1899; Marcus, 1976; Molnar & Kurz, 1997) it has not been established whether the two size classes represent inter- or intraspecific variation within the genus. Thus, the purpose of this paper is to investigate the taxonomy of Diprotod. As both Diprotod size morphs appear to be temporally coeval, hypotheses that may explain the occurrence of multiple size classes include: (1) the occurrence of more than one species, or (2) that a single Diprotod species was sexually dimorphic. Accurate taxonomic information for Diprotod is critical for adequately determining how many species actually suffered extinction during the late Pleistocene.

SEXUAL DIMORPHISM IN FOSSIL MAMMALS

The determination of sexual dimorphism in fossil mammals is generally difficult to substantiate owing to small sample sizes, and difficulties discriminating between sexually dimorphic and size-related allometric traits (Gingerich, 1974). Within extant placental mammals, obvious differences in some skeletal elements reflect gender. For example, the pelvic structure of female placentals is absolutely or relatively larger than that of males (Tague, 2003). Such morphological differences are related to parturition in females. Placental newborn body mass ranges from 0.23 to 34.5% of maternal body mass (Leitch, Hytten & Billewics, 1959). Thus, biometric and morphological features of extant sexually dimorphic placental taxa may serve as analogues to establish the potential of sexual dimorphism in abundant and well-preserved placental fossil taxa (Averianov, 1996; Lister, 1996). Sexual dimorphism has been interpreted in a range of fossil placentals, such as mesonychians, rhinocerids, proboscideans, perissodactyla, felids and primates (Gingerich, 1981; Turner, 1987; Averianov, 1996; Lister, 1996; Cameron, 1997; Mead, 2000; O'Leary, Lucas & Williamson, 2000; Schrein, 2006).

Sexual determination of fossil non-placental mammals such as marsupials is more difficult. Marsupials give birth to relatively small young (newborn body mass ranges from 0.002 to 0.07% of maternal body mass) (Parker, 1977). Thus, females do not show significant skeletal adaptations related to parturition. Differences in pelvic size between male and female marsupials are related to asynchrony of skeletal growth cessation and sexual differences in adult growth (Tague, 2003). Although some extant marsupials such as kangaroos do show differences in skeletal limb proportions between genders (e.g. disproportionately larger forelimbs in males than females, being related to male–male combat) (Jarman, 1989), other than allometrically related differences, there are few osteological differences that allow gender separation within marsupials. Therefore, establishing criteria to examine the occurrence of sexual dimorphism in fossil marsupials is difficult. The problem is especially compounded when examining fossil taxa that have no modern analogues, such as Diprotod. However, despite such problems, sexual dimorphism has been suggested for a variety of fossil marsupial taxa. For example, Cooke (2000) suggested that cranial and dental morphometric variation in the Oligo-Miocene kangaroo, Balbaroo fangarCooke, 2000, reflected sexual dimorphism. Wells & Tedford (1995) suggested that biometric differences in limb bone proportions of Pleistocene short-faced kangaroos, Sthenur spp., are similar to proportional differences of limb bones in sexed extant kangaroos and therefore reflected sexual differences. Following such reasoning, Wells & Tedford (1995) assigned associated cranial elements to gender. Similarly, Murray et a (2000) suggested that biometric bimodality and morphological differences in cranial and post-cranial elements of the mid-Miocene diprotodontid, Neohelos stirtoMurray et a, 2000, reflected sexual dimorphism.

In each of those examples and in fossil mammals in general, dentitions figure significantly in taxonomic classifications. Teeth are generally among the best-preserved elements in fossil mammals, are commonly identifiable to taxon, and have been used in a variety of ecological and evolutionary studies (Dayan, Wool & Simberloff, 2002). Teeth are commonly used as a surrogate of body size (Gould, 1975), and thus are useful for assessing sexual dimorphism in fossil taxa. In sexually dimorphic populations, body size is bimodally distributed. Hence, the best criteria for assessing sexual dimorphism in fossil species are body size (as commonly implied by tooth size) and associated skeletal robustness. Therefore, a hypothesis of sexual dimorphism in a fossil mammal would be supported if: (1) dental morphology, particularly for systematically important teeth (e.g. upper premolars for diprotodontoids; Longman, 1921; Stirton et a, 1967; Hand et a, 1993; Black & Archer, 1997; Murray et a, 2000), provide evidence for only a single morphospecies in a given population or assemblage; (2) surrogates for body size indicate a bimodal distibution, not reflecting an ontogenetic series; and (3) both size-forms are invariably temporally and spatially coeval over the entire range of the morphospecies. Conversely, sexual dimorphism would not be supported if a bimodal distribution of body size exists in a given population or assemblage, but morphological variation is sufficient to warrant separation of two or more morphospecies. However, if a morphospecies of a single body size class exists in a given population or assemblage, it cannot be unequivocally determined if that taxon is either sexually monomorphic or sexually dimorphic. Although such evidence could provide support for a single, monomorphic taxon hypothesis, given the tendency for extant sexually dimorphic mammals to occur in gender-biased social groups (Demment & Van Soest, 1985; Owen-Smith, 1988; Mysterud, 2000; Berger et a, 2001; Ruckstuhl & Neuhaus, 2002), such a monomorphic fossil population could alternatively represent a single-sex social group of a sexually dimorphic taxon. Thus, body-size ratios, when used as a proxy for sex ratios, are not necessarily useful indicators of sexual dimorphism or monomorphism in fossil taxa.

Generally, sexual dimorphism is distinguished from interspecific dimorphism when morphological differences can be explained as purely size-related, and do not reflect ontogeny. Such reasoning follows Gause's principle (i.e. ‘competitive exclusion principle’) (Gause, 1934), which states that two competing species cannot coexist indefinitely in a stable environment if they both have identical ecological requirements. If two or more sympatric species occur in a single population or assemblage, they presumably would exhibit morphological differences that reflect different habitat or resource usage (Gingerich, 1974).

METHODS

Although it is essential to document the variability within fossil mammal species, small sample sizes of individuals commonly restrict the significance in most statistical analyses (Mead, 2000). Therefore, specific localities yielding high numbers of Diprotod specimens were examined to test hypotheses relating to morphological and morphometric variation within and between different assemblages. Large collections were examined from spatially distributed localities and include the eastern Darling Downs (Queensland), Bacchus Marsh (Victoria), Lake Callabonna (South Australia), Myall Creek (New South Wales), Lancefield Swamp (Victoria) and Reddestone Creek (New South Wales) (Fig. 2).

Figure 2

Map of Australia indicating localities where Diprotod fossils have been recorded (modified from Horton, 1984) and specific assemblages mentioned in the text.

Figure 2

Map of Australia indicating localities where Diprotod fossils have been recorded (modified from Horton, 1984) and specific assemblages mentioned in the text.

For reasons outlined above, teeth formed the focus of the morphometric, morphological and taxonomic component of the investigation. Dental nomenclature follows Luckett (1993) where the posterior premolar is numbered P3 and molars are numbered M1–4. Anatomical terminology follows Murray et a (2000). Higher systematics follows Aplin & Archer (1987).

Dental and dentary measurements were made using digital callipers, and were taken to the nearest 0.1 mm. The mean, range, standard deviation, coefficient of determination (R2) and coefficient of variation (COV) were calculated predominantly for dental measurements. The COV is a dimensionless number derived from the mean and standard deviation, and was calculated in order to allow the comparison of variation between populations that have potentially different mean values.

Diprotod material (including all type specimens) was examined from several institutions, including the British Museum of Natural History (London; prefix BM), Queensland Museum (Brisbane; prefix QMF), South Australian Museum (Adelaide; prefix SAMP), Museum Victoria (Melbourne; prefix NMVP) and Australian Museum (Sydney; prefix AMF).

GEOLOGICAL AND GEOGRAPHICAL SETTINGS OF THE DEPOSITS

EASTERN DARLING DOWNS

The eastern Darling Downs, south-eastern Queensland, encompass low rolling hills and plains west of the Great Dividing Range (27°33′S, 151°57′E; Fig. 2). The poorly indurated sediments generally consist of clay, silt and sand derived from the erosion of underlying basalts and sandstones (Woods, 1960; Gill, 1978). The region contains a variety of Pleistocene–Holocene fluvial deposits. Fluvial deposition modes range from high-energy channel deposits to low-energy overbank deposits (Price & Sobbe, 2005; Price & Webb, 2006). Radiometric and optical dating of the fluvial deposits is mostly restricted to the Kings Creek catchment and its surrounds. All dating attempted thus far have suggested that the megafauna-bearing deposits are late Pleistocene (Gill, 1978; Roberts et a, 2001; Price, 2005; Price & Sobbe, 2005; Price, Tyler & Cooke, 2005; Price & Webb, 2006; Webb et a, 2007).

Fossil preservation ranges from articulated, well-preserved specimens to disarticulated, broken and fragmented, and commonly unidentifiable remains (Price & Webb, 2006). Taphonomic investigations in the Kings Creek catchment suggested that distance of bone transport prior to final deposition was minimal (Price & Sobbe, 2005; Price & Webb, 2006). Molnar & Kurz (1997) suggested that there was little faunal regionalization within the Pleistocene Darling Downs to suggest that more than one local fauna existed. Diprotod remains are common in the deposits with both size classes undoubtedly represented in the Kings Creek, Gowrie Creek and Jimbour Creek subcatchments (Molnar & Kurz, 1997).

BACCHUS MARSH

Bacchus Marsh is located in southern Victoria (37°40′S, 144°26′E; Fig. 2). The sediments consist of sands and silts, interspersed with clay lenses (Long & Mackness, 1994). Basalt, sandstone and kaolinite nodules suggest that the sediments were derived from weathering of the underlying Werribee Formation (sandstone and kaolinitic clays) and a nearby basalt plain (Long & Mackness, 1994). The stratigraphic succession is dominated by discontinuous bedding with lenticular and festoon cross-stratification, with the sediments deposited in the proximal end of an ephemeral stream (Long & Mackness, 1994). Long & Mackness (1994) suggested that the deposit may be late Pleistocene, but no analytical dates have been published.

Fossil material is generally poorly preserved due to preburial weathering and subsequent diagenesis (Long & Mackness, 1994). However, Diprotod remains are particularly abundant and there is a high instance of articulated or associated specimens in the deposit. Bone abrasion and orientation data suggest that fluvial transport was minimal (Long & Mackness, 1994). Sedimentological and taphonomic data suggest that the deposit represents a mass death assemblage of Diprotod individuals (Long & Mackness, 1994). Only small-form individuals are represented in the deposit.

LAKE CALLABONNA

Lake Callabonna is located in central eastern South Australia (29°40′S, 140°05′E; Fig. 2). It is one of a series of large, dry saltpans that form a ‘horseshoe’ around the Flinders Ranges. Today the lake remains completely dry for many years at a time, only filling after heavy tropical rains to the north feed the many streams and creeks that flow into the salt lakes. The Pleistocene deposits consist of cyclical couplets of sand and clay (Pledge, 1994; Wells & Tedford, 1995). Optical stimulated luminescence (OSL) dating of the megafauna deposits provided burial ages of 75 ± 9 kya (Roberts et a, 2001).

The accumulation of Diprotod was rather slow (possibly over several thousand years) as small groups of animals attempted to cross the lake during periods of low water (Tedford, 1973). The preservation of post-cranial bones is exceptionally good. Limb bones sank deeply into the mud and suffered the least from weathering effects. Dorsal skeletal elements such as crania that were invariably exposed to weathering were commonly crushed and severely distorted (Stirling, 1907). Both size classes of Diprotod are represented in the deposits (Stirling & Zietz, 1899).

MYALL CREEK

Myall Creek is a small meandering channel, with minor tributaries (e.g. Bone Camp Gully), 180 km south-east of the Darling Downs, located near Bingara, north-eastern New South Wales (29°48′S, 150°33′E; Fig. 2). The sediments consist of clay, sand and gravel derived from the erosion of Tertiary basalts (Marcus, 1976). The deposit has not been analytically dated, but biostratigraphic markers (e.g. Diprotod and Procoptod) indicate that the deposit is Pleistocene (Marcus, 1976).

Fossil material was recovered horizontally in si, predominantly from a single grey, sandy claystone horizon (Anderson, 1890). Preservation of fossil material ranges from being relatively well-preserved but disarticulated and broken, to small, water-worn bone pebbles. Diprotod remains (predominantly isolated teeth) are abundant in the deposit, with both size classes represented (Marcus, 1976).

LANCEFIELD SWAMP

Lancefield Swamp is located near Lancefield in southern Victoria, approximately 50 km north-east of Bacchus Marsh (37°165′S, 144°44′E; Fig. 2). Stratigraphic successions consist of clays and gravels, with abundant granite, quartz, feldspar and pisolitic laterite nodules as clasts (Van Huet, 1999). Fluvial deposition modes include both horizontally (channel) and vertically accreted deposits (Van Huet, 1999). Radiocarbon, electron spin resonance and amino-acid dating indicates that the deposit is late Pleistocene (Gillespie et a, 1978; Van Huet et a, 1998).

Fossil material is disarticulated and fragmentary. Bone orientation, weathering and abrasion, and skeletal element representation data suggest that the fossil material was washed into the deposits from areas proximal to the swamp (Van Huet, 1999). Diprotod is represented by dentary and maxillary fragments, and isolated teeth.

REDDESTONE CREEK

Reddestone Creek is a small, meandering channel, 170 km south of the Darling Downs, located near Glen Innes, north-eastern New South Wales (29°35′S, 151°42′E; Fig. 2). The sediments consist of clay, sand and gravel that probably were derived from underlying basaltic bedrock, with disturbed soils overlying the megafauna deposits (Horton & Connah, 1981). The megafauna-bearing unit is Pleistocene on the basis of biostratigraphic correlation, but no radiometric dates have been published.

Fossil material is predominantly disarticulated and broken. Post-cranial elements are uncommon in proportion to teeth, suggesting predepositional sorting as a result of fluvial processes (Horton & Connah, 1981). Diprotod remains (predominantly teeth) are particularly abundant in the deposit (Horton & Connah, 1981).

OTHER LOCALITIES

Diprotod specimens were also examined from several other localities. However, most of those localities represent ‘spot’ localities and yielded few specimens that would allow the meaningful examination of inter- or intraspecific population dynamics from single stratigraphic successions. Those ‘spot’ localities include Diprotod material from Queensland, New South Wales, South Australia, Victoria or Australian regions otherwise unspecified in institutional collections (see supplementary Appendix S1).

DARLING DOWNS DIPROTOD

DESCRIPTION

General morphology of dentary (Fig. 3)

Figure 3

Diprotod dentaries from the Darling Downs. A, lateral aspect of a large-form individual (QMF319). B, lateral aspect of a small-form individual (QMF36129). C, internal aspect of same small-form individual.

Figure 3

Diprotod dentaries from the Darling Downs. A, lateral aspect of a large-form individual (QMF319). B, lateral aspect of a small-form individual (QMF36129). C, internal aspect of same small-form individual.

Dentary long with slender to deep horizontal ramus, length ∼500–650 mm; dentary body tapers posteriorly from M1 to M4; alveolar border nearly straight to slightly concave; horizontal ramus convex to angular anterior to ascending ramus; diastemal crest short relative to length of dentary; mental foramen anteroinferior to P3 anterior alveolus; incipient smaller foramen may also be expressed; incisive foramen inferior to incisor alveolus; symphysis elongated, ovoid in outline, posterior portion inferior to M1 alveolus; vertical ramus half length of total dentary; coronoid process high, blade-like; coronoid notch very deep; condyle high above occlusal tooth line; posterior edge of ascending ramus constricted at neck of condyle; condyle very wide, convex posteriorly, anteriorly inclined, rounded articular surface; digastric fossa wide, elongated, shallow, extending anteriorly inferior to transverse valley of M3; angular process projects much higher than tooth cheek tooth row; mandibular foramen wide, ovoid, open above tooth crown height.

