Gliridae (Rodentia, Mammalia) with a simple dental pattern: a new genus and new species from the European Early and Middle Miocene

A new genus of Gliridae, Simplomys gen. nov. is proposed. It contains glirids with a simplified dental pattern from the European Early and Middle Miocene. Simplomys gen. nov. includes several species originally described as Pseudodryomys such as Simplomys simplicidens, Simplomys robustus, Simplomys julii, and Simplomys aljaphi. In addition, a new species, Simplomys meulenorum sp. nov., is proposed from the Spanish Miocene. The species of this genus share not only a very reduced and simplified dental morphology, but also unique dental proportions that clearly separate them from any other genera of Gliridae. Simplomys gen. nov. is recorded in most of the fossil faunas from the Early and Middle Miocene of the Iberian Peninsula, and shows the maximum diversity in this area during Mammal Neogene Zones MN 3 and MN 4. The genus has been also recorded in other European countries such as France, Germany, and Switzerland, conferring to this very characteristic taxon an important role for biochronological correlations within the European continent.



© 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 157, 622–652.


INTRODUCTION
The Early and Middle Miocene comprise one of the most important intervals in the evolutionary history of the 'Old World' family Gliridae (Rodentia, Mammalia). During this period, they reached their maximum diversity and relative abundance, becoming the most important group in European rodent faunas (Daams & de Bruijn, 1995). This maximum diversity of the family was not only related to an increase in the redundancy of existing dental morphologies but also because of evolution towards new morphological configurations that probably represented an increase in niches occupied by this family. Amongst the new morphological configurations, the evolution towards a more simple dental morphology that occurred during the Early Miocene mainly in the Iberian Peninsula stands out. The glirids with a simple dental morphology are especially abundant in Spanish Lower and Middle Miocene micromammal assemblages, representing the most diverse group of glirids in this area. Most of the species having a simple dental pattern are included in the genera Armantomys De Bruijn, 1966, Praearmantomys De Bruijn, 1966, and Pseudodryomys De Bruijn, 1966. The first two genera, in addition to their trend towards dental simplification, show relatively high crowned teeth, which is a unique character amongst the glirids, and are endemic to the Iberian Peninsula, as was pointed out by Daams (1991) in his revision of these two genera (for comments about correct date of publications and various species described in the latter paper see García-Paredes & van den Hoek Ostende, 2007). The species included in Pseudodryomys have more brachyodont teeth, interpreted as less specialized, and had a wider distribution through Europe.
The genus Pseudodryomys was defined based on material from Miocene localities of the Calatayud-Montalbán basin (Spain) and contained two different morphologies. The first morphology is represented by the type species Pseudodryomys ibericus De Bruijn 1966, which shows a rather conservative morphology. The second one is represented by Pseudodryomys simplicidens De Bruijn, 1966 which shows a derived simplified morphology and proportional reduction of the outer dental elements of the cheek teeth row.
Despite the important morphological differences shown by these two species (Daams, 1974), subsequently described species having affinities with either of these two morphologies have been assigned to Pseudodryomys without questioning the homogeneity of the genus (Hugueney et al., 1978;Daams, 1989;Martín-Suárez, Freudenthal & Agustí, 1993;Daams & de Bruijn, 1995;Daams, 1999). Daams (1974) gave detailed descriptions of Pseudodryomys from ten Lower and Middle Miocene Spanish localities. Since then, no further detailed descriptions were made until the unpublished work of García-Paredes (2006), even though the number of micromammal localities from the Spanish Lower and Middle Miocene has increased enormously. In the Gliridae assemblages, Pseudodryomys (sensu Daams & de Bruijn, 1995) is particularly abundant (García-Paredes, 2006). This increase in information and available material, which remained unstudied for a long time, allows us to define better the different species and evaluate changes in size and morphology amongst the various assemblages.
Several species of Pseudodryomys with a simple dental pattern form a well-defined group with particular dental proportions of premolars and third lower molars and more simplified morphology of those elements. These derived characters can be used as diagnostic criteria to propose a new genus, Simplomys gen. nov. from Lower and Middle Miocene localities of Spain, Portugal, France, Germany, and Switzerland. Revision of the species assigned to the new genus allows to solve taxonomic problems, to specify the stratigraphical distribution of species and the recognition of a new species of this genus, Simplomys meulenorum sp. nov.

