An exceptionally well-preserved herbaceous eudicot from the Early Cretaceous (late Aptian–early Albian) of Northwest China

Abstract A fossil eudicot, Gansufructus saligna gen. et sp. nov., is reported from the Early Cretaceous (late Aptian–early Albian) of the Gansu Province, Northwest China, based on numerous well-preserved axes with attached leaves and infructescences. The leaves are alternate, short petiolate and linear-lanceolate with low rank pinnate to reticulate venation. The infructescences are loose panicles bearing fruits in different stages of maturity, each containing four partly free carpels borne in a whorled arrangement. Each carpel has three to five seeds borne along its ventral margin. The nature of the leaves and axes indicates a terrestrial, herbaceous habit. In general organization, Gansufructus is closely similar to the fruit-bearing axes of Sinocarpus decussatus from the Early Cretaceous Jehol Biota, as well as other more or less contemporaneous angiosperms from the Far East, which together provide evidence of diverse eudicot angiosperms of low stature colonizing areas close to environments of deposition.


INTRODUCTION
Angiosperms (flowering plants) represent the largest and most successful clade of vascular plants, with >350 000 extant species distributed all over the world [1,2], but their origin, evolution, early diversification, as well as the habitat preferences and ecology of early forms, are still poorly understood [3][4][5]. Some molecular studies suggest a pre-Cretaceous origin for angiosperms, perhaps Late Triassic [5][6][7], but there are no reliable fossil angiosperms in Triassic or Jurassic deposits [8,9], and the rise of angiosperms during the Early and mid-Cretacea has been regarded as a trigger for the Cretaceous Terrestrial Revolution (KTR) [10,11]. Rapid diversification of angiosperms in habit, morphology, anatomy, physiology and reproductive biology, may have been important in promoting the diversification of insects, amphibians, mammals, ferns and many other terrestrial organisms [12][13][14][15].
The Early Cretaceous terrestrial Jehol Biota is widely distributed in East Asia (northern China, southeastern Mongolia, Siberia, Korea and Japan), and is characterized by the Eosestheria-Ephemeropsis-Lycoptera assemblage [16]. Well-preserved and informative Jehol Biota fossils are particularly abundant in western Liaoning, eastern Heilongjiang, northern Hebei and southeastern Inner Mongolia, and include crucial specimens of feathered dinosaurs, early birds, eutherian mammals and early flowering plants [17][18][19]. Lower Cretaceous strata are also widely distributed in Northwest China, especially in the western part of Gansu Province, where numerous fossils document a rich Jehol fauna and flora [20] that includes fishes [21], turtles [22], insects [23], birds [24], dinosaurs [25] and plants [26][27][28], although no angiosperms have been described. Here, we report an early angiosperm from the late Early Cretaceous (late Aptian-early Albian, 115-112 Ma) [20,23,[29][30][31] Zhonggou Formation of the Jiuquan Basin in Northwest China (Fig. 1). The fossil specimens are assigned to the eudicots clade based on the morphology of both vegetative and reproductive organs.   Leaves are simple, symmetrical, deciduous or persistent and alternately arranged. They vary in size, typically being ∼1-2.5 cm long and 0.2-0.4 cm wide, but are larger toward the base of the plant (Fig. 2). Leaves are narrow-ovate, lanceolate or ovoid-lanceolate in shape (Figs 2 and 3I). Leaf apex is acute, and the leaf margin is entire (Figs 2 and 3I-K). Leaf base is decurrent with a short petiole (Fig. 3I).