I1(Fig. 3)

Single paired lower incisors, chisel-like, slightly forked, procumbent, hypselodont; straight to slightly curved dorsally; occludes with posterior surface of I1 and horizontal surface of I2−3; enamel confined to lateral external surface, curving around ventrally to lower mesial quarter, and extended to anterior ventral surface tip; longitudinal groove on mesial side wide and shallow; tooth ovate in cross-section, narrowing towards anterior tip.

P3(Fig. 4A, B)

Figure 4

Diprotod P3 and M1. A–B, QMF6633, occlusal and lingual aspects of P3–M1(Darling Downs). C–D, SAMP36367, occlusal and lingual aspects of P3 –M1 (Strezlecki Creek). E–F, QMF11136, occlusal and lingual aspects of P3 –M1 (mirrored; Darling Downs).

Figure 4

Diprotod P3 and M1. A–B, QMF6633, occlusal and lingual aspects of P3–M1(Darling Downs). C–D, SAMP36367, occlusal and lingual aspects of P3 –M1 (Strezlecki Creek). E–F, QMF11136, occlusal and lingual aspects of P3 –M1 (mirrored; Darling Downs).

Bilophodontid; subrectangular to subtriangular in occlusal outline; hypolophid wider than protolophid; protolophid angled anterobuccally; hypolophid angled perpendicular to length of tooth; protolophid higher crowned than hypolophid; metalophid bulbous swelling, positioned on anterobuccal portion of hypoconid, connects to midline of posterior face of protolophid; paralophid crest descends anterolingually to small, pocket-like anterior cingulid; premetacristid weakly expressed, descends from halfway up metaconid; postmetacristid and preentocristid small, variably expressed, meeting at lingual opening of transverse valley, connecting entoconid and metaconid; bucccal cingula on transverse valley weakly developed; postentocristid descends entoconid posterobuccally to form pocket-like posterior cingulid; posterior cingulid well developed with slight thickening of enamel just lingual to midline; posthypocristid descends hypoconid to connect to posterior cingulid, one-third from buccal extremity of cingulid; posterior cingulid sits below height of anterior cingulid of succeeding tooth.

M1(Figs 4A, B, 5A)

Figure 5

Diprotod cheek teeth (M1–4, lacking premolars) from the Darling Downs. A, QMF1517, occlusal aspect of lower right molar row. B, QMF10783, occlusal aspect of upper left molar row.

Figure 5

Diprotod cheek teeth (M1–4, lacking premolars) from the Darling Downs. A, QMF1517, occlusal aspect of lower right molar row. B, QMF10783, occlusal aspect of upper left molar row.

Bilophodontid; subrectangular in occlusal outline; hypolophid longer and wider than protolophid; lophids slightly concave anteriorly, angled perpendicular to molar row, transverse valley V-shaped in lingual view, U-shaped in buccal view; hypolophid wider and longer than protolophid; paralophid crest and preprotocrista weakly developed, descends anteriorly from halfway up protoconid, connecting to anterobuccal portion of anterior cingulum; premetacristid weakly developed, descends anteriorly from halfway up metaconid, connecting to anterolingual portion of anterior cingulum; anterior cingulid small, horizontal; metalophid weakly expressed, approximates as a swelling on posterior surface of the midline of the protolophid, descends anterolingually from hypoconid; postmetacristid and preentocristid faint, meet at lingual opening of transverse valley; postprotocristid and prehypocristid absent to weakly expressed, meeting at buccal opening of transverse valley; transverse valley narrow, deep; postentocristid small, descends entoconid to form lingual margin of posterior cingulum; posthypocristid descends hypoconid to form buccal margin of posterior cingulum; posterior cingulum wide, well developed, crosses tooth transversely horizontal to midline then descends buccally to base of hypoconid; slight thickening of posterior cingulid at midline as its connection to the hypolophid becomes more crest-like; posterior cingulid overlaps anterior cingulid of succeeding tooth.

M2−4(Fig. 5A)

Progressively larger than M1; M2−4 generally similar in morphology to M1, however: protolophid and hypolophid more similar in length and width; paralophid crest, preprotocristid and premetacristid rarely expressed; postcristids linking protolophids and hypolophids at lingual and buccal openings of transverse valley more weakly expressed or absent; transverse valleys of M3−4 more V-shaped in buccal view.

General morphology of cranium (Fig. 6)

Figure 6

Diprotod cranium (small form) in lateral view collected from the Darling Downs (note that the maxilla is broken and anteroventrally displaced and angled slightly vertically).

Figure 6

Diprotod cranium (small form) in lateral view collected from the Darling Downs (note that the maxilla is broken and anteroventrally displaced and angled slightly vertically).

Long, narrow, ranges from 730 mm to slightly greater than 1000 mm in condylobasal length; premaxilla elongated; rostrum long, tapered anteriorly; diastema long, narrow, deflected slightly anteroventrally relative to cheek tooth row; upper incisors outline trapezoidal in occlusal aspect; buccinator fossa extremely deep; premaxillary septum high; nasals retracted; narial aperture deeper than broad; palate closed, narrowing anteriorly; palatal vacuities short, terminating posterior to cheek tooth row; zygomatic arch elongate, masseteric processes well developed, deep; lateral maxillary fossa very deep; cranial vault dome-like in posterior profile, convex in lateral profile; basicranial plane markedly elevated above level of palatal plane; postglenoid process elongated, fused with tympanic process and mastoid–squamosal posteriorly; occiput region broad, angled anteriorly; lambdoid crest short, smoothly arching; occipital condyles very large, 60 mm to > 110 mm in depth; braincase positioned posteriorly, very small, surrounded by air-filled sinuses.

I1 (Fig. 7)

Figure 7

Diprotod upper incisors (QMF2485). A, lateral aspect. B, occlusal aspect.

Figure 7

Diprotod upper incisors (QMF2485). A, lateral aspect. B, occlusal aspect.

Strap-like, curved ventrally, hypselodont; occlusal notch present on most teeth; I1 wears on posterior surface, tip extends below occlusal plane of I2−3; enamel confined to dorsal, lateral and upper one-third of mesial portion; shallow, wide longitudinal groove on dorsal surface; shallow longitudinal groove on mesial surface; wide, shallow groove on ventral, lateral margin; mesial portion deeper than lateral portion.

I2 (Fig. 7)

Left and right I2 converge at occlusal tip; occlusal surface of crown horizontal, ovoid; anterior angle acute, posterior angle obtuse; curves mesially; narrows towards tip; pointed anterior margin overlaps posterolingual surface of I1.

I3 (Fig. 7)

Similar in morphology to I2, but narrower in relation to length; left and right I3 splayed; crown much more elongate relative to width.

P3 (Fig. 4)

Lophodont; subovoid to subtriangular in occlusal outline; parastyle most anterior cusp lying just buccal to midline forming anterobuccal margin of tooth, variably expressed ranging from slight enamel bulge to large bulbous cusp; protocone wide, transverse, forming lingual portion of tooth, horizontal or slightly posteolingual to paracone; cleft between protocone and paracone shallow to deep; metacone transverse to protocone, posterobuccal to paracone, connected to protocone by protoloph; preparacrista descends anteriorly from paracone to buccal side of parastyle; preprotocrista descends anteriorly from posterior or anterior of protocone to form lingual corner of anterior cingulum; anterior cingulum small, connected either to anterior face of parastyle or connects to preparacrista; secondary anterocrista commonly expressed descending anterior face of paracone to connect to midline of parastyle; postprotocrista and postmetacrista descend posteriorly from apex of protocone and metacone, respectively, forming lingual and buccal corners of posterior cingula; posterior cingula well developed running transversely, curved, lowest at midline; ectoflexus between paracone and metacone variably curved, or with distinct vertical ridges; buccal cingula absent to small, variably expressed.

M1 (Figs 4, 5B)

Bilophodont; subrectangular to subtrapezoidal in occlusal outline; metaloph wider than protoloph; transverse lophs anteriorly convex, angled perpendicular to molar row, V-shaped in buccal and lingual views; preparacrista descends anteriorly from paracone to buccal portion of anterior cingulum; preprotocista descends anteriorly from protocone to form lingual portion of anterior cingulum; anterior cingulum well developed, forming a shallow basin across anterior margin of tooth; incipient forelink variably expressed in basin of anterior cingulum in midline of tooth; postprotocrista descends protocone, consistent with a small lingual cingulum, on lingual opening of transverse valley; postparacrista small to absent; postmetacrista descends posteriorly from high on posterior face of metacone to a small metastyle; metastyle variably expressed, forming posterior buccal corner of tooth; postmetaconulecrista small, descends posterior face of metaconule, forming posterior lingual corner of tooth; posterior cingulum low, running transversely along posterior of tooth, connecting to postmetaconulecrista and metastyle.

M2 (Fig. 5B)

Larger than M1; M2 generally similar in morphology to M1, however: protoloph and metaloph more similar in width; postprotocrista and postparacrista variably less developed or absent; lingual cingulum less developed or absent; forelink absent; metastyle not expressed.

M3−4 (Fig. 5B)

Progressively larger than M1−2; generally similar in morphology to M2, but protoloph wider than metaloph, particularly in M4 where metaloph is lingually offset; more U-shaped in lingual view; lophs angled more anterobuccally relative to molar row; lingual cingula absent; posterior cingula less developed.

MORPHOMETRIC AND MORPHOLOGICAL COMPARISON

Dentary

Both Diprotod size classes are abundantly represented in Darling Downs fossil assemblages. To demonstrate those size classes, basic measurements were taken of the depth and width at specific points along the dentary. The morphometric results indicate that there is a bimodal distribution of dentary sizes within Darling Downs Diprotod (Fig. 8). Such differences reflect large and small form individuals.

Figure 8

Dimensions of Diprotod dentaries at position below M3 for the Darling Downs (N = 34), Lake Callabonna (N = 19) and Bacchus Marsh (N = 23) assemblages. Closed markers indicate large-form individuals, open markers indicate small-form individuals.

Figure 8

Dimensions of Diprotod dentaries at position below M3 for the Darling Downs (N = 34), Lake Callabonna (N = 19) and Bacchus Marsh (N = 23) assemblages. Closed markers indicate large-form individuals, open markers indicate small-form individuals.

There are several observable morphological differences in the shape and form of the dentary of large and small form individuals (Fig. 9). The dentary of mature large-form individuals is generally much longer, deeper and wider than that of similarly aged small-form individuals. Large-form individuals may also develop a very distinctive ‘chin’ where the symphysis is greatly expanded and is much steeper anteriorly (Fig. 9C–E). The ‘chin’ apparently developed early in the life of the large form as exhibited in a juvenile eastern Darling Downs individual (Fig. 9F). The ventral margin of the horizontal ramus is commonly more concave in the large form than the small form. The posterior portion of the dentary is angular in the large form, and less inclined and rounded in small-form individuals. In the large form, the coronoid notch is deeper and coronoid process taller in relation to the condyle than in the small form. The leading edge of the ascending ramus is inclined slightly posteriorly in the large form but is angled more vertically in the small form. Otherwise, the dentaries are very similar.

Figure 9

Line drawings of morphological variation in Diprotod dentary shape (all specimens from the Darling Downs). A, QMF36129, small form. B, QMF1517, small form. C, QMF319, large form. D, QMF6569, large form. E, QMF10311, large form. F, QMF6633, juvenile large form. Numerals on line drawings denote character variations: 1, anterior portion of ramus gently curved in lateral aspect; 2, anterior portion of ramus expanded and angular in lateral aspect; 3, ventral margin of horizontal ramus straight; 4, ventral margin of horizontal ramus slightly concave; 5, posterior portion of dentary gently curved; 6, posterior portion of dentary angular.

Figure 9

Line drawings of morphological variation in Diprotod dentary shape (all specimens from the Darling Downs). A, QMF36129, small form. B, QMF1517, small form. C, QMF319, large form. D, QMF6569, large form. E, QMF10311, large form. F, QMF6633, juvenile large form. Numerals on line drawings denote character variations: 1, anterior portion of ramus gently curved in lateral aspect; 2, anterior portion of ramus expanded and angular in lateral aspect; 3, ventral margin of horizontal ramus straight; 4, ventral margin of horizontal ramus slightly concave; 5, posterior portion of dentary gently curved; 6, posterior portion of dentary angular.

Teeth

Although there are clear differences in dentary sizes between both morphs (Figs 8, 9), those differences are less apparent in analyses of isolated teeth. Therefore, Diprotod dentaries were scored as to whether they represented the large or small form, and a series of tooth measurements (length, anterior and posterior width) were taken for in si molars. The morphometric results indicate that there is an obvious difference in tooth size between large- and small-form Darling Downs Diprotod (Table 1; Fig. 10). The means of most cheek teeth measurements are ∼4–17% larger in the large form than the small form (Table 1).

Table 1

Cheek tooth dimensions (mm) for large- and small-form Darling Downs Diprotod

Tooth Size class Length Anterior width Posterior width N 
P3 Large form 20.23 13.84 14.88 
 Small form 19.4 (0.8, 4.1) 13.1 (0.6, 4.6) 13.3 (0.3, 2) 
M1 Large form 37.1 (0.7, 1.8) 23.1 (1.3, 5.8) 24.7 (0.7, 2.7) 
 Small form 34.5 (2, 5.9) 20.2 (1.8, 8.8) 23.1 (2.8, 12) 11 
M2 Large form 45.2 (2.5, 5) 28.4 (2.8, 9.8) 29.3 (1.9, 6.5) 
 Small form 42.4 (1.9, 4.7) 28.2 (1.7, 6.1) 28.3 (1.4, 4.9) 12 
M3 Large form 54.9 (3.3, 6) 36.1 (2.6, 7.1) 37.2 (2.1, 5.5) 10 
 Small form 52.5 (2.5, 4.7) 35.4 (2.6, 7.2) 36.2 (2.2, 6.2) 16 
M4 Large form 57.4 (2.9, 5.1) 37.4 (3.2, 8.4) 37.3 (2.1, 5.6) 
 Small form 55.5 (2.5, 4.5) 37.9 (2.4, 6.3) 35.8 (1.9, 5.2) 18 
P3 Large form 22.8 18 19.6 
 Small form 23.2 (0.6, 2.5) 17.7 (1.2, 6.9) 19.6 (0.6, 2.9) 
M1 Large form 38.1 (1.7, 4.6) 33.5 (1.6, 4.7) 37.4 (1.3, 3.6) 
 Small form 34.1 (2.1, 6.2) 30.0 (1.8, 5.9) 34.7 (3.9, 11.3) 
M2 Large form 46.9 (2.7, 5.7) 41.2 (0.6, 1.4) 42.8 (1.5, 3.5) 
 Small form 40.9 (1.6, 3.9) 38.9 (2.8, 7.1) 38.9 (1.8, 4.7) 
M3 Large form 55.4 (2.2, 3.9) 44.9 (2.1, 4.6) 42.3 (2, 4.6) 
 Small form 49.6 (1.9, 3.9) 42.1 (1.8, 4.2) 40.2 (2.2, 5.4) 
M4 Large form 58.22 42.82 – 
 Small form 49.8 (2.7, 5.3) 42.6 (4.2, 9.4) 36.4(4.3, 11.7) 
Tooth Size class Length Anterior width Posterior width N 
P3 Large form 20.23 13.84 14.88 
 Small form 19.4 (0.8, 4.1) 13.1 (0.6, 4.6) 13.3 (0.3, 2) 
M1 Large form 37.1 (0.7, 1.8) 23.1 (1.3, 5.8) 24.7 (0.7, 2.7) 
 Small form 34.5 (2, 5.9) 20.2 (1.8, 8.8) 23.1 (2.8, 12) 11 
M2 Large form 45.2 (2.5, 5) 28.4 (2.8, 9.8) 29.3 (1.9, 6.5) 
 Small form 42.4 (1.9, 4.7) 28.2 (1.7, 6.1) 28.3 (1.4, 4.9) 12 
M3 Large form 54.9 (3.3, 6) 36.1 (2.6, 7.1) 37.2 (2.1, 5.5) 10 
 Small form 52.5 (2.5, 4.7) 35.4 (2.6, 7.2) 36.2 (2.2, 6.2) 16 
M4 Large form 57.4 (2.9, 5.1) 37.4 (3.2, 8.4) 37.3 (2.1, 5.6) 
 Small form 55.5 (2.5, 4.5) 37.9 (2.4, 6.3) 35.8 (1.9, 5.2) 18 
P3 Large form 22.8 18 19.6 
 Small form 23.2 (0.6, 2.5) 17.7 (1.2, 6.9) 19.6 (0.6, 2.9) 
M1 Large form 38.1 (1.7, 4.6) 33.5 (1.6, 4.7) 37.4 (1.3, 3.6) 
 Small form 34.1 (2.1, 6.2) 30.0 (1.8, 5.9) 34.7 (3.9, 11.3) 
M2 Large form 46.9 (2.7, 5.7) 41.2 (0.6, 1.4) 42.8 (1.5, 3.5) 
 Small form 40.9 (1.6, 3.9) 38.9 (2.8, 7.1) 38.9 (1.8, 4.7) 
M3 Large form 55.4 (2.2, 3.9) 44.9 (2.1, 4.6) 42.3 (2, 4.6) 
 Small form 49.6 (1.9, 3.9) 42.1 (1.8, 4.2) 40.2 (2.2, 5.4) 
M4 Large form 58.22 42.82 – 
 Small form 49.8 (2.7, 5.3) 42.6 (4.2, 9.4) 36.4(4.3, 11.7) 

Mean (standard deviation, coefficient of variation).