MATERIAL AND METHODS
The nomenclature for the parts of the occlusal surface of the cheek teeth of Gliridae ( Fig. 1) is modified after De Bruijn (1966) and Daams (1981). P/p and M/m is used for upper/lower premolars and upper/lower molars respectively. The measurements of specimens were taken with a Nikon Measuroscope 10 microscope with digital micrometer and are given in mm units. Width represents the maximum width of the crown, taken parallel to the contact facet of the posterior wall, in the case of premolars and first molars, or the anterior wall in second and third molars. Length represents the maximum length of the crown and was measured at right angles to the width. Whenever possible, two indexes were calculated: 'surface' as length multiplied by width in mm 2 , and the length-: width ratio, a nondimensional index that gives an idea of the slenderness of the tooth.
Additional indices for the relative size of P4/p4 and M3/m3 were calculated by dividing the surface of P4/p4 and M3/m3 by the surface of adjacent teeth (M1/m1 and M2/m2, respectively). These indices represent the relative dental proportions. The ratios were calculated from the average values for surface (Appendix). As a rule, this was only carried out when at least five measurable specimens of each element were available. However, exceptions were made in order to include type localities of important species and/or small collections. Localities, species, and values used to calculate those indices are shown in the Appendix. Scatter diagrams presented in Figure 2A and B and Figure 3A and B show the length : width ratios of P4/p4 and M3/m3 versus their relative dental proportions. Table 1 lists the material studied, including abbreviations for fossil sites and institutions where collections are stored (see below). Descriptive statistics for measurements of the studied assemblages are given in Tables 2  and 3.
Size comparisons between Simplomys gen. nov. species were carried out by performing Student's t-test in localities containing the two species to compare and at least five data per dental element of each species (Table 4).
The chronological framework and correlations used in this paper, as well as the stratigraphical distribution of species of Simplomys gen. nov., are shown in Table 5.
The photographs of specimens were taken on the Environmental Scanning Electron Microscope FEI Quanta 200 in environmental mode at the 'Museo Nacional de Ciencias Naturales -CSIC' (National Museum of Natural Sciences -CSIC) in Madrid (Spain). All specimens in Figures 4-9 are figured as if they are from the left side. If the original is from the right side its label is underlined on the plates. Holotypes are indicated by an italic character.
Lists of synonymies of species include only those references that imply a change in name in order to avoid very long lists of complete references, which are given in Supporting Information Appendix S1.    Bruijn (1966) and Daams (1981). Drawings represent molars from the left side and names in italics indicate main cusps. localities from the Calatayud-Montalbán basin. The most common dental pattern in Ps. ibericus consists of upper molars with four main ridges, two centrolophs and sometimes one or two extra ridges inside the trigon. Lower molars always have one posterior extra ridge and a relatively long centrolophid in addition to the four main ridges. In contrast, Ps. simplicidens is characterized by a simple dental pattern. The most common morphology shows only the four main ridges in the upper molars and lower molars with a short centrolophid as the only additional crestid. Subsequent species assigned to Pseudodryomys have been recognized as part of the two main morphological groups: Pseudodryomys aljaphi Hugueney et al.,  1978, Pseudodryomys rex (García-Moreno, 1986in Álvarez Sierra & García-Moreno, 1986, and Pseudodryomys granatensis Agustí, 1993in Martín-Suárez et al., 1993, are placed with Ps. ibericus in the more complex group (Daams & de Bruijn, 1995;Daams, 1999). Pseudodryomys robustus De Bruijn, 1967; Pseudodryomys julii Daams, 1989 and Ps. simplicidens are included in the simple dental pattern group (Daams & de Bruijn, 1995;Daams, 1999). The latter group was recognized on the basis of its particular dental morphology. Pseudodryomys simplicidens and Ps. robustus were considered as morphologically identical species (De Bruijn, 1967;Daams, 1974) Figure 3. A, scatter diagram of the ratio of length : width of the p4 and the surface of p4/m1 in several Gliridae assemblages. B, scatter diagram of the ratio of length : width of the m3 and the surface of m3/m2 in several Gliridae assemblages (see Appendix for details). The circles denote the ranges of the Simplomys gen. nov. assemblages (continuous line) and the Pseudodryomys assemblages (discontinuous line).
Álvarez Sierra et al. (1991) raised the question as to whether or not Ps. simplicidens and Ps. robustus should be included in the genus Pseudodryomys. A discussion on that subject was not included, because it was beyond the scope of their paper (for comments about the correct date of publication of various species described in Álvarez Sierra et al., 1991, see García-Paredes & van den Hoek Ostende, 2007. No further mention has been made about the option to exclude the species with a simple dental pattern from Pseudodryomys until the unpublished work of García-Paredes (2006), in which the designation of a new genus to include them was proposed.
In this paper we formally propose the new genus Simplomys gen. nov. to include all the species that show a highly simplified dental morphology and/or with a reduction in the size proportions of the upper and lower premolars and third molars relative to the first and second molars, as will be discussed below, that were previously included in the genus Pseudodryomys. Diagnosis: Small to large-sized Gliridae with simple dental pattern and cheek teeth with moderately concave occlusal surface. The crowns of the cheek teeth are of medium height and the ridges are wide. Premolars are small and very simplified in morphology compared to molars. The upper molars generally consist of the four main ridges only; one centroloph may be present and in some cases even two. Centrolophs are usually not longer than half the width of the occlusal surface. In M1 and M2 the anteroloph ends freely on the lingual side. The protoloph and metaloph are connected before reaching the lingual border, forming an Y-shaped trigon. The lower molars have four or five ridges. If present, the centrolophid is small; its morphology varies from a short prolongation on the anterior lingual border to a well-developed ridge that extends diagonally into the central valley. Its length never exceeds the central axis of the occlusal surface. The mesolophid of the m3 is reduced and does not reach the labial border. The extension of the posterior valley varies from half the width of the occlusal surface to absent.
Differential diagnosis: Simplomys gen. nov. differs from Pseudodryomys De Bruijn, 1966, Peridyromys Stehlin & Schaub, 1951, Miodyromys Kretzoi, 1943, and Prodryomys Mayr, 1979 by its simpler dental pattern and by having relatively smaller and simplified premolars and lower third molars. In Simplomys gen. nov. the mesolophid of the m3 and the p4 is reduced and the posterior valley may be absent.
Simplomys gen. nov. differs from Pseudodryomys and Peridyromys by having a less concave occlusal surface in the upper molars.  Bruijn, 1966 andPraearmantomys De Bruijn, 1966 by having molars with thinner and lower ridges, by having upper molars with protoloph and metaloph always connected at the lingual half of the occlusal surface forming an Y-shaped trigon. In Simplomys gen. nov. the molars are not progressively smaller from M1/m1 through to M3/m3 as in Armantomys and Praearmantomys.
Simplomys gen. nov. differs from Tempestia Van de Weerd, 1976 by having molars with thinner and lower ridges, M1 and M2 with Y-shaped trigon, m1 and m2 longer than wider and the posterior part of the m3 more reduced and simplified.
Simplomys gen. nov. differs from Altomiramys Díaz Molina & López Martínez, 1979 by having cheek teeth with thicker and slanting ridges and narrower valleys, upper molars with less rounded outline and an anterior valley that does not extend along the lingual border to the protocone.
Etymology: After the simplicity of its dental pattern.