Figure 10

Length versus posterior width for Darling Downs Diprotod lower molar teeth. A, M1. B, M2. C, M3. D, M4.

Figure 10

Length versus posterior width for Darling Downs Diprotod lower molar teeth. A, M1. B, M2. C, M3. D, M4.

Over 170 teeth were examined in the Darling Downs Diprotod assemblage. The molar teeth of Darling Downs Diprotod are somewhat variable in morphology. Generally, upper molar teeth vary in the degree of development of cingula and stylar cusps. Lower molars also vary in the degree of development of cingulae, and anterior molars vary in the development of the metalophid, which is small, but distinct in some individuals, and reduced in others.

Within Darling Downs Diprotod, P3 is one of the more variable teeth in terms of morphology. The upper premolar (P3) is one of the most important teeth for determining relationships between diprotodontids (Longman, 1921; Stirton et a, 1967; Hand et a, 1993; Black & Archer, 1997; Murray et a, 2000), and thus its variability is considered here in more detail. The crowns vary in the expression of the cusps, cingula and occlusal outline. The parastyle is the most variable cusp, ranging from being large and bulbous, forming the anterior margin of the tooth, to being small, and confluent with the anterior cingulum. The anterior cingulum (and associated preparacrista and preprotocrista) varies in its development and range around the anterior margin of the P3. The anterior cingulum may connect either to the buccal or anterior face of the parastyle (especially if the parastyle is large and bulbous), or be confluent and connect to the preparacrista. The preprotocrista may extend anteriorly from the anterior or posterior portion of the protocone. Where arising from the posterior portion of the protocone, it may form a small, pocket-like lingual cingulum. The ectoflexus between the paracone and metacone is also variable, being smooth, rounded, or well defined and angular in occlusal outline. Buccal cingula may be absent to moderately developed. The cleft between the paracone and protocone may be either deep or shallow. The cleft between the paracone and metacone may also be deep and shallow. Where deep, a characteristic horseshoe-shaped wear pattern is developed on the occlusal surface. Where shallow, the occlusal surface of the tooth may wear into a ring-shaped pattern. P3 morphology is variable in both large- and small-form individuals, and no single morphology is restricted to either size class.

Lower incisors of large-form Darling Downs Diprotod tend to be wider and deeper, and less curved than in small-form individuals. For cheek teeth, there are no consistent morphological differences between large- and small-form individuals. However, as demonstrated above, there is an observable morphometric overlap between cheek teeth size of the large- and small-form Diprotod (Fig. 10).

BACCHUS MARSH DIPROTOD

DENTAL MORPHOMETRIC AND MORPHOLOGICAL COMPARISON

The Bacchus Marsh Diprotod assemblage is dominated by dentary and maxillary fragments, with at least 18 individuals represented. Interestingly, the assemblage is represented solely by small-form individuals (Fig. 8). Tooth eruption and wear patterns indicate that the assemblage is dominated by subadult to young adult individuals. Juvenile and mature individuals were not observed in the assemblage. The dentaries are morphologically similar to small-form individuals from the Darling Downs.

More than 230 teeth were examined from the Bacchus Marsh Diprotod assemblage. All teeth are morphologically similar to the corresponding teeth of the Darling Downs Diprotod assemblage discussed above and thus do not warrant additional description. The mean dimensions of most cheek teeth are smaller than corresponding teeth of all other undifferentiated size class localities examined in this study (Tables 2, 3). However, most means overlap at one standard deviation, and all overlap at two standard deviations (Tables 2, 3). Mean tooth dimensions of the Bacchus Marsh Diprotod small-form assemblage are similar to the corresponding means of teeth of the size class differentiated small-form sample from the Darling Downs (Tables 1–3).

Table 2

Upper cheek teeth dimensions (mm) for Diprotod

Tooth Locality Length Anterior width Posterior width N 
P3 All localities 23.0 (2.1, 9.1) 18.5 (2.4, 13) 20.1 (1.8, 9) 113 
 Darling Downs 22.5 (0.9, 4) 17.8 (1, 5.6) 19.2 (1, 5.2) 13 
 Bacchus Marsh 20.8 (1.6, 7.7) 17.9 (1.6, 8.9) 20.0 (1.5, 7.5) 20 
 Lake Callabonna 24.4 (2.4, 9.8) 18.5 (2, 10.8) 20.9 (1.8, 8.6) 
 Myall Creek 23.5 (1.8, 7.7) 18.7 (1.8, 9.6) 20.0 (1.9, 9.5) 25 
 Lancefield Swamp 24.3 (0.6, 2.5) 20.1 (1.6, 8) 21.3 (1.1, 5.2) 
 Reddestone Creek – – – 
M1 All localities 34.7 (2.8, 8.1) 31.3 (2.2, 7) 34.7 (2.6, 7.5) 124 
 Darling Downs 34.9 (2.6, 7.4) 31.6 (2.2, 7) 35.5 (2.8, 7.9) 17 
 Bacchus Marsh 32.6 (2.4, 7.4) 29.7 (1.5, 5.1) 32.9 (1.7, 5.2) 19 
 Lake Callabonna 35.1 (1.3, 3.7) 31.3 (2.1, 6.7) 36.0 (2.3, 6.4) 
 Myall Creek 35.5 (2.6, 7.3) 31.7 (2.2, 6.9) 34.6 (2, 5.8) 28 
 Lancefield Swamp 36.7 (1.5, 4.1) 33.3 (0.7, 2.1) 36.2 (1, 2.8) 
 Reddestone Creek 33.6 (2.1, 6.3) 31.4 (1.5, 4.8) 34.2 (2.5, 7.3) 14 
M2 All localities 43.6 (3.7, 8.5) 39.9 (2.5, 6.3) 40.6 (2.4, 5.9) 111 
 Darling Downs 42.9 (3.6, 8.4) 38.7 (2.3, 5.9) 39.8 (2.5, 6.3) 16 
 Bacchus Marsh 40.9 (2, 4.9) 38.5 (1.9, 4.9) 39.4 (2.2, 5.6) 20 
 Lake Callabonna 42.6 (3.5, 8.2) 40.0 (3, 7.5) 41.5 (2.6, 6.3) 
 Myall Creek 45.9 (3.1, 6.8) 40.7 (2, 4.9) 41.2 (1.9, 4.6) 25 
 Lancefield Swamp 44.6 (0.9, 2) 41.7 (1.1, 2.6) 41.5 (1.7, 4.1) 
 Reddestone Creek – 37.4 39 
M3 All localities 53.2 (3.4, 6.4) 45.5 (3, 6.6) 42.8 (2.9, 6.8) 126 
 Darling Downs 52.7 (3.2, 6.1) 44.3 (2.6, 5.9) 42.3 (3, 7.1) 14 
 Bacchus Marsh 52.4 (3, 5.7) 44.2 (2.3, 5.2) 42.0 (2.1, 5) 20 
 Lake Callabonna 54.3 (2.6, 4.8) 48.1 (1.4, 2.9) 45.7 (1.4, 3.1) 
 Myall Creek 55.2 (3.6, 6.5) 46.0 (3.5, 7.6) 42.7 (2.4, 5.6) 18 
 Lancefield Swamp 54.6 (3.3, 6) 45.5 (2.6, 5.7) 43.2 (2.3, 5.3) 
 Reddestone Creek 52.1 (2.6, 5) 44.8 (2.1, 4.7) 42.3 (2.6, 6.1) 15 
M4 All localities 53.8 (3.9, 7.2) 45.0 (3, 6.7) 37.8 (3.4, 9) 119 
 Darling Downs 53.0 (4.9, 9.2) 45.7 (4.2, 9.2) 39.0 (3.9, 10) 15 
 Bacchus Marsh 51.5 (3.8, 7.4) 43.7 (2, 4.6) 37.0 (2.2, 5.9) 13 
 Lake Callabonna 57.6 (4.4, 7.6) 49.0 (1.4, 2.9) 42.1 (2.4, 5.7) 
 Myall Creek 53.6 (3.2, 6) 44.3 (2, 4.5) 37.0 (3, 8.1) 35 
 Lancefield Swamp 57.3 (2.5, 4.4) 46.2 (2.1, 5.5) 40.1 (3.1, 7.7) 
 Reddestone Creek 52.9 (3.5, 6.6) 44.0 (3.3, 7.5) 36.3 (2.9, 8) 
Tooth Locality Length Anterior width Posterior width N 
P3 All localities 23.0 (2.1, 9.1) 18.5 (2.4, 13) 20.1 (1.8, 9) 113 
 Darling Downs 22.5 (0.9, 4) 17.8 (1, 5.6) 19.2 (1, 5.2) 13 
 Bacchus Marsh 20.8 (1.6, 7.7) 17.9 (1.6, 8.9) 20.0 (1.5, 7.5) 20 
 Lake Callabonna 24.4 (2.4, 9.8) 18.5 (2, 10.8) 20.9 (1.8, 8.6) 
 Myall Creek 23.5 (1.8, 7.7) 18.7 (1.8, 9.6) 20.0 (1.9, 9.5) 25 
 Lancefield Swamp 24.3 (0.6, 2.5) 20.1 (1.6, 8) 21.3 (1.1, 5.2) 
 Reddestone Creek – – – 
M1 All localities 34.7 (2.8, 8.1) 31.3 (2.2, 7) 34.7 (2.6, 7.5) 124 
 Darling Downs 34.9 (2.6, 7.4) 31.6 (2.2, 7) 35.5 (2.8, 7.9) 17 
 Bacchus Marsh 32.6 (2.4, 7.4) 29.7 (1.5, 5.1) 32.9 (1.7, 5.2) 19 
 Lake Callabonna 35.1 (1.3, 3.7) 31.3 (2.1, 6.7) 36.0 (2.3, 6.4) 
 Myall Creek 35.5 (2.6, 7.3) 31.7 (2.2, 6.9) 34.6 (2, 5.8) 28 
 Lancefield Swamp 36.7 (1.5, 4.1) 33.3 (0.7, 2.1) 36.2 (1, 2.8) 
 Reddestone Creek 33.6 (2.1, 6.3) 31.4 (1.5, 4.8) 34.2 (2.5, 7.3) 14 
M2 All localities 43.6 (3.7, 8.5) 39.9 (2.5, 6.3) 40.6 (2.4, 5.9) 111 
 Darling Downs 42.9 (3.6, 8.4) 38.7 (2.3, 5.9) 39.8 (2.5, 6.3) 16 
 Bacchus Marsh 40.9 (2, 4.9) 38.5 (1.9, 4.9) 39.4 (2.2, 5.6) 20 
 Lake Callabonna 42.6 (3.5, 8.2) 40.0 (3, 7.5) 41.5 (2.6, 6.3) 
 Myall Creek 45.9 (3.1, 6.8) 40.7 (2, 4.9) 41.2 (1.9, 4.6) 25 
 Lancefield Swamp 44.6 (0.9, 2) 41.7 (1.1, 2.6) 41.5 (1.7, 4.1) 
 Reddestone Creek – 37.4 39 
M3 All localities 53.2 (3.4, 6.4) 45.5 (3, 6.6) 42.8 (2.9, 6.8) 126 
 Darling Downs 52.7 (3.2, 6.1) 44.3 (2.6, 5.9) 42.3 (3, 7.1) 14 
 Bacchus Marsh 52.4 (3, 5.7) 44.2 (2.3, 5.2) 42.0 (2.1, 5) 20 
 Lake Callabonna 54.3 (2.6, 4.8) 48.1 (1.4, 2.9) 45.7 (1.4, 3.1) 
 Myall Creek 55.2 (3.6, 6.5) 46.0 (3.5, 7.6) 42.7 (2.4, 5.6) 18 
 Lancefield Swamp 54.6 (3.3, 6) 45.5 (2.6, 5.7) 43.2 (2.3, 5.3) 
 Reddestone Creek 52.1 (2.6, 5) 44.8 (2.1, 4.7) 42.3 (2.6, 6.1) 15 
M4 All localities 53.8 (3.9, 7.2) 45.0 (3, 6.7) 37.8 (3.4, 9) 119 
 Darling Downs 53.0 (4.9, 9.2) 45.7 (4.2, 9.2) 39.0 (3.9, 10) 15 
 Bacchus Marsh 51.5 (3.8, 7.4) 43.7 (2, 4.6) 37.0 (2.2, 5.9) 13 
 Lake Callabonna 57.6 (4.4, 7.6) 49.0 (1.4, 2.9) 42.1 (2.4, 5.7) 
 Myall Creek 53.6 (3.2, 6) 44.3 (2, 4.5) 37.0 (3, 8.1) 35 
 Lancefield Swamp 57.3 (2.5, 4.4) 46.2 (2.1, 5.5) 40.1 (3.1, 7.7) 
 Reddestone Creek 52.9 (3.5, 6.6) 44.0 (3.3, 7.5) 36.3 (2.9, 8) 

Mean (standard deviation, coefficient of variation). ‘All localities’ combines the Darling Downs, Bacchus Marsh, Lake Callabonna, Myall Creek, Lancefield Swamp and Reddestone Creek data sets, as well as morphometric data for specimens from other localities available for study.

LAKE CALLABONNA DIPROTOD

DENTAL MORPHOMETRIC AND MORPHOLOGICAL COMPARISON

Both size classes of Diprotod are undoubtedly represented in the Lake Callabonna Diprotod assemblage (Fig. 8). More than 100 teeth were investigated from the deposits. Both dentary and tooth morphology are similar to that of the Darling Downs, Reddestone Creek and Myall Creek Diprotod assemblages and thus do not warrant additional description.

The mean dimensions of most cheek teeth are similar to those of other assemblages examined in this study, and most means overlap at one standard deviation (Tables 2, 3). However, the mean dimensions of some teeth from the Lake Callabonna assemblage are 5–6% larger than corresponding teeth of other assemblages (e.g. M4; Table 2). In the case of the M4, the mean is based on a small sample size (N = 6). The dimension of the M4 is similar to that of the large form from the Darling Downs (Tables 1–3). Thus, such results may be the product of sampling bias; those teeth could conceivably be solely represented by the large-form Diprotod, therefore skewing the mean. Dimensions of other teeth from the Lake Callabonna assemblage do not vary more than 1–2% in comparison with other assemblages (Tables 2, 3). Additionally, there is an observable morphometric overlap in teeth size between large- and small-form Lake Callabonna Diprotod (Fig. 11).

Figure 11

Length versus posterior width for Lake Callabonna Diprotod lower molar teeth. A, M1. B, M2. C, M3. D, M4.

Figure 11

Length versus posterior width for Lake Callabonna Diprotod lower molar teeth. A, M1. B, M2. C, M3. D, M4.

MYALL CREEK DIPROTOD

DENTAL MORPHOMETRIC AND MORPHOLOGICAL COMPARISON

There were no well-preserved dentaries or maxillae in the Myall Creek Diprotod assemblage to allow comprehensive description or comparison with other localities. The single dentary available for study (AMF49681) belonged to a large-form individual that is morphologically similar to large-form individuals from other assemblages examined.

Over 160 teeth were examined from the assemblage. Tooth morphology is similar to that of all other Diprotod assemblages examined in this study, and thus does not warrant additional description. Like the Darling Downs assemblage, P3 morphology is particularly variable in cusp, cingula and shape expression (Fig. 12). The means of cheek teeth dimensions are similar to those of corresponding teeth of other Diprotod assemblages, and all means overlap at one standard deviation (Tables 2, 3).

Figure 12

Photographs and line drawings of morphological variation in P3 of Diprotod (all specimens from Myall Creek). A, AMF49697, RP3 (mirrored). B, AMF49698, LP3. C, AMF51864, LP3. D, AMF49700, RP3 (mirrored). E, AMF497100, LP3. F, AMF49703, LP3. G, AMF51863, RP3 (mirrored). H, AMF49695, LP3. I, AMF86825, RP3 (mirrored). J, AMF86826, RP3 (mirrored). Numerals on line drawings denote character variations: 1, parastyle absent or reduced, concurrent with anterior cingulum; 2, parastyle large or bulbous; 3, anterior groove poorly developed; 4, anterior groove moderately developed; 5, cleft between paracone and protocone deep; 6, cleft between paracone and protocone shallow; 7, preprotocrista originates at anterobuccal portion of protocone; 8, preprotocrista originates at mid-buccal to posterobuccal portion of protocone; 9, buccal cingulum moderately to well developed; 10, buccal cingulum poorly developed to absent.

Figure 12

Photographs and line drawings of morphological variation in P3 of Diprotod (all specimens from Myall Creek). A, AMF49697, RP3 (mirrored). B, AMF49698, LP3. C, AMF51864, LP3. D, AMF49700, RP3 (mirrored). E, AMF497100, LP3. F, AMF49703, LP3. G, AMF51863, RP3 (mirrored). H, AMF49695, LP3. I, AMF86825, RP3 (mirrored). J, AMF86826, RP3 (mirrored). Numerals on line drawings denote character variations: 1, parastyle absent or reduced, concurrent with anterior cingulum; 2, parastyle large or bulbous; 3, anterior groove poorly developed; 4, anterior groove moderately developed; 5, cleft between paracone and protocone deep; 6, cleft between paracone and protocone shallow; 7, preprotocrista originates at anterobuccal portion of protocone; 8, preprotocrista originates at mid-buccal to posterobuccal portion of protocone; 9, buccal cingulum moderately to well developed; 10, buccal cingulum poorly developed to absent.

LANCEFIELD SWAMP DIPROTOD

DENTAL MORPHOMETRIC AND MORPHOLOGICAL COMPARISON

The Diprotod assemblage is represented by dentary and maxillary fragments, but is dominated by isolated teeth. Both size classes are undoubtedly represented in the deposit and the ratio of large- to small-form individuals is approximately 1:1. One dentary (NMVP151805-6) represents a juvenile small-form individual. The individual appears to be of similar morphological age to the juvenile large-form individual (QMF6633; Fig. 9F) from the Darling Downs (as determined by similar molar eruption patterns). The Lancefield Swamp specimen lacks the distinctive ‘chin’; the ventral margin of the horizontal ramus is more convex; and the posterior portion of the dentary is curved, similar to that of more mature individuals. Generally, from the limited number of dentaries available, no specific morphological differences were observed in comparison with either large- and/or small-form individuals of the Darling Downs, Bacchus Marsh and Lake Callabonna assemblages. Some dentaries show evidence of abrasion, possibly suggesting post-mort fluvial transport.

More than 70 teeth were examined in the Lancefield Swamp collection. Tooth morphologies are identical to those of all other aforementioned localities, and thus do not warrant additional description. As the assemblage is dominated by isolated teeth, it is not possible to assign the majority of teeth to specific size classes. The mean molar teeth dimensions vary little in comparison with other assemblages, and the means overlap at less than one standard deviation for most measurements (Tables 2, 3). However, similar to aforementioned examples from the Lake Callabonna Diprotod assemblage, some teeth (e.g. P3 and M4) are larger than corresponding teeth in other populations. However, like those from Lake Callabonna, such teeth are represented by few specimens, and for reasons described above, the differences are probably the result of sampling biases.

REDDESTONE CREEK DIPROTOD

DENTAL MORPHOMETRIC AND MORPHOLOGICAL COMPARISON

The Reddestone Creek Diprotod assemblage is similar to that of the Myall Creek assemblage in that most of the material is represented by isolated teeth rather than dentaries or maxillae. Almost 50 Diprotod teeth (mostly isolated) were examined from the assemblage. Reddestone Creek Diprotod teeth are morphologically similar to those of other assemblages examined and do not warrant additional description. The means for most measurements of molar dimensions vary less than 1–2% in comparison with those of the Darling Downs, and the means overlap at less than one standard deviation (Tables 2, 3).

DISCUSSION

DARLING DOWNSDIPROTODON

Molnar & Kurz's (1997) audit of the Queensland Museum Darling Downs collections suggested that the large-form Diprotod is eight times more common than the small form. However, that audit did not challenge the identifications previously placed on specimens, and the ratio could be inaccurate. The present data suggest that the small form is represented by at least 30 individuals and the large form by 21 individuals. The differing results demonstrate the high degree of morphological similarity between the large- and small-form Diprotod, and highlights the difficulties in assigning isolated specimens to a specific size class.

Teeth of Darling Downs Diprotod large-form individuals are slightly larger than corresponding teeth of small-form individuals. A similar morphometric relationship is exhibited in sexually dimorphic extant grey kangaroo (Macropus giganteShaw, 1790) populations collected from the Warwick region (see supplementary Appendix S2 for list of specimens examined), southern Darling Downs (Fig. 13). In terms of dentary cheek teeth dimensions, grey kangaroo males are mostly ∼4–10% larger than females, although a considerable degree of overlap exists between sexes. On average, COV and R2 values indicate that the correlation between length and width of Darling Downs Diprotod lower molar teeth (Fig. 10) is similar to values for the corresponding teeth of the modern grey kangaroo population (Fig. 13) from the Darling Downs (Tables 3, 4). However, examination of COV values shows that Darling Downs Diprotod upper molar teeth are slightly more variable than for corresponding teeth of modern populations of grey kangaroos.

Figure 13

Length versus posterior width for sexed eastern grey kangaroos (Macropus gigante) lower molar teeth from Warwick, Darling Downs. A, M1. B, M2. C, M3. D, M4.

Figure 13

Length versus posterior width for sexed eastern grey kangaroos (Macropus gigante) lower molar teeth from Warwick, Darling Downs. A, M1. B, M2. C, M3. D, M4.

Table 3

Lower cheek teeth dimensions (mm) for Diprotod

Tooth Locality Length Anterior width Posterior width N 
P3 All localities 18.6 (1.3, 7) 12.8 (1.2, 9.4) 13.6 (1, 7.4) 25 
 Darling Downs 19.6 (0.8, 4.1) 13.3 (0.6, 4.5) 13.7 (0.8, 4.5) 
 Bacchus Marsh 18.0 (1.3, 7.2) 12.4 (1.2, 9.7) 13.6 (0.8. 5.9) 14 
 Lake Callabonna – – – 
 Myall Creek 19.5 (1.8, 9.2) 14.1 (1.2, 8.5) 13.9 (1.8, 12.9) 
 Lancefield Swamp 19.1 (0.5, 2.6) 12.8 (0.2, 1.6) 14.1 (0.2, 1.4) 
 Reddestone Creek – – – 
M1 All localities 35.4 (2.7, 7.6) 20.6 (1.6, 7.8) 23.4 (1.7, 7.3) 70 
 Darling Downs 35.2 (2, 5.7) 20.9 (2.1, 10) 23.5 (2.5, 10.6) 15 
 Bacchus Marsh 34.4 (2.4, 7) 20.1 (1.1, 5.5) 23.1 (1.2, 5.2) 27 
 Lake Callabonna 37.6 (1.9, 5.1) 19.3 (1.7, 8.8) 23.2 (1.2, 5.2) 
 Myall Creek 36.4 (2.2, 6) 21.5 (1, 4.7) 24.1 (0.7, 2.9) 
 Lancefield Swamp 37.3 (1.5, 4) 21.9 (0.7, 3.2) 21.1 (0.8, 3.8) 
 Reddestone Creek – – – 
M2 All localities 43.5 (2.8, 6.4) 28.4 (2.4, 8.5) 29.0 (1.9, 6.6) 120 
 Darling Downs 43.4 (2.3, 5.3) 28.0 (2.3, 8.2) 28.6 (1.7, 5.9) 24 
 Bacchus Marsh 41.8 (2.4, 5.7) 27.6 (1.4, 5.1) 29.1 (1.8, 6.2) 32 
 Lake Callabonna 44.8 (2.9, 6.5) 28.3 (2.8, 9.9) 28.7 (2.3, 8) 16 
 Myall Creek 43.9 (0.8, 1.8) 31.8 (2.4, 7.5) 31.2 (2.9, 9.3) 
 Lancefield Swamp 45.5 (1.9, 4.2) 29.2 (2.8, 9.6) 29.7 (1.1, 3.7) 15 
 Reddestone Creek 45.6 (0.4, 0.9) 33.0 (4.7, 14.2) 29.6 
M3 All localities 53.4 (3.8, 7.1) 35.6 (2.9, 8.1) 36.4 (2.9, 8) 140 
 Darling Downs 53.3 (3.2, 6) 35.6 (2.9, 8.1) 36.4 (2.5, 6.9) 31 
 Bacchus Marsh 52.0 (2.8, 5.4) 35.1 (2.3, 6.6) 36.5 (1.9, 5.2) 34 
 Lake Callabonna 55.2 (4.4, 8) 36.1 (2.8, 7.8) 37.3 (2.6, 7) 19 
 Myall Creek 53.5 (2.6, 4.9) 34.6 (2.3, 6.6) 34.8 (2.2, 6.3) 
 Lancefield Swamp 54.5 (1.9, 3.5) 36.3 (1.3, 3.6) 37.0 (1.7, 4.6) 11 
 Reddestone Creek 52.1 34 35.6 
M4 All localities 56.9 (2.8, 4.9) 37.8 (2.7, 7.1) 36.9 (2.8, 7.6) 138 
 Darling Downs 55.7 (3.1, 5.6) 37.4 (2.8, 7.5) 36.1 (2.2, 6.1) 29 
 Bacchus Marsh 55.0 (2.7, 4.9) 37.6 (2.4, 6.4) 36.6 (1.9, 5.2) 30 
 Lake Callabonna 57.5 (3.7, 6.4) 37.8 (2.7, 7.1) 36.8 (3.7, 10.1) 18 
 Myall Creek 59.2 (2.5, 4.2) 38.0 (1.9, 5) 38.2 (1.4, 3.7) 13 
 Lancefield Swamp 59.1 (2.8, 4.7) 39.1 (2.3, 5.9) 38.1 (2.1, 5.5) 10 
 Reddestone Creek 58.0 (2.3, 4) 38.6 (1.9, 4.9) 37.2 (4, 10.8) 
Tooth Locality Length Anterior width Posterior width N 
P3 All localities 18.6 (1.3, 7) 12.8 (1.2, 9.4) 13.6 (1, 7.4) 25 
 Darling Downs 19.6 (0.8, 4.1) 13.3 (0.6, 4.5) 13.7 (0.8, 4.5) 
 Bacchus Marsh 18.0 (1.3, 7.2) 12.4 (1.2, 9.7) 13.6 (0.8. 5.9) 14 
 Lake Callabonna – – – 
 Myall Creek 19.5 (1.8, 9.2) 14.1 (1.2, 8.5) 13.9 (1.8, 12.9) 
 Lancefield Swamp 19.1 (0.5, 2.6) 12.8 (0.2, 1.6) 14.1 (0.2, 1.4) 
 Reddestone Creek – – – 
M1 All localities 35.4 (2.7, 7.6) 20.6 (1.6, 7.8) 23.4 (1.7, 7.3) 70 
 Darling Downs 35.2 (2, 5.7) 20.9 (2.1, 10) 23.5 (2.5, 10.6) 15 
 Bacchus Marsh 34.4 (2.4, 7) 20.1 (1.1, 5.5) 23.1 (1.2, 5.2) 27 
 Lake Callabonna 37.6 (1.9, 5.1) 19.3 (1.7, 8.8) 23.2 (1.2, 5.2) 
 Myall Creek 36.4 (2.2, 6) 21.5 (1, 4.7) 24.1 (0.7, 2.9) 
 Lancefield Swamp 37.3 (1.5, 4) 21.9 (0.7, 3.2) 21.1 (0.8, 3.8) 
 Reddestone Creek – – – 
M2 All localities 43.5 (2.8, 6.4) 28.4 (2.4, 8.5) 29.0 (1.9, 6.6) 120 
 Darling Downs 43.4 (2.3, 5.3) 28.0 (2.3, 8.2) 28.6 (1.7, 5.9) 24 
 Bacchus Marsh 41.8 (2.4, 5.7) 27.6 (1.4, 5.1) 29.1 (1.8, 6.2) 32 
 Lake Callabonna 44.8 (2.9, 6.5) 28.3 (2.8, 9.9) 28.7 (2.3, 8) 16 
 Myall Creek 43.9 (0.8, 1.8) 31.8 (2.4, 7.5) 31.2 (2.9, 9.3) 
 Lancefield Swamp 45.5 (1.9, 4.2) 29.2 (2.8, 9.6) 29.7 (1.1, 3.7) 15 
 Reddestone Creek 45.6 (0.4, 0.9) 33.0 (4.7, 14.2) 29.6 
M3 All localities 53.4 (3.8, 7.1) 35.6 (2.9, 8.1) 36.4 (2.9, 8) 140 
 Darling Downs 53.3 (3.2, 6) 35.6 (2.9, 8.1) 36.4 (2.5, 6.9) 31 
 Bacchus Marsh 52.0 (2.8, 5.4) 35.1 (2.3, 6.6) 36.5 (1.9, 5.2) 34 
 Lake Callabonna 55.2 (4.4, 8) 36.1 (2.8, 7.8) 37.3 (2.6, 7) 19 
 Myall Creek 53.5 (2.6, 4.9) 34.6 (2.3, 6.6) 34.8 (2.2, 6.3) 
 Lancefield Swamp 54.5 (1.9, 3.5) 36.3 (1.3, 3.6) 37.0 (1.7, 4.6) 11 
 Reddestone Creek 52.1 34 35.6 
M4 All localities 56.9 (2.8, 4.9) 37.8 (2.7, 7.1) 36.9 (2.8, 7.6) 138 
 Darling Downs 55.7 (3.1, 5.6) 37.4 (2.8, 7.5) 36.1 (2.2, 6.1) 29 
 Bacchus Marsh 55.0 (2.7, 4.9) 37.6 (2.4, 6.4) 36.6 (1.9, 5.2) 30 
 Lake Callabonna 57.5 (3.7, 6.4) 37.8 (2.7, 7.1) 36.8 (3.7, 10.1) 18 
 Myall Creek 59.2 (2.5, 4.2) 38.0 (1.9, 5) 38.2 (1.4, 3.7) 13 
 Lancefield Swamp 59.1 (2.8, 4.7) 39.1 (2.3, 5.9) 38.1 (2.1, 5.5) 10 
 Reddestone Creek 58.0 (2.3, 4) 38.6 (1.9, 4.9) 37.2 (4, 10.8) 

Mean (standard deviation, coefficient of variation). ‘All localities’ combines the Darling Downs, Bacchus Marsh, Lake Callabonna, Myall Creek, Lancefield Swamp and Reddestone Creek data sets, as well as morphometric data for specimens from other localities available for study.