COMPARISON WITH OTHER GENERA
The particular characteristics used to define Simplomys gen. nov. are the combination of a simplified dental morphology with unusual dental proportions amongst the different elements of the tooth row. In our opinion, the dentition should be understood as a complete organ and hence a functional entity. It is important to study the variability of all cheek teeth. The importance of premolars and third molars for taxonomy has usually been underestimated. Several works pointed out the difficulties in their specific or even generic assignation (Daams, 1974(Daams, , 1981(Daams, , 1999b. In our experience, most of the premolars and third molars can be assigned and provide useful systematic characters.   Table 1. Type locality in bold.

Mean
Max.     Mein (1975Mein ( , 1990  To evaluate relative dental proportions in 'Pseudodryomys' (sensu Daams & de Bruijn, 1995), various assemblages of Miocene species included in that genus and other common species assigned to different genera have been analysed (Appendix).

SD
Several indices have been calculated to evaluate relative dental proportions: (1) the length : width ratios of upper and lower premolars and third molars; (2) the ratios between the surface of the upper and lower premolars and the first molar per species; and (3) the ratios between the surface of the upper and lower third molars and the second molar per species.
The results are shown in scatter diagrams (Figs 2A-B, 3A-B) that represent the slenderness of premolars and third molars (length : width ratio) versus their relative size to adjacent teeth.
In the case of P4 ( Fig. 2A), most assemblages fall with in a well-defined range of length : width ratio (0.7-0.9) showing small differences amongst most analysed species of Gliridae. Lower values of P4 length : width ratio are for species of Armantomys, Armantomys jasperi being the lowest, also for the surface P4 : surface M1 ratio. Similar values of surface P4 : surface M1 ratio are occupied by species of Simplomys gen. nov. but with higher P4 length-: width ratios, being clearly separated from the other species and particularly from the species of Pseudodryomys. The highest values of P4 length : width ratio are for Simplomys julii, which, with the most rounded P4 of all studied species, is outside of the abovementioned Gliridae range.
For the M3 length : width ratio and surface M3 : surface M2 ratio (Fig. 2B), lower values for both indices are for species of Simplomys gen. nov., species of Armantomys, and Praearmantomys crusafonti. They form a clearly differentiated group from the rest of species despite the small overlap between higher values of species of Simplomys gen. nov. (S. julii) and lower values of Pseudodryomys ibericus.
In the case of the p4 (Fig. 3A), there is a similar situation as in the P4 ( Fig. 2A). Most analysed species of Gliridae have a p4 length : width ratio of approximately 1.0. The lowest values of the p4 length : width ratio are for Simplomys simplicidens. For the p4 : m1 surface ratio there is a clear low values group including species of Simplomys gen. nov. and species of Armantomys. This group is clearly separated from Pseudodryomys ibericus and other Pseudodryomys species. Figure 3B shows the m3 length : width ratio and surface m3 : surface m2 ratio. The lowest values for both indexes are only for species of Simplomys gen. nov. with a small overlap between the highest values of species of Simplomys gen. nov. (Simplomys aljaphi) and the lowest values of Armantomys aragonensis. Two Simplomys robustus assemblages with low values of the m3 length : width ratio and medium values of the surface ratio are slightly out of the main Simplomys gen. nov. area being near to those of Praearmantomys crusafonti.
Based on the obtained results there is a clear differentiation in relative dental proportions amongst the genera with a very simple dental morphology (Armantomys, Praearmantomys, and Simplomys gen. nov.) and those with a somewhat conservative dental morphology (Pseudodryomys, Peridyromys, Prodryomys) or the more complicated ones (Microdyromys De Bruijn, 1966 andGlirudinus De Bruijn, 1966). All simplepatterned species have relatively small and slender upper and lower premolars and third molars compared to the more complicated species. The differences amongst the three genera with simple dental morphology are the following: Praearmantomys has in general less reduced premolars and third molars than Simplomys gen. nov. and Armantomys. Simplomys gen. nov. and Armantomys differ mainly by the length-: width ratio of the P4, being in general on average smaller in Simplomys gen. nov. than in Armantomys ( Fig. 2A) and the relatively more reduced m3 in Simplomys gen. nov. (Fig. 3B). In addition, there is a clear difference in hypsodonty amongst these genera, Simplomys gen. nov. being significantly lower crowned than Armantomys and Praearmantomys. The clear differences between the dental proportions of Simplomys gen. nov. species and those of Pseudodryomys are remarkable. The species assigned to the latter genus show dental proportions closer to the genera Peridyromys, Prodryomys, and even Microdyromys and Glirudinus than to Simplomys gen. nov.
The morphology of Simplomys aljaphi is intermediate in complexity between S. simplicidens and Pseudodryomys ibericus, as will be discussed below, but it already shows a high degree of reduction in the upper and lower premolars and third molars, and therefore has been included in Simplomys gen. nov. Pseudodryomys robustus from Armantes 3 in Daams, 1974 Pseudodryomys robustus from Armantes 3 in Daams, Freudenthal & van de Weerd, 1977Pseudodryomys meini Adrover, 1978 Pseudodryomys robustus in Alberdi, Morales & Sesé, 1980 Pseudodryomys robustus in Alberdi et al., 1981 Pseudodryomys robustus in Alberdi et al., 1984 Pseudodryomys robustus in Alberdi et al., 1985Pseudodryomys robustus in Sesé et al., 1985 Pseudodryomys robustus in Antunes et al., 1987 Pseudodryomys robustus in López Martínez, Sesé & Herráez, 1987Pseudodryomys robustus in Herráez, 1993 Pseudodryomys robustus in Cerdeño & Íñigo, 1997 Original diagnosis (De Bruijn, 1966: 10): 'Pseudodryomys with cheek teeth that are slightly larger than those of the previous species [Pseudodryomys iberi- Figure 6. Simplomys aljaphi (Hugueney et al., 1978) from Montaigu-le-Blin (modified after Hugueney et al., 1978). cus]. The dental pattern is very simple. The M1-2 have neither centrolophs nor extra ridges. The m3 is reduced. M1/m1 smaller than M2/m2.' Emended diagnosis by Daams (1974: 234): 'Pseudodryomys-assemblages with a simple dental pattern. The M1,2 has the posterior centroloph only, or no centroloph at all. The main ridges are wide. In the lower molars extra ridges are absent. The m3 is reduced.' Emended diagnosis: Simplomys species with a simple dental pattern, but the posterior centroloph may be present. In some cases, mainly in M2, posterior and anterior centrolophs are present, the posterior one being usually longer. Centrolophs are at right angles to the labial border and not longer than half the width of the occlusal surface. The dental pattern of m1 and m2 shows the four main ridges and a well-developed centrolophid extending diagonally into the central valley and never reaching more than half way along the occlusal surface. The m3 has the posterior part small and simplified in morphology and the posterior valley may be completely absent in the younger occurrences.  Geographical distribution: Simplomys simplicidens is widely known from Spain and is also present in Portugal, France, Switzerland and Germany (see Supporting Information Appendix S1).