Following criteria established in the Introduction, the data collected from the assemblage are consistent with the hypothesis that Darling Downs Diprotod is represented by a single, sexually dimorphic species. That interpretation is supported by the following observations: (1) other than differences in the size and shape of the dentary, there are no significant or consistent dental morphological differences (particularly in the systematically important upper premolar) between large- and small-form Darling Downs Diprotod ; (2) both teeth and dentary size are consistent with a bimodal body size distribution within the assemblage; and (3) both size classes are temporally and spatially coeval.

Table 4

Coefficient of variation (COV) values for cheek teeth of extant grey kangaroos (Macropus gigante) from Warwick, Darling Downs

Tooth Length COV Width COV N 
P3 11.2 11.4 40 
M1 6.1 6.5 74 
M2 6.7 5.7 82 
M3 5.1 5.6 65 
M4 6.1 6.5 51 
P3 7.0 9.8 35 
M1 6.1 4.7 50 
M2 5.7 4.5 58 
M3 4.5 4.4 44 
M4 3.9 4.3 29 
Tooth Length COV Width COV N 
P3 11.2 11.4 40 
M1 6.1 6.5 74 
M2 6.7 5.7 82 
M3 5.1 5.6 65 
M4 6.1 6.5 51 
P3 7.0 9.8 35 
M1 6.1 4.7 50 
M2 5.7 4.5 58 
M3 4.5 4.4 44 
M4 3.9 4.3 29 

In sexually dimorphic mammals, males are generally larger than females (Andersson, 1994). However, there are a few rare cases where the female is the larger sex (e.g. extant spotted hyena, Crocuta crocu, although in that case, dimorphism is reflected in body size only and does not extend to tooth morphometrics) (Van Horn, McElhinny & Holekamp, 2003). Within Darling Downs Diprotod and other Diprotod assemblages in general, it is not possible unambiguously to assign gender on the basis of morphometric or morphological differences in dentary and tooth size. However, by drawing analogy to all extant megaherbivores, the male is almost invariably larger than the female (although extant black rhinos, Diceros bicorn, are slightly more monomorphic) (Owen-Smith, 1988). Additionally, the male is significantly larger than the female in all sexually dimorphic extant marsupials greater than 5 kg in mean sexed body weight (predominantly macropodoids) (Jarman, 1989). By drawing such analogies for Diprotod, it is most likely that the large form is male and the small form is female. However, that hypothesis remains difficult to test. Although placentals show morphological and morphometric skeletal differences reflective of parturition and thus of gender (Averianov, 1996; Lister, 1996), such differences are less obvious in marsupials (Tague, 2003). Some extant marsupials do show gender-related differences in skeletal limb portions (Jarman, 1989); however, demonstrating similar post-cranial, gender-related osteological differences in Diprotod is difficult because there are very few (if any) completely articulated or associated Diprotod skeletons for comparison.

BACCHUS MARSHDIPROTODON

In most cases, COV values for Bacchus Marsh Diprotod cheek teeth dimensions are more similar to those of the modern grey kangaroo population than to the Darling Downs Diprotod assemblage (Tables 2–4). That may reflect: (1) a different temporal sampling range at Bacchus Marsh than at the Darling Downs (presumably of less duration for Bacchus Marsh); (2) a condition inherent to either the Bacchus Marsh or the Darling Downs Diprotod assemblages; and/or (3) simply reduced variation due to one size class being represented. Testing such hypotheses is difficult due to the paucity of material available for study. However, an extended temporal period of sampling would support recent sedimentological and taphonomic interpretations regarding the attritional nature of the majority of Darling Downs fossil assemblages (Price & Sobbe, 2005; Price & Webb, 2006). Additionally, taphonomic data for Bacchus Marsh indicate that the Diprotod assemblage accumulated during a single, mass mortality event, suggesting that biotic sampling occurred over a particularly short temporal scale (Long & Mackness, 1994).

At face value, the data compiled for Bacchus Marsh Diprotod are not entirely consistent with an interpretation of sexual dimorphism within the assemblage. As with the Darling Downs, there are no consistent dental morphologies to warrant separation of more than one morphospecies in the assemblage. However, morphometric data suggest that the single morphospecies was monomorphic in body size, but similar in size to the small-form (?female) Darling Downs Diprotod (Fig. 8; Tables 1–3). It would be expected that a sexually dimorphic assemblage would contain members of both sexes (i.e. both size classes). However, biological and taphonomic information provides some evidence that the assemblage could represent a gender-segregated herd. Extant sexually dimorphic megafauna commonly occur in gender-segregated herds (Owen-Smith, 1988; Haynes, 1991; Berger et a, 2001). As the assemblage is thought to represent individuals trapped in muds of a drying marsh (Long & Mackness, 1994), it is possible that a single herd was sampled. Long & Mackness (1994) suggested that the preponderance of subadult to adult individuals over juvenile and mature individuals in the Bacchus Marsh Diprotod assemblage reflected a population that was stressed by drought. Similar demographic profiles have been observed in drought-stressed sexually dimorphic marsupials such as the extant red kangaroo, Macropus rufDesmarest, 1817, and the grey kangaroo (Newsome, 1965; Kirkpatric & McEvoy, 1966). Red and grey kangaroo fecundity decreases in drought conditions and adults tend to be the dominant surviving group (Newsome, 1965; Kirkpatric & McEvoy, 1966). Thus, the over-abundance of small-form (?female) individuals in the Bacchus Marsh Diprotod assemblage may be explained by drought effects. Norbury, Coulson & Walters (1988) observed that male-biased mortality is extenuated by drought conditions in sexually dimorphic marsupials such as the western grey kangaroo, Macropus fuliginosDesmarest, 1817. Such biased mortality patterns are similar in all age classes and reflect disparate energy costs imposed by sex differences in body size and mobility (Norbury et a, 1988). Thus, at Bacchus Marsh, large-form (?male) Diprotod individuals of all age classes may have a succumbed to drought conditions before small-form (?female) Diprotod individuals, and hence were not sampled in this chance event where one herd died around a drying water course. The other assemblages that contain both large- and small-form individuals all represent broader attritional accumulation processes that would be unlikely to sample only a single herd.

LAKE CALLABONNADIPROTODON

At least six large- and six small-form Lake Callabonna individuals were examined in this study, derived mostly from collections of the South Australian Museum. That result contrasts with Stirling & Zietz (1899) who suggested that the large form was significantly more common than the small form. The difference in perceived abundance of the large form over the small form may be a result of misidentification of size classes in the 1899 investigations, or alternatively, a greater number of specimens (dominated by large-form individuals) were available to Stirling & Zietz (1899). However, it is pertinent to note that there are several unprepared and unregistered specimens (including several large-form individuals) in the South Australian Museum collections that were not examined in this study.

The Lake Callabonna Diprotod assemblage is relatively autochthonous in comparison with most other assemblages examined in this study. Tedford (1973) suggested that several small groups of Diprotod crossed the lake at various times of low water levels, eventually becoming mired in the muddy substrate. Both mature large- and small-form individuals undoubtedly occur in the deposits (Fig. 8). However, previous collecting procedures have mixed the samples and it cannot be determined if the social groups were body size (i.e. gender) segregated.

As with the Darling Downs, there are no significant or consistent dental morphological differences to warrant separation of more than one Diprotod morphospecies in the Lake Callabonna assemblage. Coupled with the observations that a bimodal body size distribution exists in the assemblage, and the high probability that both size forms were temporally coeval, the data suggest that a single, sexually dimorphic Diprotod morphospecies is represented in the Lake Callabonna assemblage. Although Stirling & Zietz (1899) suggested that there were at least two species represented at Lake Callabonna (based on the occurrence of two size classes), they did not discount the possibility that the differences in body size represented intraspecific, rather than interspecific variation within the Diprotod assemblage.

Again, as with other mentioned assemblages, it is not possible unambiguously to assign gender to either Lake Callabonna Diprotod size form. Interestingly, at Lake Callabonna, neonatal remains of an individual Diprotod were recovered in association with an adult individual, and from the position where the pouch would have been located (Tedford, 1973). Although some remains of the neonatal individual were accessioned into museum collections, they were separated from the putative mother (Pledge, 1994). Thus, the question of gender within D. optat remains unresolved and ‘the most wonderful discovery ever made in the world’, according to George Hurst, discoverer of those Lake Callabonna specimens (fiTedford, 1973), was squandered.

Lake Callabonna Diprotod dental morphometric COV values are higher relative to Bacchus Marsh and extant populations of grey kangaroos, but are more similar to those of the Darling Downs (Tables 2–4). That may indicate that there was a different temporal sampling range at Lake Callabonna (presumably of longer duration) than those other assemblages. Again, that hypothesis is difficult to test, but it would support Tedford's (1973) suggestion that the accumulation of individuals was quite slow, possibly occurring over several thousands of years, and would be consistent with sampling from more than one herd.

MYALL CREEKDIPROTODON

As there were no dentaries sufficiently preserved to allow definitive distinction of large- and small-form individuals, it is not possible to separate isolated teeth into form class. However, the means of Myall Creek Diprotod cheek teeth are similar to the undifferentiated size class samples from the Darling Downs and Lake Callabonna (Tables 2, 3). Additionally, the morphometric range of tooth sizes encompasses that of both large- and small-form individuals of body size differentiated assemblages (e.g. Darling Downs and Lake Callabonna; Figs 14, 15). Thus, those observations are in agreement with those of Marcus (1976), and suggest that both Diprotod size classes are represented in the Myall Creek assemblage. Additionally, there are no consistent morphologies sufficient to warrant distinction of more than one morphospecies in the assemblage. Therefore, the data suggest that a single, sexually dimorphic Diprotod species is present in the Myall Creek assemblage.

Figure 14

Diprotod lower molar tooth area from major assemblages examined, arranged from left to right in order of increasing latitude. Horizontal line represents the mean, thick vertical line represents two standard deviations, and thin vertical line represents the range of measurements. A, M1. B, M2. C, M3. D, M4.

Figure 14

Diprotod lower molar tooth area from major assemblages examined, arranged from left to right in order of increasing latitude. Horizontal line represents the mean, thick vertical line represents two standard deviations, and thin vertical line represents the range of measurements. A, M1. B, M2. C, M3. D, M4.

Coefficient of variation values of the Myall Creek Diprotod assemblage, and extant grey kangaroos, are lower than that for the Darling Downs and Lake Callabonna Diprotod assemblages (Tables 2–4). Thus, the data may suggest that the temporal sampling range of the Myall Creek assemblage was less than that for the Darling Downs and Lake Callabonna.

LANCEFIELD SWAMPDIPROTODON

There are no consistent dental morphological differences to suggest that more than one morphospecies exists in the Lancefield Swamp Diprotod assemblage. Additionally, both Diprotod size classes are undoubtedly represented in the deposit. Thus, the data support an interpretation of sexual dimorphism within Lancefield Swamp Diprotod.

Horton & Connah (1981) suggested that the mortality profile of the Lancefield Swamp Diprotod, which is dominated by mature individuals and lacks young individuals, reflects a drought assemblage. However, Van Huet (1999) identified fluvial regimes with variable flow and deposition, and suggested that post-mort selective sorting was equally plausible in explaining the accumulation of the Diprotod assemblage. COV values for Lancefield Swamp Diprotod cheek teeth are similar to those of the extant population of grey kangaroos (Tables 2–4), suggesting that the accumulation occurred over a discrete temporal period. Thus, although the data provide some support for a rapidly accumulated drought assemblage (Horton & Connah, 1981), variable abrasion on some Diprotod dentaries also supports suggestions of secondary reworking of some components of the assemblage (Van Huet, 1999).

Figure 15

Diprotod upper molar tooth area from major assemblages examined, arranged from left to right in order of increasing latitude. Horizontal line represents the mean, thick vertical line represents two standard deviations, and thin vertical line represents the range of measurements. A, M1. B, M2. C, M3. D, M4.

Figure 15

Diprotod upper molar tooth area from major assemblages examined, arranged from left to right in order of increasing latitude. Horizontal line represents the mean, thick vertical line represents two standard deviations, and thin vertical line represents the range of measurements. A, M1. B, M2. C, M3. D, M4.

REDDESTONE CREEKDiprotodon

As with the Myall Creek assemblage, it is not possible definitively to separate isolated Diprotod teeth into large-and small-form individuals. However, dental morphometric data of Reddestone Creek Diprotod are similar to morphometric data of corresponding teeth of undifferentiated size class samples of other assemblages (Tables 2, 3), and the range of teeth sizes encompasses that of both large- and small-form individuals of body size differentiated samples (Figs 14, 15). Those data suggest that both Diprotod size classes were sampled in the Reddestone Creek assemblage. Additionally, there is little basis to discriminate more than one Diprotod morphospecies in the assemblage. Thus, the data suggest that a single, sexually dimorphic species is represented at Reddestone Creek.

Horton & Connah (1981) observed that there were very few young or very old Diprotod individuals represented at Reddestone Creek, and suggested that the Diprotod assemblage represented a rapid accumulation of individuals from a drought-stressed population. COV values of Reddestone Creek Diprotod cheek teeth are more similar to values of corresponding teeth of extant grey kangaroos and Bacchus Marsh, Lancefield Swamp and Myall Creek Diprotod assemblages, than Lake Callabonna and Darling Downs Diprotod assemblages. Thus, the data suggest that the assemblage was sampled over a shorter temporal scale than at Lake Callabonna and the Darling Downs, thus supporting Horton & Connah's (1981) suggestion that the accumulation of Diprotod at Reddestone Creek took place over a relatively short time.

TAXONOMY OFDIPROTODON

Differences in dentary size and morphology exist within several of the assemblages examined (Fig. 8). However, those morphologies are also exhibited in all other assemblages where dentaries are well preserved. Where dentaries can be unequivocally assigned as either large- or small-form Diprotod, associated cheek teeth reflect a bimodal size distribution, although a considerable degree of overlap exists between measurements (e.g. Table 1, Figs 10, 11, 14, 15). On the basis of dental morphological and morphometric data, there is currently little basis to discriminate more than one Diprotod morphospecies within or between the Darling Downs, Reddestone Creek, Myall Creek, Lake Callabonna, Lancefield Swamp and Bacchus Marsh assemblages. Thus, dimorphism in Diprotod body size, as suggested by dentary and teeth size, can be explained as being purely size-related, not reflecting an ontogenetic series. Therefore, following criteria established in the Introduction, the differences between the large- and small-form Diprotod, within and between all major assemblages examined, reflect intraspecific differences, i.e. sexual dimorphism within a single morphospecies, rather than interspecific differences. That interpretation has significant implications for the taxonomy of most currently recognized Diprotod species. All studied material is considered here to belong to Diprotodon optatOwen, 1838(see Appendix 1). Diprotodon australOwen, 1844, D. annexta McCoy, 1861, D. minHuxley, 1862, D. longiceMcCoy, 1865, D. lodeKrefft, 1873a, D. bennettKrefft, 1873b (nec D.‘bennettOwen, 1877) and D. ‘bennettOwen, 1877 (nec D. bennettKrefft, 1873b) are here considered junior synonyms of D. optat. Generally, most diagnostic features originally suggested for the synonymized species were related to size or slight morphological variations between single, isolated specimens collected from isolated geographical areas. The range of dental morphological and morphometric variation exhibited in both large- and small-form individuals, and for previously erected Diprotod species, is, however, easily encompassed within single Diprotod assemblages of the eastern Darling Downs, Reddestone Creek, Myall Creek, Lake Callabonna, Lancefield Swamp and Bacchus Marsh. A major implication of this taxonomic interpretation is that only one species of Diprotod, rather than eight, lived during, and suffered extinction, sometime in the late Pleistocene.

Sexual dimorphism is characteristic of a wide variety of extant mammals and is particularly prevalent in megaherbivores. For example, within extant African and Asian elephants, adult males may grow to be almost twice as heavy as adult females (Owen-Smith, 1988). While there are obvious differences in the shape of the skull, dentary and tusk between genders (Haynes, 1991), cheek teeth are morphologically identical (Roth, 1992). Although male elephants have slightly larger cheek teeth than females, the difference is not marked (Roth, 1992). Within large extant sexually dimorphic marsupials such as the grey kangaroo, there is also a similar overlap in teeth dimensions between sexes (Fig. 13), but with no major differences in dental morphology. An identical relationship is obvious between the large- and small-form Diprotod (Figs 4, 7; Table 1), thereby providing additional support for the interpretation of sexual dimorphism.

SPATIAL AND TEMPORAL DISTRIBUTION

Stirling & Zietz (1899), Gill (1954), Murray (1991) and Molnar & Kurz (1997) suggested that the small-form Diprotod was less common, and geographically restricted in comparison with the large-form Diprotod. However, the proportions of large- to small-form individuals in the Darling Downs, Lake Callabonna and Lancefield Swamp Diprotod assemblages are fairly close to parity or slightly biased to the small form, suggesting that both size classes were sympatric, equally abundant and widespread. For reasons suggested above, the difficulties involved in accurately identifying a large- or small-morph D. optat may be polarized when analysing only fragmentary and isolated remains, and have previously led to misinterpretations regarding the distribution of Diprotod size classes.

As here recognized, Diprotodon optat was extremely wide-ranging, with a near continent-wide distribution (Fig. 2). Thus, the Pleistocene geographical distribution of D. optat was similar to that for extant megaherbivores such as African elephants. In the historical period, African elephants had a near-continental distribution and occupied almost every habitat south of the Sahara (although, more recently, the geographical distribution has decreased significantly) (Douglas-Hamilton & Michaelmore, 1996). Like African elephants, D. optat was probably also a habitat generalist. That interpretation is supported by stable isotope data that suggest that Diprotod had the ability to utilize a variety of C3 and C4 food resources over wide geographical areas (Gröcke, 1997).

Although only one-half of the sites examined in this paper are well dated, there is little evidence of significant temporal morphological and morphometric variation between Diprotodon optat assemblages. Given the trend for diprotodontoids to increase in body size over time from the early Miocene to the late Pleistocene (Murray, 1991), the similarities in teeth size between all Diprotod assemblages examined suggests that those localities may be temporally coeval. Drawing from analytical dating of Darling Downs, Lake Callabonna and Lancefield Swamp deposits (Gill, 1978; Gillespie et a, 1978; Van Huet et a, 1998; Roberts et a, 2001; Price, 2005; Price & Sobbe, 2005; Price et a, 2005; Webb et a, 2007), that interpretation would suggest that the Bacchus Marsh, Myall Creek and Reddestone Creek assemblages may also be late Pleistocene. However, that hypothesis may obscure potential physiological geographical body size effects that could relate to spatial habitat or climatic differences, such as Bergmann's rule (Bergmann, 1847; Meiri & Dayan, 2003). Bergmann's rule suggests that animals in cooler climates tend to be larger than congeners that occur in warmer climates (Bergmann, 1847). Such geographical morphoclines may be the result of a taxon's response to latitudinal differences in primary productivity and seasonality, or physiological adaptations to ambient temperature, although the mechanisms driving the underlying patterns remain unclear (Meiri & Dayan, 2003; Blackburn & Hawkins, 2004). Regardless of the cause, for extant Australian marsupials, Bergmann's rule suggests that species in southern (cooler) regions are larger than species that occur in northern (warmer) regions.Yom-Tov & Nix (1986) observed Bergmann-type responses of taxa within several Australian marsupial groups. For D. optat, there does not appear to be a latitudinal morphocline in body size (Figs 14, 15). However, that hypothesis relies on the assumption that all D. optat assemblages examined in this study are temporally coeval. Ideally, that hypothesis can be tested only by securely dating material from all Diprotod localities examined.

Diprotodon optat is so far reliably known only from Pleistocene deposits. However, Tedford (1994) reported Diprotod from Pliocene deposits of Fisherman's Cliff & Dog Rocks, Victoria.Archer (1977) regarded those records as doubtful, and Mackness & Godthelp (2001) suggested that such specimens may actually belong to Euryzygo. The youngest reliable records place Diprotod in late Pleistocene deposits (Roberts et a, 2001; Pledge, Prescott & Hutton, 2002).

PALAEOBIOLOGY

Mating strategy

If it is provisionally accepted that the large-form Diprotodon optat is male and small form is female, there are some very interesting implications for behaviour and life strategy. In many large extant marsupials such as kangaroos, there is a strong correlation between body size and degree of sexual dimorphism. That is, larger-sized species tend to be more sexually dimorphic than smaller-sized species (Jarman, 1989). That is also evident here, where the degree of dimorphism within the ∼2500-kg D. optat (∼4–17% difference in cheek teeth morphometrics between genders) is greater relative to that of the extant ∼50-kg grey kangaroo (∼4–10% difference in cheek teeth morphometrics between genders). For ungulates, Loison et a (1999) suggested that allometry was unimportant in shaping sexual dimorphism, but rather, the degree of polygyny can almost entirely account for the relationship between increasing size dimorphism and body size. Most notably, polygynous mating systems are dominant in all sexually dimorphic extant megaherbivores (Owen-Smith, 1988). Thus, by drawing analogy to extant groups, it is most likely that D. optat exhibited a polygynous breeding strategy.

Social groups

In cases of large-sized extant taxa exhibiting extreme sexual dimorphism, mature males and females commonly live in separate social groups (Owen-Smith, 1988). Importantly, the frequency of sexual segregation increases with increasing levels of sexual body size dimorphism (Mysterud, 2000). Such gender segregation may be a consequence of differential exploitation of resources (i.e. ecological segregation) relating to asynchrony of activity budgets between sexes (Isacc, 2005). However, reasons for ecological segregation are less clear. There is a clear link between body size and the nutritional ecology of large herbivores (Demment & Van Soest, 1985). Larger-sized herbivores can survive on lower quality food than smaller-sized herbivores. Thus, within large-sized taxa exhibiting extreme sexual dimorphism, larger-sized males may accept lower quality forage than smaller-sized females (Demment & Van Soest, 1985; Mysterud, 2000). Alternatively, gender segregation may affect both performance and survival of the sexes. Large males may select high-quality forage in order to improve body condition and growth, factors that may affect fighting ability and therefore reproductive success (Mysterud, 2000). Females may select habitats that maximize their ability to raise young (Mysterud, 2000). Haynes (1991) observed that in modern bone assemblages of extant sexually dimorphic megaherbivores such as elephants, adult females and young are always the most abundant, and males are comparatively rare. For sexually dimorphic ungulates, Ruckstuhl & Neuhaus (2002) suggested that gender segregation is related to incompatibilities of activity budgets and movement rates. Males and females are commonly found in the same geographical area (thus do not segregate by habitat), select the same plants, but have differential temporal patterning of foraging (Ruckstuhl & Neuhaus, 2002). Examining food resource partitioning between the large-and small-form D. optat may be possible using stable isotope analysis. However, current research in such dietary studies has not differentiated between large- and small-form individuals (e.g. Gröcke, 1997).

By drawing analogy to large-sized extant sexually dimorphic herbivores, Diprotodon optat probably did exhibit gender segregation among social groups. However, extracting data relating to social structure from the examined Diprotod assemblages is difficult due to the allocthonous nature of most deposits. The Bacchus Marsh deposit is one of the most autochthonous deposits examined in this study, and the Diprotod assemblage indicates that the small form (?female) was abundantly represented (N = 18 individuals) whilst the large form (?male) was absent. Other assemblages with large numbers of individuals such as the Darling Downs (N > 50 individuals) are also biased (∼ 1.4:1) towards small-form (?female) individuals. Thus, such evidence provides some support for gender social segregation within D. optat herds.

EVOLUTION

Diprotodon optat and the Pliocene Euryzygoma dunen (de Vis, 1888a) are united within diprotodontines by the unique morphology of the basicranium where posterior elongation and fusion of the postglenoid process with the tympanic process and mastoid–squamosal forms a complete auditory meatus (Murray et a, 2000). However, D. optat is more derived than E. dunen by possessing: (1) higher-crowned molars; (2) molar lophs that are angled more perpendicular relative to the length of the dentary; and (3) by being larger (an example of phyletic giantism; senGould & MacFadden, 2004). Additionally, although highly variable, D. optat P3 morphology where the paracone and metacone are divided is an autapomorphy within the Diprotodontinae (Archer, 1977). The morphological and morphometric variability of D. optat premolars may be the result of their small relative size (Tables 1, 2), associated with their presumed slight functional importance to the animal (Marcus, 1976). The interpretation here of sexual dimorphism within Diprotod raises the possibility that sexual dimorphism also may have occurred in other members of the family. Thus far, sexual dimorphism has previously only been reported in the zygomaturine Neohelos stirto (Murray et a, 2000).

CONCLUSION

Previously erected Diprotod species were described predominantly from single specimens that were collected from wide geographical areas. This study examined large numbers of Diprotod remains (> 1000 teeth) from six large, geographically dispersed assemblages, as well as ‘spot’ collections from Queensland, New South Wales, South Australia and Victoria. A comparison of morphological and morphometric variation within and between single assemblages suggests that there is little basis for the discrimination of more than one morphospecies, indicating that Diprotod was monotypic. That interpretation has implications for determining rates of prehistoric megafaunal losses in Australia, and suggests that only one Diprotod morphospecies, rather than eight, suffered extinction during the late Pleistocene. The single morphospecies, D. optat, had a near-continental geographical distribution, similar to that of extant megaherbivores (e.g. African elephants) from other parts of the world. Such an extensive geographical distribution suggests that D. optat was a habitat generalist.

There are two distinct size classes within Diprotodon optat. Such dimorphism is distinguished here from interspecific and ontogenetic differences on the basis of dental morphological and morphometric data. Therefore, D. optat is interpreted as being sexually dimorphic. By drawing analogy to extant megaherbivores and marsupials that exhibit extreme sexual dimorphism, it is probable that the large-form D. optat was male and the small form was female. Diprotodon optat was probably polygynous and was probably a herding species that exhibited social gender segregation. Those results further our understanding of the palaeoecology of Diprotod.

ACKNOWLEDGEMENTS

I thank G. Webb and B. Cooke who guided the early development of this paper and provided many constructive comments on earlier drafts. S. Hocknull and A. Cook are thanked for their constructive comments and discussions on Diprotod taxonomy. For access to institutional collections, I thank R. Molnar, A. Cook, S. Hocknull, K. Spring, J. Wilkinson, D. Lewis and S. Parfrey (Queensland Museum); J. McNamara and N. Pledge (South Australia Museum); A. Currant and J. Hooker (British Museum of Natural History); R. Jones (Australia Museum); K. Piper, D. Pickering and E. Fitzgerald (National Museum of Victoria). For additional access to resources or material I thank I. Sobbe and family, T. Sutton and family, O. McCaw, D. and J. Price, D. Norman, J.-X. Zhao and Y.-X. Feng. This research was funded in part by the Queensland University of Technology, the Queensland Museum, and an Australian Research Council Linkage Grant LP0453664.

REFERENCES

Anderson
W.
1890
.
On the post-Tertiary ossiferous clays, near Myall Creek, Bingera
.
Records of the Geological Survey of New South Wales
 
1
:
116
126
.
Andersson
M.
1994
.
Sexual selection
 .
Princeton, NJ
:
Princeton University Press
.
Anon
1861
.
The Argus
 
4783
: October 1st, 1861, p.
5
, cols 1–2.
Aplin
KP
,
Archer
M.
1987
.
Recent advances in marsupial systematics with a new syncretic classification
. In:
Archer
M
ed.
,
Possums and opossums: studies in evolution
 .
Sydney
:
Surrey Beatty & Sons and the Royal Zoological Society of New South Wales
,
xv
lxxii
.
Archer
M
1977
.
Origins and subfamilial relationships of Diprotod (Diprotodonidae, Marsupialia)
.
Memoirs of the Queensland Museum
 
18
:
37
39
.
Archer
M
1984
.
Background to the search for Australia's oldest mammals
. In:
Archer
M
,
Clayton
G
eds.
,
Vertebrate zoogeography and evolution in Australasia
 .
Carlisle
:
Hesperian Press
,
517
565
.
Averianov
AO
1996
.
Sexual dimorphism in the mammoth skull, teeth, and long bones
. In:
Shoshani
J
,
Tassy
P
eds.
,
The Proboscidea: evolution and palaeoecology of elephants and their relatives
 .
Oxford
:
Oxford University Press
,
260
267
.
Bartholomai
A
1973
.
Stratigraphy, skeletal morphology and evolution of the Upper Cainozoic and recent Macropodidae of Queensland
 . DPhil Thesis, University of Queensland.
Bartholomai
A
1976
.
Notes of the fossiliferous Pleistocene fluviatile deposits of the eastern Darling Downs
.
Bureau of Mineral Resources, Geology and Geophysics, Bulletin
 
166
:
153
154
.
Bartholomai
A
,
Woods
JT
1976
.
Notes of the vertebrate fauna of the Chinchilla Sand
.
Bureau of Mineral Resources, Geology and Geophysics, Bulletin
 
166
:
151
152
.
Berger
J
,
Dulamtseren
S
,
Cain
S
,
Enkkhbileg
D
,
Lichtman
P
,
Namshir
Z
,
Wingard
G
,
Reading
R
2001
.
Back-casting sociality in extinct species: new perspectives using mass death assemblages and sex ratios
.
Proceedings of the Royal Society of London Series B Biological Sciences
 
268
:
131
139
.
Bergmann
C
1847
.
Ueber die verhältnisse der wärmeökonomie der thiere zu ihrer grösse
.
Göttinger Studien
 
3
:
595
708
.
Black
K
,
Archer
M
1997
.
Silvabesti, a new genus and two new species of primitive zygomaturines (Marsupialia, Diprotodontidae) from Riversleigh, northwestern Queensland
.
Memoirs of the Queensland Museum
 
41
:
181
208
.
Blackburn
TM
,
Hawkins
BA
2004
.
Bergmann's rule and the mammal fauna of northern North America
.
Ecography
 
27
:
715
724
.
Brook
BW
,
Bowman
DMJS
,
Burney
DA
,
Flannery
TF
,
Gagan
MK
,
Gillespie
R
,
Johnson
CN
,
Kershaw
P
,
Magee
JW
,
Martin
PS
,
Miller
GH
,
Peiser
B
,
Roberts
RG
2007
.
Would the Australian megafauna have become extinct if humans had never colonised the continent? Comments on ‘A review of the evidence for a human role in the extinction of Australian megafauna and an alternative explanation’ by S. Wroe and J. Field
.
Quaternary Science Reviews
 