REMARKS ON SIMPLOMYS SIMPLICIDENS
Simplomys simplicidens was proposed by De Bruijn (1966) for glirid material with a rather simple dental pattern from Miocene localities of the Calatayud-Montalbán basin. This author named an assemblage Ps. ibericus if more than 75% out of at least ten M1-2 had the 'ibericus -morphotype' and Ps. simplicidens if more than 75% of at least ten M1-2 had the 'simplicidens -morphotype'. If one of the two patterns did not exceed 75% of the total number of M1-2 or if the assemblage consisted of fewer than ten specimens, the indication Ps. ibericus -simplicidens was used (De Bruijn, 1967). According to De Bruijn & Moltzer (1974) it was not clear whether the specimens with 'ibericus' morphology and those with 'simplicidens' morphology were morphotypes occurring in one  Bruijn (1967). Daams (1974) studied Pseudodryomys material from ten Miocene localities of Spain, analysing the distribution of standard morphotypes of the cheek teeth. The results show that Ps. ibericus and Ps. simplicidens are separated species and the original diagnoses were emended. Later field campaigns, mainly in the Aragonian and Ramblian type areas (Calatayud-Montalbán basin), have increased the number of localities with Pseudodryomys and Simplomys gen. nov. enormously (see Supporting Information Appendix S1).
Based on more than 2700 specimens from 59 localities of the type area of the Aragonian, García-Paredes (2006) demonstrated trends toward size increase of the cheek teeth (Fig. 10), toward simplification of the p4 and toward simplification of the posterior part in the m3 of S. simplicidens. This study also demonstrated that morphological features, such as simplification degree of the m3, can be used to distinguish S. simplicidens from S. robustus, even though size ranges of S. simplicidens and S. robustus overlap (Fig. 10).  However, when these species occur in the same locality, S. robustus is always larger than S. simplicidens.
The situation with the material from Middle Miocene locality Armantes 3 in the Calatayud-Montalbán basin described as Pseudodryomys cf. simplicidens by De Bruijn (1967) and assigned to Pseudodryomys robustus by Daams (1974) and Daams et al. (1977) is similar and is here assigned to Simplomys simplicidens.
Pseudodryomys meini Adrover, 1978 described from Navarrete del Río (local biozone Z, MN1, Lower Miocene) was considered a junior synonym of Ps. simplicidens by Agustí (1983) as well as by Daams & de Bruijn (1995). However, these authors did not give any argument to justify this decision. The morphology of the specimens described as Ps. meini fits the diagnosis of Simplomys simplicidens. Adrover (1978) noted that the Navarrete del Río material is smaller than the type material of Pseudodryomys [= Simplomnys] simplicidens. Because of the trend toward size increase in S. simplicidens, this species is smaller in older than younger localities (Fig. 10). Therefore, Ps. meini Adrover (1978) is considered a subjective synonym of S. simplicidens.
The material from the Lower and Middle Miocene fissure fillings in southern France listed by Aguilar et al. (1999) is of special interest. Several of these localities (Ste. Catherine 3, Ste. Catherine 4, Ste. Catherine 5, and Ste. Catherine 9) contain both S. simplicidens and S. robustus. As a result of the nature of these fossil sites, the material assigned to S. simplicidens and to S. robustus in Aguilar et al. (1999), may represent the same species at different stages of evolution (Fig. 10). Therefore, further analysis is needed in order to establish whether this material belongs to S. robustus, S. simplicidens, or to Simplomys meulenorum sp. nov. (DE BRUIJN, 1967) (FIG. 5) Original diagnosis (De Bruijn, 1967: 259 Emended diagnosis by Daams (1974: 236): 'Pseudodryomys-assemblages consisting of large teeth which differ from P. simplicidens in size and in the less reduced m3. The dental patterns of the other molars are identical of those of P. simplicidens'.

SIMPLOMYS ROBUSTUS
Emended diagnosis: Large Simplomys in which the dental pattern in m1 and m2 is formed by the four main ridges and a well-developed centrolophid. The latter extends diagonally into the central valley but never exceeds the middle part of the occlusal surface. The m3 always has a posterior valley and shows little reduction in its posterior part. The M1 may rarely present a small anterior centroloph.
Differential diagnosis: Simplomys robustus differs from S. simplicidens by having the premolars and the m3 less reduced in size and less simplified in morphology. The posterior valley of m3 is, in contrast to S. simplicidens, always present. When both species occur in the same assemblage, S. robustus is always larger than S. simplicidens.

SIMPLOMYS ALJAPHI (HUGUENEY ET AL., 1978) (FIG. 6)
Original diagnosis (Hugueney et al., 1978: 28, translated from French): '[Pseudodryomys similar in size to P. ibericus and P. simplicidens, close to P. ibericus by the evolutionary degree of its M1-2, which also possesses centrolophs and sometimes accessory ridges, but with a protoloph almost parallel to the metaloph, which gives a quite less pointed trigon on the upper molars; this new species on the whole has more simple molars than P. ibericus, with P4 and M3 much more reduced and very simplified. The reduction of the P4 and M3 is not as extreme as in P. simplicidens, which, in addition, has upper molars without centrolophs]'. Differential diagnosis: Simplomys aljaphi differs from S. simplicidens by its less reduced and simplified premolars and m3. When centrolophs are present in the M1 and M2, they extend diagonally into the central valley. The anterior one is always present and longer than the posterior one in S. aljaphi, whereas in S. simplicidens the centrolophs are closer to being parallel to protoloph and/or metaloph and the posterior one is longer. Simplomys aljaphi differs from S. simplicidens by having, in general, a more complicated dental pattern in the lower molars that may include tiny and low extra ridges.
Simplomys aljaphi differs from S. robustus by its smaller size and its more complicated dental pattern.
Geographical distribution: Spain, France, and Switzerland.

REMARKS ON SIMPLOMYS ALJAPHI
This species is included in Simplomys gen. nov. because of its degree of reduction and simplification of the premolars and m3. The morphology of the premolars and the m3 of S. aljaphi, and their size relative to the molars are more similar to S. simplicidens than to Ps. ibericus. The morphology of m1 and m2 is also close to that of S. simplicidens. Most specimens from Montaigu-le-Blin described by Hugueney et al. (1978) have a relatively simple morphology and a similar number of ridges as in the most complicated S. simplicidens upper molars. The complicated ones are, however, more similar to Ps. ibericus than to S. simplicidens.
Emended diagnosis: Small Simplomys species with molars that have a flat to slightly convex occlusal surface and simple pattern. Anteroloph and posteroloph are situated slightly below the plane of the trigon (protoloph-metaloph). The protoloph and metaloph form a composite 'Y' shape with a bent connection. A line connecting the labial end of the metaloph and the lingual end of the protoloph is aligned at right angles to the anteroposterior axis of the tooth. The occlusal NEW GENUS AND SPECIES OF MIOCENE GLIRIDAE 641 surface of the M1 and M2 has a square outline. In M1 the anteroloph and the anterior valley are straight and parallel to the anterior border of the tooth. In the lower molars, the centrolophid is absent or poorly developed and it does not extend into the central valley. The m3 has the posterior part reduced in size and in most cases the posterior valley is absent.
Differential diagnosis: Simplomys julii differs from S. simplicidens, S. robustus, and S. aljaphi in its smaller size, less concave cheek teeth, and by having the centrolophid less developed.

Stratigraphical distribution: Local zones A to Db (MN 3-MN 4, Lower and Middle Miocene).
Geographical distribution: Spain and France (see Supporting Information Appendix S1).

REMARKS ON SIMPLOMYS JULII
Simplomys julii is a rare species in the fossil record. It has not been studied since Daams (1989).Material from other than the type locality was not described until the revision of S. julii from the Aragonian type area by García-Paredes (2006). This study demonstrated a trend toward size increase of the cheek teeth (Fig. 10) and gives for the first time detailed descriptions of the premolars and lower third molars of this species, showing its high degree of simplification and reduction (Fig. 7).
Simplomys julii has been considered to have a short stratigraphical distribution restricted to the lower Aragonian (Lower Miocene) (Daams & de Bruijn, 1995;Daams, 1999). García-Paredes (2006) extended the stratigraphical distribution of this species to the upper Ramblian (Lower Miocene) and part of the middle Aragonian (Middle Miocene) ( Table 5). Simplomys julii has been found only in Spain and France (see Supporting Information Appendix S1 for details on localities and references).
Diagnosis: Medium to large-sized Simplomys with simple dental pattern. Centrolophs are absent. The anteroloph is interrupted in the middle in half of the M1 and may be connected medially to the protoloph. The centrolophid is absent or poorly developed and does not extend into the central valley. The mesolophid of the m3 is reduced and limited to the lingual part (entoconid). In most cases the mesolophid and posterolophid are fused as a unique ridge and the posterior valley is absent.
Differential diagnosis: Simplomys meulenorum sp. nov. differs from S. simplicidens by having a less developed centrolophid and by its less reduced and simplified premolars. When these species occur in the same association, S. meulenorum sp. nov. is larger, and the m3 is more reduced and simplified. When the mesolophid and posterolophid of the m3 are fused in a single ridge, in S. meulenorum sp. nov. the last ridge is mainly composed of the posterolophid and the mesolophid is almost completely reduced (Fig. 8L, P), whereas in S. simplicidens the last ridge is equally composed of the lingual part of the mesolophid and the labial part of the posterolophid (Fig. 4H). Simplomys meulenorum sp. nov. differs from S. robustus by having a less-developed centrolophid and by the simpler morphology of the posterior part of the m3. When both species co-occur in the same locality S. meulenorum sp. nov. is smaller than S. robustus.
Simplomys meulenorum sp. nov. differs from S. aljaphi by its larger cheek teeth and by having in general a less complicated dental pattern, less developed centrolophid, and by having the posterior part of the m3 more reduced and simplified.
Simplomys meulenorum sp. nov. differs from S. julii by its larger cheek teeth and by having a more concave occlusal surface. In S. meulenorum sp. nov. the central valley of M1 and M2 is narrower and its protoloph-metaloph connection is more pointed than in S. julii. In S. meulenorum sp. nov. the anteroloph and the anterior valley of M1 are not straight as in S. julii.
Etymology: After our friend and colleague Dr Albert J. van der Meulen (Utrecht University, The Netherlands) and his family. Material and measurements from localities in the Aragonian type area: Tables 1-3.