26
:
560
564
.
Cameron
DW
1997
.
Sexual dimorphic features within extant Great Ape faciodental skeletal anatomy and testing the single species hypothesis
.
Zeitschrift für Morphologie und Anthropologie
 
81
:
253
288
.
Cooke
BN
2000
.
Cranial remains of a new species of Balbarine kangaroo (Marsupialia: Macropodoidea) from the Oligo-Miocene freshwater limestone deposits of Riversleigh World Hertiage Area, Northern Australia
.
Journal of Paleontology
 
74
:
317
326
.
Dawson
L
,
Augee
ML
1997
.
The late Quaternary sediments and fossil vertebrate fauna from Cathedral Cave, Wellington Caves, New South Wales
.
Proceedings of the Linnean Society of New South Wales
 
117
:
51
78
.
Dayan
T
,
Wool
D
,
Simberloff
D
2002
.
Variation and covariation of skulls and teeth: modern carnivores and the interpretation of fossil mammals
.
Paleobiology
 
28
:
508
526
.
Demment
MW
,
Van Soest
PJ
1985
.
A nutritional explanation for body size patterns of ruminant and non-ruminant herbivores
.
American Naturalist
 
125
:
641
672
.
Desmarest
AG
1817
.
Nouveau dictionnaire d'histoire naturelle appliquée aux arts
 .
Paris
:
C. Deterville.
Douglas-Hamilton
I
,
Michaelmore
F.
1996
.
Loxodonta africa : range and distribution, past and present
. In:
Shoshani
J
,
Tassy
P
eds.
,
The Proboscidea: evolution and palaeoecology of elephants and their relatives
 .
Oxford
:
Oxford University Press
,
321
326
.
Gause
GF
1934
.
The struggle for existence
 .
Baltimore, MD
:
Williams & Wilkins
.
Gill
T.
1872
.
Arrangement of the families of mammals with analytical tables
.
Smithsonian Miscellaneous Collection
 
11
:
1
98
.
Gill
ED.
1954
.
The range and extinction of Diprotodon min Huxley
.
Proceedings of the Royal Society of Victoria
 
66
:
225
228
.
Gill
ED.
1978
.
Geology of the late Pleistocene Talgai cranium from S.E. Queensland, Australia
.
Archaeology and Physical Anthropology in Oceania
 
13
:
177
197
.
Gillespie
R
,
Horton
DR
,
Ladd
P
,
Macumber
PG
,
Rich
TH
,
Thorne
R
,
Wright
RVS.
1978
.
Lancefield Swamp and the extinction of the Australian megafauna
.
Science
 
200
:
1044
4048
.
Gingerich
PD.
1974
.
Size variability of the teeth of living mammals and the diagnosis of closely related sympatric fossil species
.
Journal of Paleontology
 
48
:
896
903
.
Gingerich
PD.
1981
.
Variation, sexual dimorphism, and social structure in the early Eocene horse Hyracotheri (Mammalia, Perissodactyla)
.
Paleobiology
 
7
:
443
455
.
Gould
SJ.
1975
.
On the scaling of tooth size in mammals
.
American Zoology
 
15
:
353
362
.
Gould
GC
,
MacFadden
BJ.
2004
.
Gigantism, dwarfism, and Cope's rule: nothing in evolution makes sense without a phylogeny
.
Bulletin of the American Museum of Natural History
 
285
:
219
237
.
Gröcke
DR
1997
.
Distribution of C3 and C4 plants in the late Pleistocene of South Australia recorded by isotope biogeochemistry of collagen in megafauna
.
Australian Journal of Botany
 
45
:
607
617
.
Guerin
C
,
Winslow
JH
,
Piboule
M
,
Faure
M
1981
.
Le prétendu rhinocéros de Nouvelle Calédonie est un marsupial (Zygomaturus diahotens nov. sp.): solution d'une énigmeet conséquences paléogéographiques
.
Geobios
 
14
:
201
217
.
Hand
SJ
,
Archer
M
,
Rich
TH
,
Pledge
NS.
1993
.
Nimbad, a new genus and three species of Tertiary zygomaturines (Marsupialia, Diprotodontidae) from northern Australia, with a reassessment of Neohelos
.
Memoirs of the Queensland Museum
 
33
:
193
210
.
Haynes
G.
1991
.
Mammoths, mastodonts, and elephants: biology, behavior, and the fossil record
 .
New York
:
Cambridge University Press
.
Horton
DR
1984
.
Red kangaroos: last of the Australian megafauna
. In:
Martin
PS
,
Klein
RG
eds.
,
Quaternary extinctions: a prehistoric revolution
 .
Tuscon, AZ
:
University of Arizona Press
,
639
690
.
Horton
DR
,
Connah
GE.
1981
.
Man and megafauna at Reddestone Creek, near Glen Innes, northern New South Wales
.
Australian Archaeology
 
13
:
35
52
.
Huxley
TH
1862
.
On the premolar teeth of Diprotod, and on a new species of that genus
.
Quarterly Journal of the Geological Society
 
18
:
422
427
.
Illiger
C
1811
.
Prodromus systematis mammalian et avium aditus terminus zoographicis utriudque classis
 .
Berlin
:
C. Salfield.
Isacc
JL.
2005
.
Potential causes and life-history consequences of sexual size dimorphism in mammals
.
Mammal Review
 
35
:
101
115
.
Jarman
PJ.
1989
.
Sexual dimorphism in Macropodoidea
. In:
Grigg
G
,
Jarman
P
,
Hume
I
eds.
,
Kangaroos, wallabies and rat-kangaroos
 .
Sydney, NSW
:
Surrey Beatty and Sons
.
432
447
.
Keble
RA.
1945
.
The stratigraphical range and habitat of the Diprotodontidae in southern Australia
.
Proceedings of the Royal Society of Victoria
 
57
:
23
48
.
Kirkpatric
TH
,
McEvoy
JS.
1966
.
Studies of the Macropodidae in Queensland. 5. Effects of drought on reproduction in the grey kangaroo (Macropus gigante)
.
Queensland Journal of Agriculture and Animal Sciences
 
23
:
439
442
.
Krefft
G.
1871
.
Natural history. The Megatheri
.
The Sydney Mail and New South Wales Advertiser
 
579
: 12, August 5th, p. 722, cols 1–2.
Krefft
G
1873a
.
Natural history. Review of Professor Owen's papers on the fossil mammals of Australia
.
Sydney Mail and New South Wales Advertiser
 
686
: 16, August 23rd, p.
238
, col. 2.
Krefft
G
1873b
.
Natural history. Mammals of Australia and their classification. Part I: Ornithodelphia and Dipelphia
.
Sydney Mail and New South Wales Advertiser
 
697
: 16, November 8th, p.
594
595
(plus supplement).
Krefft
G
1875
.
Remarks on the working of the molar teeth of the diprotodons
.
Quarterly Journal of the Geological Society
 
31
:
317
318
.
Leitch
I
,
Hytten
FE
,
Billewics
DWZ
1959
.
The maternal and neonatal weights of some mammalia
.
Proceedings of the Zoological Society of London
 
133
:
11
28
.
Lister
AM
1996
.
Sexual dimorphism in the mammoth pelvis: an aid to gender determination
. In:
Shoshani
J
,
Tassy
P
eds.
,
The Proboscidea: evolution and palaeoecology of elephants and their relatives
 .
Oxford
:
Oxford University Press
,
254
259
.
Loison
A
,
Gaillard
JM
,
Pélabon
C
,
Yoccoz
NG
1999
.
What factors shape sexual size dimorphism in ungulates?
Evolutionary Ecology Research
 
1
:
611
633
.
Long
J
,
Mackness
BS
1994
.
Studies of the late Cainozoic diprotodontid marsupials of Australia. 4. The Bacchus Marsh diprotodons – Geology, sedimentology and taphonomy
.
Records of the South Australian Museum
 
27
:
95
110
.
Longman
HA
1921
.
A new genus of fossil marsupials
.
Memoirs of the Queensland Museum
 
3
:
65
80
.
Longman
HA
1924
.
Some Queensland fossil vertebrates
.
Memoirs of the Queensland Museum
 
8
:
16
28
.
Luckett
WP
1993
.
An ontogenetic assessment of dental homologies in therian mammals
. In:
Szalay
FS
,
Novacek
MJ
,
McKenna
MC
eds.
,
Mammal phylogeny
 .
New York
:
Springer
,
182
204
.
McCoy
F
1865
.
On the bones of a new gigantic marsupial
.
Transactions and Proceedings of the Royal Society of Victoria
 
6
:
25
.
McCoy
F
1876
.
Prodromus of the palaeontology of Victoria; or figures and descriptions of Victorian organic remains – decade IV
 .
Victoria
:
Geological Survey of Victoria
.
Mackness
BS
,
Godthelp
H
2001
.
The use of Diprotod as a biostratigraphic marker of the Pleistocene
.
Transactions of the Royal Society of South Australia
 
125
:
155
156
.
Mahoney
JA
,
Ride
WDL
1975
.
Index to the genera and species of fossil Mammalia described from Australia and New Guinea between 1838 and 1968
.
Western Australian Museum Special Publication
 
6
:
1
250
.
Marcus
LF
1976
.
The Bingara fauna, a Pleistocene vertebrate fauna from Murchinson County, New South Wales, Australia
.
University of California Publications in Geological Sciences
 
114
:
1
146
.
Mead
AJ
2000
.
Sexual dimorphism and paleoecology in Teleocer, a North American Miocene rhinoceros
.
Paleobiology
 
26
:
689
706
.
Meiri
S
,
Dayan
T
2003
.
On the validity of Bergmann's rule
.
Journal of Biogeography
 
30
:
331
351
.
Mitchell
TL
1838
.
Three expeditions into the interior of Eastern Australia, with descriptions of the recently explored region of Australia Felix, and of the present colony of New South Wales
 .
London
:
T. and W. Boone
, 1–2.
Molnar
RE
,
Kurz
C
1997
.
The distribution of Pleistocene vertebrates on the eastern Darling Downs, based on the Queensland Museum collections
.
Proceedings of the Linnean Society of New South Wales
 
117
:
107
134
.
Murray
PF
1991
.
The Pleistocene megafauna of Australia
. In:
Vickers-Rich
P
,
Monaghan
JM
,
Baird
RF
,
Rich
TH
eds.
,
Vertebrate palaeontology of Australasia
 .
Melbourne
:
Pioneer Design Studio & Monash University
,
1071
1164
.
Murray
PF
,
Megirian
D
,
Rich
TH
,
Plane
MD
,
Vickers-Rich
P
2000
.
Neohelos stirto, a new species of Zygomaturinae (Diprotodonta: Marsupialia) from the mid-Tertiary of the Northern Territory, Australia
.
Records of the Queen Victoria Museum
 
105
:
1
47
.
Mysterud
A
2000
.
The relationship between ecological segregation and sexual body size dimorphism in large herbivores
.
Oecologia
 
124
:
40
54
.
Newsome
AE
1965
.
Reproduction in natural populations of the red kangaroo, Megaleia ru (Demarest), in central Australia
.
Australian Journal of Zoology
 
13
:
735
759
.
Norbury
GL
,
Coulson
GM
,
Walters
BL
1988
.
Aspects of the demography of the western grey kangaroo, Macropus fuliginosus melano, in semiarid northwest Victoria
.
Wildlife Research
 
15
:
257
266
.
O'Leary
MA
,
Lucas
SG
,
Williamson
TE
2000
.
A new specimen of Ankalag (Mammalia, Mesonychia) and evidence of sexual dimorphism in mesonychians
.
Journal of Vertebrate Paleontology
 
20
:
387
393
.
Owen
R
1843
.
Additional evidence proving the Australian pachyderm described in a former number of the ‘Annals’ to be a Dinotheri, with remarks on the nature and affinities of that genus
.
Annals and Magazine of Natural History
 
11
:
329
332
.
Owen
R
1844
.
Description of a fossil molar tooth of a Mastod discovered by Count Strzlecki (sic) in Australia
.
Annals and Magazine of Natural History
 
14
:
268
271
.
Owen
R
1845
.
Report on the extinct mammals of Australia, with descriptions of certain fossils indicative of the former existence in that continent of large marsupial representatives of the Order Pachydermata
.
Report of the British Association for the Advancement of Science
 
14
:
223
240
.
Owen
R
1866
.
On the anatomy of vertebrates 11, birds and mammals
 .
London
:
Longmans, Green, and Co.
Owen
R
1870
.
On the fossil mammals of Australia. Part III. Diprotodon austral, Owen
.
Philosophical Transactions of the Royal Society of London
 
160
:
519
578
.
Owen
R
1877
.
Researches on the extinct fossil mammals of Australia; with a notice of the extinct marsupials of England
 .
London
:
J. Erxleben.
Owen
R
1838
. In:
Mitchell
TL
ed.
,
Three expeditions into the interior of Eastern Australia, with descriptions of the recently explored region of Australia Felix, and of the present colony of New South Wales
 .
London
:
T. and W. Boone
, 2:
362
363
.
Owen-Smith
RN
1988
.
Megaherbivores: the influence of very large body size on ecology
 .
Cambridge
:
Cambridge University Press
.
Parker
P
1977
.
An ecological comparison of marsupial and placental patterns of reproduction
. In:
Stonehouse
B
,
Gilmore
D
eds.
,
The biology of marsupials
 .
London
:
MacMillan Press
,
273
286
.
Pledge
NS
1994
.
Fossils of the Lake: a history of the Lake Callabonna excavations
.
Records of the South Australian Museum
 
27
:
65
77
.
Pledge
NS
,
Prescott
JR
,
Hutton
JT
2002
.
A late Pleistocene occurrence of Diprotod at Hallett Cove, South Australia
.
Transactions of the Royal Society of South Australia
 
126
:
39
44
.
Price
GJ
20052004
.
Fossil bandicoots (Marsupialia, Peramelidae) and environmental change during the Pleistocene on the Darling Downs, southeastern Queensland, Australia
.
Journal of Systematic Palaeontology
 
2
:
347
356
.
Price
GJ
,
Sobbe
IH
2005
.
Pleistocene palaeoecology and environmental change on the Darling Downs, southeastern Queensland, Australia
.
Memoirs of the Queensland Museum
 
51
:
171
201
.
Price
GJ
,
Tyler
MJ
,
Cooke
BN
2005
.
Pleistocene frogs from the Darling Downs, south-eastern Queensland, and their palaeoenvironmental significance
.
Alcheringa
 
29
:
171
182
.
Price
GJ
,
Webb
GE
2006
.
Late Pleistocene sedimentology, taphonomy and megafauna extinction on the Darling Downs, southeastern Queensland
.
Australian Journal of Earth Sciences
 
53
:
947
970
.
Ride
WDL
,
Cogger
HG
,
Dupuis
C
,
Kraus
O
,
Minelli
A
,
Thompson
FC
,
Tubbs
PK
1999
.
International code of zoological nomenclature
 .
London
: The International Trust for Zoological Nomenclature.
Roberts
RG
,
Flannery
TF
,
Ayliffe
LK
,
Yoshida
H
,
Olley
JM
,
Prideaux
GJ
,
Laslett
GM
,
Baynes
A
,
Smith
MA
,
Jones
R
,
Smith
BL.
2001
.
New ages for the last Australian megafauna: continent-wide extinction about 46,000 years ago
.
Science
 
292
:
1888
1892
.
Roth
VL.
1992
.
Quantitative variation in elephant dentitions: implications for the delimitation of fossil species
.
Paleobiology
 
18
:
184
202
.
Ruckstuhl
KE
,
Neuhaus
P.
2002
.
Sexual segregation in ungulates: a comparative test of three hypotheses
.
Biological Reviews
 