Description of Simplomys meulenorum sp. nov.
from Artesilla P4: The occlusal surface is slightly concave and it has an oval or subtriangular outline. It is small in comparison to the molars. The dental pattern is simple and all specimens have four ridges. The anteroloph is usually short and situated in the middle part of the anterior border. Its labial and lingual parts end free in all the specimens. The anteroloph may be interrupted in its medial part and shows a medial connection with the protoloph in three out of eight specimens. The protoloph is oblique and its lingual end shows a cusp-like shape (protocone). The metaloph is short and its lingual end extends just to the centre of the occlusal surface. Protoloph and metaloph are connected in seven out of eight specimens forming a composite Y-shape. The short isolated posteroloph is situated medially and reaches neither the lingual nor the labial border.

M1:
The occlusal surface of the M1 is concave and has a square outline with rounded corners. The most common morphotype shows four main ridges, with the metaloph connected to the protoloph, forming an asymmetric Y-shape, and the anteroloph interrupted and medially connected to the protoloph. The anteroloph is well developed and its labial and lingual parts end free in all specimens. The anteroloph is interrupted in seven out of nine specimens. There is a longitudinal connection between the anteroloph and protoloph in six out of nine specimens. The protoloph is the longest ridge; its lingual part is more oblique than the anteroloph and reaches the posterolingual corner of the occlusal surface. The labial end of the protoloph is curved anteriorly near the border of the tooth. There are no centrolophs. The lingual end of the metaloph shows a slight bend and its labial end is curved backwards. The isolated posteroloph is relatively straight, it reaches neither the lingual nor the labial border.

M2:
The occlusal surface of the M2 has a more rectangular outline than the M1. The anteroloph is not divided and there is no longitudinal connection with the protoloph. The anteroloph ends free lingually and it is connected labially to the protoloph in one out of ten specimens. As in the M1, the protoloph is the longest ridge. Its lingual end reaches the posterolingual corner of the occlusal surface. The metaloph is relatively straight. The protoloph and metaloph are connected, forming an asymmetrical Y-shape. Centrolophs are absent. The posteroloph is relatively straight; it reaches neither the lingual nor the labial border. It is labially connected to the metaloph in one out of ten specimens and it is lingually connected to the protoloph in four out of 11 specimens.

M3:
The M3 has a concave occlusal surface and trapezoidal outline. The posterior part is narrower than the anterior one and it is slightly rounded. All specimens have four ridges. The labial and lingual ends of the anteroloph are curved backwards, and all specimens have the anteroloph connected labially to the protoloph. The anteroloph and protoloph are always connected labially, whereas they are lingually connected in only four out of ten specimens. As in the M2, the anteroloph is neither divided nor medially connected to the protoloph. The protoloph is curved posterolingually. Its lingual and labial ends are aligned at right angles to the anteroposterior axis of the tooth. Most specimens (eight out of ten) show a strongly asymmetric Y-shaped trigon but in two specimens the protoloph and metaloph are joined on the lingual border and the trigon shows an asymmetric V-shape. Centrolophs are absent. The curve of the metaloph is variable. Its labial part may be directed backwards. The posteroloph is the shortest ridge and ends free labially. The lingual end of the posteroloph reaches the border of the occlusal surface. It is lingually connected to the protoloph in seven out of ten specimens (forming an asymmetric Y-shaped trigon) and to the metaloph in two out of ten specimens (forming a V-shaped trigon). Only in one specimen is the posteroloph lingually free-ending.

NEW GENUS AND SPECIES OF MIOCENE GLIRIDAE 643
p4: The outline of the occlusal surface is oval and slightly concave. The p4 is small compared to the molars and its morphology is simpler than these. The anterior part is occupied by a curved ridge which is assumed to be formed by the union of the anterolophid and metalophid. The mesolophid is very short and connected to the posterolophid near the lingual border in three out of four specimens. There is a narrow trace of the posterior valley.

m1:
The occlusal surface is slightly concave, with a subrectangular outline. The anterior part is narrower than the posterior one. The short anterolophid is slightly curved. Its labial part ends free in all specimens and does not reach the labial border of the tooth. The metalophid is sharply bent and its isolated labial end constitutes the anterolabial corner of the tooth, whereas its lingual border does not reach the anterolingual corner, but is connected to the anterolophid in 11 out of 14 specimens. The centrolophid is absent (three out of 14 specimens) or is poorly developed. This ridge, interpreted as a residual centrolophid, extends along the lingual border instead of into the central valley. The centrolophid, metalophid, and anterolophid are connected at the lingual border of the tooth in six out of 14 specimens. Only two specimens show a short isolated centrolophid. The mesolophid is bent and its lingual part is directed anteriorly. The posterolophid is the longest ridge and is more curved than the mesolophid. These ridges are connected lingually in eight out of 14 specimens. m2: The m2 differs from m1 in having a more rectangular outline, but its morphology is very similar. In general, the anterolophid is not as short as the m1 and its labial end neither reaches the border of the tooth nor joins the metalophid. The angle of the metalophid, situated in its middle part, is less acute than in the m1. The metalophid ends behind the anterolingual corner of the tooth and is connected to the anterolophid in eight out of 12 specimens. In 11 out of 12 specimens a small ridge, interpreted as a residual centrolophid, extends along the lingual border, sometimes almost reaching the mesolophid. In one specimen this structure is absent. The centrolophid, metalophid, and anterolophid are connected at the lingual border in four out of 12 specimens. In the remaining specimen, the centrolophid is connected to the metalophid. The centrolophid is isolated in four out of 12 specimens but connected to the lingual border of the anterolophid in three out of 12 specimens. The mesolophid is less bent than in the m1 and is lingually connected to the posterolophid in six out of 12 specimens. The posterolophid is as described in the m1. m3: The occlusal surface is not very concave and its outline forms a half-ellipse. The anterolophid and metalophid are connected lingually in eight out of nine specimens. The centrolophid is more variable than in m1 and m2 and it is absent in five out of nine specimens. The centrolophid is isolated in one specimen; in the remaining specimens with a centrolophid (three out of nine) it is connected to the metalophid and the anterolophid at the lingual border of the tooth. The posterior part of the m3 is rounded, narrow, and very simplified in all specimens. The mesolophid and posterolophid are fused, forming a single composite posterior ridge and the posterior valley is absent in eight out of nine specimens. In the remaining specimen, the mesolophid is a very small, cusp-like, ridge connected to the posterolophid, but separated from it by a narrow trace of the posterior valley.

REMARKS ON SIMPLOMYS MEULENORUM SP. NOV.
In the localities of the Aragonian type area, S. meulenorum sp. nov. does not show much morphological variation but does show morphological simplification of the m3. In the older localities of zone B, the m3 is already quite reduced and most specimens develop a single posterior ridge and full reduction of the posterior valley. In localities from the middle part of zone B until the end of zone C, only this completely reduced morphology is found.
Ongoing research shows the presence of this species in the Ramblian. These assemblages will allow us to get a better understanding of the primitive character state of the m3.
The m3 of S. meulenorum sp. nov. and S. simplicidens have the mesolophid and posterolophid fused into a single ridge, but this union is achieved in different ways in both species. In S. meulenorum sp. nov. the mesolophid is almost completely reduced so the last ridge is mainly composed of the posterolophid (Fig. 8L, P). In S. simplicidens the labial part of the mesolophid and the lingual part of the posterolophid are short as is shown in specimens with intermediate simplification; thus, the last ridge is equally composed of the lingual part of the mesolophid and the labial part of the posterolophid separated by an inflexion (Fig. 4H). This inflexion is interpreted as a narrow trace of the posterior valley and is situated medially, whereas in S. meulenorum sp. nov. it is near the lingual border. Other morphological differences between the m3 of these species have been observed. In S. meulenorum sp. nov. the central valley is deeper than in S. simplicidens. In S. meulenorum sp. nov. the posterior part is lower than the anterior one and the posterior wall of the ridges is more inclined than in S. simplicidens.
In the P4 of Simplomys meulenorum sp. nov. the main cusps are less distinguishable than in S. simplicidens.
An unusual morphological feature in Gliridae is found in S. meulenorum sp. nov. In the M1, the anteroloph may be interrupted and connected medially to the protoloph (Fig. 8B, F). The taxonomical or functional value of this particular morphology is not clear, but it is present in half of the M1 of S. meulenorum sp. nov. from all localities, whereas it is much less common in the M1 of S. simplicidens.
Simplomys meulenorum sp. nov. and S. simplicidens show statistically significant differences in length, width, and surface for all dental elements when these two species occur in the same locality (Table 4); S. meulenorum sp. nov. is the larger one.
Simplomys meulenorum sp. nov. shows a trend to size increase through time in the studied area (García-Paredes, 2006) (Fig. 10). As a result of this trend, size ranges of S. meulenorum sp. nov. may overlap with S. robustus. However, different morphologies allow us to distinguish them.
Original diagnosis (De Bruijn, 1966: 7): 'Gliridae of intermediate size. Occlusal surface of cheek teeth concave. Crowns of cheek teeth low. P4/p4 small relative to M1/m1 and rounded; M1/m1 smaller than M2/m2. The Spanish representatives of the genus Pseudodryomys are characterized by a gradual simplification of the dental pattern through time'.
Emended diagnosis: Medium to large-sized Gliridae, which have cheek teeth with a concave occlusal surface. The dental pattern is formed by the four main ridges and a variable number of extra ridges, usually between two and eight, including centrolophs and centrolophids. The ridges are relatively high, thick, and slanting and the valleys are narrow. In P4, M1, and M2 the trigon is very asymmetric forming a composite Y-shape. The upper molars generally have both centrolophs, the anterior one usually being longer. The lingual parts of the centrolophs may be joined to each other and/or also to the metaloph. The P4 has an oval outline; it is not very simplified and may present one or even two short centrolophs. Lower molars have a relatively long centrolophid and in m1 and m2 there is at least one extra ridge in the posterior valley. The mesolophid of the m3 is not reduced and labially is not connected to the posterolophid.
We consider Prodryomys a genus with clear distinctive features that can be used to distinguish it from Pseudodryomys as has been pointed out by García-Paredes (2006). In Pseudodryomys the ridges are higher, thicker, more slanting, and the valleys are narrower than in Prodryomys. The upper molars of Pseudodryomys show a more asymmetrical trigon and centrolophs than Prodryomys. In the latter genus, the P4 is relatively larger and its morphology is more similar to that of the molars than in Pseudodryomys.