77
:
77
96
.
Schrein
CM.
2006
.
Metric variation and sexual dimorphism in the dentition of Ouranopithecus macedoniens
.
Journal of Human Evolution
 
50
:
460
468
.
Shaw
G.
1790
.
The naturalist's miscellany: or coloured figures of natural objects, drawn and described immediately from nature
 .
London
:
G. Kearsley.
Simpson
GG.
1930
.
Post-Mesozoic Marsupialia. Fossilium Catalogus 1: Animalia. Pars 47
 .
Berlin
:
W. Junk
.
Stephenson
NG.
1967
.
Phylogenetic trends and speciation among wombats
.
Australian Journal of Zoology
 
15
:
873
880
.
Stirling
EC.
1907
.
Reconstruction of Diprotod from the Callabonna deposits, South Australia
.
Nature
 
76
:
543
544
.
Stirling
EC
,
Zietz
AHC.
1899
.
Fossil remains of Lake Callabonna: part 1- Description of the manus and pes of Diprotodon austral, Owen
.
Memoirs of the Royal Society of South Australia
 
1
:
1
40
.
Stirton
RA
,
Woodburne
MO
,
Plane
MD.
1967
.
Tertiary Diprotodontidae from Australia and New Guinea
.
Bureau of Mineral Resources, Geology and Geophysics, Bulletin
 
85
:
1
149
.
Tague
RG.
2003
.
Pelvic sexual dimorphism in a metatherian, Didelphis virginia : implications for eutherians
.
Journal of Mammology
 
84
:
1464
1473
.
Tedford
RH.
1973
.
The diprotodons of Lake Callabonna
.
Australian Natural History
 
17
:
349
354
.
Tedford
RH
1994
.
Succession of Pliocene through medial Pleistocene mammal faunas of southeastern Australia
.
Records of the South Australian Museum
 
27
:
79
93
.
Turnbull
WD
,
Lundelius
EL
Jr
,
Tedford
RH
1992
.
A Pleistocene marsupial fauna from Limeburner's Point, Victoria, Australia
.
Beagle, Records of the Northern Territory Museum of Arts and Sciences
 
9
:
143
172
.
Turner
A
1987
.
Megantereon cultride (Cuvier) (Mammalia, Felidae, Machairodontinae) from Plio-Pleistocene deposits in Africa and Eurasia, with comments on dispersal and the possibility of a New World origin
.
Journal of Paleontology
 
61
:
1256
1268
.
Van Horn
RC
,
McElhinny
TL
,
Holekamp
KE
2003
.
Age estimation and dispersal in the spotted hyena (Crocuta crocu)
.
Journal of Mammology
 
84
:
1019
1030
.
Van Huet
S
1999
.
The taphonomy of the Lancefield swamp megafaunal accumulation, Lancefield, Victoria
.
Records of the Western Australian Museum, Supplement
 
57
:
331
340
.
Van Huet
S
,
Grün
R
,
Murray-Wallace
CV
,
Redvers-Newton
N
,
White
PJ
1998
.
Age of the Lancefield megafauna: a reappraisal
.
Australian Archaeology
 
46
:
5
11
.
De Vis
CW
1888a
.
On a supposed new species of Nototheri
 . Abstract of Proceedings of the Linnean Society of New South Wales for December 28, 1887: V.
De Vis
CW
1888b
.
On Diprotodon min- Hux
.
Proceedings of the Royal Society of Queensland
 
4
:
38
44
.
Webb
GE
,
Price
GJ
,
Nothdurft
LD
,
Deer
L
,
Rintoul
L
2007
.
Cryptic meteoric diagenesis in freshwater bivalves: implications for radiocarbon dating
 .
Geology in press
.
Wells
RT
,
Tedford
RH
1995
.
Sthenur (Macropodidae: Marsupialia) from the Pleistocene of Lake Callabonna, South Australia
.
Bulletin of the American Museum of Natural History
 
255
:
1
111
.
Woods
JT
1958
.
The extinct marsupial genus Palorchest Owen
.
Memoirs of the Queensland Museum
 
13
:
177
193
.
Woods
JT.
1960
.
Fossiliferous fluviatile and cave deposits
. In:
Hill
D
,
Denmead
AK
eds.
, The geology of Queensla.
Journal of the Geological Society of Australia
 
393
403
.
Woods
JT
1968
.
The identity of the extinct marsupial genus Nototheri
.
Memoirs of the Queensland Museum
 
15
:
111
116
.
Wroe
S
,
Crowther
M
,
Dortch
J
,
Chong
J
2003
.
The size of the largest marsupial and why it matters
.
Proceedings of the Royal Society of London B (Supplement)
 
271
:
S34
S36
.
Wroe
S
,
Field
J
2006
.
A review of the evidence for a human role in the extinction of Australian megafauna and an alternative interpretation
.
Quaternary Science Reviews
 
25
:
2692
2703
.
Yom-Tov
Y
,
Nix
H
1986
.
Climatological correlates for body size of five species of Australian mammals
.
Biological Journal of the Linnean Society
 
29
:
245
262
.

Appendices

APPENDIX 1: SYSTEMATIC PALAEONTOLOGY

MARSUPIALIAILLIGER, 1811 DIPROTODONTIAOWEN, 1866 DIPROTODONTIDAEGill, 1872 DiprotodontinaeStirton et a, 1967

DiprotodOwen, 1838(Figs 3, 6, 8–12).

Type species:

Diprotodon optatOwen, 1838.

Holotype and type locality:

BM10796 (Fig. 1), anterior extremity of right mandibular ramus with an incomplete incisor, collected from the Wellington Caves, New South Wales. Age of type locality is (?late) Pleistocene (Dawson & Augee, 1997).

Referred specimens:

See supplementary Appendix S1.

Generic diagnosis:

Large-sized diprotodontid characterized by a combination of characters including: I1 chisel-like, straight to slightly curved dorsally and/or slightly curved laterally, procumbent, hypselodont; I1 broad, strap-like, curved ventrally, hypselodont; P3 small in comparison with molars, paracone and metacone divided, protocone connected to metacone, parastyle reduced to well developed, lacking hypocone; P3 bilophodont; molars bilophodont, high crowned, commonly with crescentic cementum, lacking conspicuous midlinks, lophids relatively perpendicular to dentary length; tooth enamel with punctate markings.

Description of holotype:

Fragment of right horizontal ramus, broken anterior to P3 alveoli, juvenile, 145 mm maximum length, 65 mm maximum depth at symphysis; diastemal crest straight; symphysis elongated, ovoid in outline, ventral border gently curved in lateral profile similar to typical small-form condition; lower incisor broken anteriorly just anterior to incisor alveolus, procumbent, narrows anteriorly, slightly curved dorsally, slightly ovoid in cross-section, enamel confined to lateral external surface, curving around ventrally to lower mesial one-third; tooth enamel punctate.

Remarks

It has commonly been suggested that the rugose, punctate appearance of Diprotod tooth enamel is diagnostic of the genus (e.g. Archer, 1977). However, dental enamel of similar appearance occurs in other diprotodontids, including some individuals of Euryzygo (e.g. QMF3369), and to a lesser degree in Nototherium ( e.g. QMF518). Thus, although that feature may be characteristic of some diprotodontids, it is not a generic diagnostic character unique to Diprotod.

Diprotodon optatOwen, 1838(Figs 3, 6, 8–12)

1843 Dinotherium austra Owen, pp. 329–332, figs 1, 2.

1844 Diprotodon austral Owen p. 268.

1845 Diprotodon austral Owen, pp. 224–231.

1861 Diprotodon annexta McCoy, p. 5 (see Anon., 1861).

1862 Diprotodon (austral ?) Huxley, pp. 422–427, pl. 21, figs 1–3.

1862 Diprotodon min Huxley, pp. 422–427, pl. 21, figs 4–6.

1865 Diprotodon longice McCoy, p. 25.

1870 Diprotodon austral Owen, pp. 519–578, pl. 35, figs 1–4; pls 36–39; pl. 40, figs 1–13, 16–18; pls 41–50.

1871 Diprotodon lode Krefft, nomen nud, p. 722.

1873a Diprotodon lode Krefft, p. 238.

1873b Diprotodon bennett Krefft (nec DiprotodbennettOwen, 1877), pp. 594–595, and supplement pl. 1, figs 1, 2, 4.

1875 Diprotodon min Krefft, p. 317, fig. 1.

1876 Diprotodon longice McCoy, pp. 7–11, pl. 31–33.

1877 Diprotodon austral Owen, pp. 507–512, pl. 72, figs 1–3; pls 73–74.

1877 Diprotodbennett’ Owen (nec Diprotodon bennettKrefft, 1873b), pp. 510–511.

1877 Nototherium victori Owen, p. 513. pl. 88, figs 15–17.

1877 Nototherium iner Owen, p. 478. pl. 88, figs 11–14.

1888b Diprotodon min de Vis, pp. 38–44, pl. unnumbered.

1888b Diprotodon austral de Vis, pp. 38–44, pl. unnumbered.

1899 Diprotodon austral Stirling and Zietz, pp. 1–40, pl. 1–18.

1924 Diprotodon australLongman, pp. 16–17, pl. 1, fig. 1; pl. 2, fig. 2.

1924 Diprotodon minLongman, pp. 17–19, pl. 1, fig. 2, pl. 2, fig. 1.

1945 Diprotodon longiceKeble, pp. 23–48.

1945 Diprotodon australKeble, pp. 23–48.

1954 Diprotodon min Gill, pp. 225–228, fig. 1.

1967 Diprotodon optat Stirton et a, fig. 2.

1976 Diprotodon min Marcus, pp. 105, 113–115, fig. 51.

1977 Diprotod sp. Archer, pp. 37–39, pl. 15.

1992 Diprotod sp. cf. D. optat Turnbull, Lundelius & Tedford, pp. 164–167, fig. 17.

2005 Diprotod sp. Price and Sobbe, p. 185, fig. 10a.

Specific diagnosis:

As for genus.

Holotype:

As for genus.

COMPARISON WITH PREVIOUSLY ERECTED SPECIES

Diprotodon optatOwen, 1838,D. australOwen, 1844

Workers have had different opinions regarding the status of Diprotodon optatOwen, 1838 versus D. austral 1844 as the type species of the genus (Mahoney & Ride, 1975). The name D. austral was regarded as valid by some workers (e.g. McCoy, 1876; Stirling & Zietz, 1899), while others have regarded it as a replacement name for D. optat and its use erroneous (Simpson, 1930). Mahoney & Ride (1975) suggested that D. australOwen, 1844 is undoubtedly a replacement name for D. optatOwen, 1838, and that Owen was possibly under the misapprehension that he had earlier used the name ‘austral’ rather than ‘optat’. In Owen's original description of Diprotod in volume II of Mitchell (1838), no specific name was given. However, there is ample indication that the species listed as ‘… Diprotodon optat Owen (extinct genus)’ in volume I of Mitchell (1838: XIX) was based on the same and sole specimen as that upon which the genus was described subsequently in volume II. Owen later discussed the D. optat holotype in his original description of D. austral (Owen 1844: 268) stating that ‘the tusk from one of the bone-caves of Wellington Valley, described by me in Sir T. Mitchell’s “Expeditions into the Interior of Australia”, vol. ii, 1838, p. 362. pl. 31. figs 1 and 2, as indicative of a new genus and species of gigantic marsupial animal, to which I gave the name of Diprotodon austral ' (see alsoOwen, 1845, 1870). Interestingly, Owen never used the name ‘optat’ beyond its original use in Mitchell's ‘Expeditions into the Interior of Australia’ (Mahoney & Ride, 1975), although it has been used frequently by other authors (e.g. Simpson, 1930; Woods, 1958; Marcus, 1976; Murray, 1991; Turnbull, Lundelius & Tedford, 1992; Tedford, 1994; Roberts et a, 2001). According to Article 23.1 of the ‘Code of Zoological Nomenclature’ (Ride et a, 1999), the oldest available name of a taxon is the valid name, unless the name has been invalidated or another name has been given precedence by any provision of the Code or by a ruling of the International Commission on Zoological Nomenclature. The name ‘optat’ is clearly the oldest available name, and thus D. optat is here given precedence, and D. austral is regarded as a junior synonym.

The Diprotodon optat holotype shows some morphological similarities to small-form individuals, although due to the fragmentary and juvenile nature of the material, it is difficult confidently to assign the specimen to either large- or small-form Diprotod. However, the specimen is morphologically similar to juvenile dentary fragments from the Darling Downs (QMF3366) and Lancefield Swamp (NMVP151805-6) Diprotod assemblages. Therefore, the single morphospecies identified from the Darling Downs, Bacchus Marsh, Lake Callabonna, Myall Creek, Lancefield and Reddestone Creek Diprotod assemblages is considered here to represent D. optat.

Diprotodon annexta McCoy, 1861/D. longiceMcCoy, 1865

Diprotodon annexta was erected on the basis of a broken mandible from a mature large-form individual collected from near Colac, Victoria (NMVP12109). The original description was presented at a meeting to the Royal Society of Victoria (Anonymous, 1861), although the original paper was subsequently lost (McCoy, 1865). However, an abstract was later published regarding that specimen, but with a new name, D. longice (McCoy, 1865), and a holotype was subsequently described (McCoy, 1876). Thus, the relationship between the names ‘annexta’ and ‘longice’ is similar to that described above for ‘optat’ and ‘austral’. However, the dentary and associated dentition of D. annexta /longice is morphologically similar to and falls within the range of variation of specimens from all major Diprotod assemblages examined above. Thus, there is no reason to separate D. annexta /longice as a distinct species of Diprotod. Additionally, when McCoy (1876) finally described the D. annexta /longice holotype, he also referred an unprepared Diprotod palate to the species (NMVP13003). However, Turnbull et a (1992) re-examined that specimen and suggested that it was morphologically similar to Diprotod collected from Lake Callabonna. The morphology of the palate (NMVP13003) and dentition compares well with specimens collected from Lake Callabonna, as well as other assemblages examined, and supports Turnbull et a 's (1992) interpretation.

Diprotodon minHuxley, 1862

Diprotodon min was erected on the basis of a portion of maxilla containing LP3 –M1−2 and an isolated M4 from a small-form individual (BM10771) from Gowrie Creek, Darling Downs. Huxley (1862) separated the species on the basis of size, as well as P3 morphology. However, as demonstrated above, the range of morphometric (Table 2) and morphological variation (Fig. 6) of Diprotod P3 is quite large. The morphology of the D. min holotype is easily encompassed within the range of variation of the single morphospecies identified from the major Diprotod assemblages. Thus, D. min is here considered a junior synonym of D. optat.

Diprotodon lodeKrefft, 1873a

Diprotodon lode was erected on the basis of a palate containing complete dentition from a large-form individual (AMF4623) collected from near Murrurundi, New South Wales (Mahoney & Ride, 1975). Diprotodon lode is morphologically similar to Diprotod material examined from all assemblages above and is here considered a junior synonym of D. optat.

Diprotodon bennettKrefft, 1873b(nec DiprotodbennettOwen, 1877)

Diprotodon bennettKrefft, 1873b was also based on material collected from Gowrie Creek, Darling Downs. The holotype (BM47855) is a near-complete mandible from a large-form individual. The mandible and associated dentition falls within the range of morphological and morphometric variation from all the major Diprotod assemblages examined above, and is here regarded a junior synonym of D. optat.

DiprotodbennettOwen, 1877(nec Diprotodon bennettKrefft, 1873b)

DiprotodbennettOwen, 1877 is a preoccupied name. The holotype is based on a right dentary with broken RM3−4 of a large-form individual (BM46056) collected from near Mendooran, New South Wales (Mahoney & Ride, 1975). The dentary and associated dentition are morphometrically and morphologically within the range of variation encompassed within all the major Diprotod assemblages. Thus, the holotype of D. ‘bennett’ is considered here to represent D. optat.