NEW GENUS AND SPECIES OF MIOCENE GLIRIDAE 645
There are more difficulties in differentiating Pseudodryomys and Miodyromys because of the possible invalidity of Miodyromys as explained by Daams (1999 , 1974). This problem has not been solved yet, and according to Heissig (2006), Miodyromys and Pseudodryomys can be distinguished by the more simple teeth and higher ridges of the latter genus. Although this distinction is applicable to the 'simplicidens-group', it does not hold true for the type species of Pseudodryomys, Ps. ibericus.
The situation involving the genera Pseudodryomys, Peridyromys, Miodyromys, and also Myomimus is a complex problem not yet solved. A separate study is necessary to establish uniform and clear diagnostic criteria for these problematic genera, but this question is out of the scope of this paper. However, the proposal of Simplomys gen. nov. makes this problem a bit less complicated.

CONCLUDING REMARKS
In general, the dental patterns of the different species of Simplomys gen. nov. are very similar because of the limited variations in their simple morphology. Nevertheless, they show different dental proportions as well as particular features in the morphology of the lower cheek teeth, and especially of the m3.
Simplomys aljaphi is the oldest species within the genus (Table 5) and it represents a primitive morphological stage. The information available indicates that Simplomys gen. nov. originated either in France or in the Iberian Peninsula. Because of the morphological similarities, S. simplicidens is probably directly related to S. aljaphi. The more primitive dental pattern shown by the latter suggests an ancestordescendant relationship. Simplomys robustus and S. simplicidens are very similar and may therefore be directly related. The timing of the morphological simplification and reduction of the premolars and third molars is different but they share the presence of a centrolophid in the lower molars that is less reduced than in the other more modern species. Simplomys meulenorum sp. nov. and S. julii show the more derived dental morphology.
The phylogenetic relationships of Simplomys gen. nov. with other genera are not clear yet. The dental morphology of S. aljaphi, the oldest species within the genus, is similar to that of Peridyromys murinus, which is considered to be the ancestor of a number of other genera of Gliridae such as Armantomys, Praearmantomys, Pseudodryomys, etc. (Daams, 1991;Daams & de Bruijn, 1995). The genera of Gliridae with simple dental morphology form a peculiar group within this family. As Armantomys and Praearmantomys presumably originated on the Iberian Peninsula, a close relationship with Simplomys gen. nov. is expected, but morphological similarities can be the result of convergent evolution as a result of living in a similar environment. The change in the morphology of the m3 of Armantomys shows a different trend than in Simplomys gen. nov. Except for its simple morphology, Armantomy looks like a group clearly separated from Simplomys gen. nov. Praearmantomys shows a reduction in the m3 more similar to Simplomys gen. nov. but their possible phylogenetic relationships are not clear at the moment.
The morphological characterization of the species assigned to the new genus allows us to solve taxonomical problems and to specify their stratigraphical distribution. This study also shows the systematic value of the outer elements of the teeth row (premolars and third molars) and its interest concerning the study of evolutionary patterns.
Simplomys gen. nov. is very common in Lower and Middle Miocene localities of the Iberian Peninsula, but it has been also recorded in other European countries such as France, Germany, and Switzerland. As we noted in this paper, there has been considerable taxonomic confusion regarding the species of the 'simplicidens-group' (i.e. Simplomys gen. nov.). Therefore, a review of the 'simplicidens-group' material from localities outside Spain is needed, using the taxonomic framework proposed in this paper and in García-Paredes (2006). This is a worthwhile exercise because Simplomys gen. nov. is a very characteristic taxon, which, in combination with its wide distribution, makes it a promising tool for biochronological correlations within Europe. also due to V. Quiralte Palomar for helping in the preparation of the manuscript and to A. J. van der Meulen for his supportive comments. We acknowledge the facilities provided by Nationaal Natuurhistorisch Museum -Naturalis (The Netherlands), Department of Earth Sciences of the Utrecht University (The Netherlands), and Museo Nacional de Ciencias Naturales -CSIC (Spain) to study their collections. We also want to thank Monique Vianey-Liaud and Hans de Bruijn for constructive review of the manuscript. We acknowledge financial support by the projects PB 98-0691-C03-02, CGL2004-02094/BTE, CGL2007-65208/BTE, CGL2008-04200/BTE, and Research Group CAM-UCM 910607. The first author (I. G.-P.) has a postdoctoral contract of the Spanish Foundation for Science and Technology (FECYT) and the Spanish Ministry of Science and Innovation. APPENDIX Species, localities, number of specimens, and values used in the scatter diagrams of Figures 2 and 3. Ratios of length : width (L/W) of premolars and third molars and their relative dental proportions are shown. The indices for the relative size of P4/p4 and M3/m3 have been calculated by dividing the surface area of P4/p4 and M3/m3 by the surface area of adjacent teeth (M1/m1 and M2/m2, respectively). As a rule, this was only carried out when at least five measurable specimens of each element were available. However, exceptions were made (grey shaded cells) for material from type localities (bold text) and/or small collections. Abbreviations: MC, F, Massif Central basin; France; C-M, S, Calatayud-Montalbán basin, Spain; T, S, Tagus basin, Spain; D, S, Duero basin, Spain; G, S, Granada basin, Spain. For details on localities see Supporting Information Appendix S1.