SYSTEMATIC PALEONTOLOGY
Leaf venation is poorly organized, pinnate to reticulate (Fig. 3I). The primary vein is prominent and straight, or slightly curved, and extends from the leaf base to the apex ( Prior to dehiscence, the closed fruits are elliptic or subglobose in shape, ∼2-3 mm long and 2-3 mm wide (Fig. 4A). After dehiscence along the ventral suture, the fruits are ∼4-5.5 mm long and 3-4.5 mm wide (Fig. 4A-G). In most fruits, the carpels are fused basally for about half or more of their length ( Fig. 4A-D). Some are completely dehisced, and the elongated elliptic-shaped carpels are arranged in a whorl ( Fig. 4E and G). The carpels are asymmetric with mucronate apices, ∼4.5-5.5 mm long and 2-2.5 mm wide ( Fig. 4B-D, F and G), and dehisce along the ventral side (Fig. 4E). Each carpel contains ∼3-5 ovules/seeds arranged longitudinally on linear placentae along the ventral suture of the carpel, both in the free and fused portion of the fruit (Fig. 4F).
Seeds are tightly packed in the carpels with their margins sometimes overlapping (Fig. 4F-I). Seeds vary in size and morphology. They are ∼1.5-1.8 mm long and 0.8-1.1 mm wide, asymmetrical and oval to elongated ovoid or reniform in shape (Fig. 4F-I), slightly pointed in the hilar region and rounded to truncate in the chalaza region ( Fig. 4F-I). Seeds are anatropous, with chalaza opposite to the funiculus, and the micropyle situated at the base of the funiculus ( Fig. 4H and I).
Epidermal cells on the carpels are irregular, polygonal or elongated rectangular, ∼50 μm long and 15-20 μm wide (Fig. 5A). Two epidermal layers are visible on the seeds. The inner layer is formed by pentagonal and hexagonal cells arranged in longitudinal rows that radiate from the chalaza towards the micropylar end of the seed (Fig. 5B and C). The cells become larger near the micropylar area, being ∼80-110 μm long and 50-80 μm wide, and narrowly elongated towards the chalaza part of the seeds (Fig. 5B-E, H and I). The cells of the outer layer are elongated ( Fig. 5D and E), with irregularly curved anticlinal walls (Fig. 5D, E and H-J), and transverse ribs and grooves on the periclinal walls ( Fig. 5I-K).

DISCUSSION
Gansufructus saligna gen. et sp. nov. is reconstructed as a small, slender plant with flexible stems, delicate leaves and paniculate infructescences (Fig. 6A). The pinnate-reticulate low rank leaf venation (Fig. 6B) together with partly syncarpous gynoecium and several completely enclosed seeds (Fig. 6C and D) securely place this ancient plant within the angiosperms. In addition, general morphological features of G. saligna, including alternate phyllotaxis, pinnate-reticulate leaf venation, partly apocarpous gynoecium and fruit with four carpels arranged in a whorl, indicate an affinity among the eudicots.
Eudicots appeared early in the diversification of angiosperms, as evidenced by worldwide discoveries of tricolpate pollen grains as well as fossil flowers, fruits and leaves from late Barremian and early Albian strata [5,6,[32][33][34]. However, few macro-fossils of eudicots have been reported from Albian or earlier rocks, and very few are known from both vegetative and reproductive organs. Among the several fossil records (Table 1), the infructescence of Sagaria cilentana is dichasium, and fruits are cupshaped, composed of at least three follicles, and leaves are lobate [35]. Achaenocarpites capitellatus is characterized by stipulate, basically ternate, pinnatisect or three-lobed leaves and the reproductive structure is preserved as a head of achenes consisting of ∼16 radially spreading achenes. Ternicarpites floribundus possesses pinnatisect leaves and an apocarpous gynoecium of two to five, usually three, follicular carpels [36,37]. Leefructus mirus is characterized by three-lobed leaves, and its fruit have five pseudo-syncarpous elongated carpels [17]. The fossil specimens described in this paper are distinguished from all these fossils by their simple and lanceolate leaves, paniculate and determinate infructescence with dehiscent fruits composed of four basally syncarpous carpels, each enclosing three to five oval ovules/seeds.
Gansufructus closely resembles Sinocarpus and Hyrcantha in gross morphology (Table 1), as well as in the paniculate infructescence and polycarpous fruits. They may be closely related despite showing different branching types and leaf characteristics. Sinocarpus decussatus differs from G. saligna by its decussate phyllotaxis, ovate or narrow-ovate leaves with serrated margin, and its greater number of seeds per carpel (10-20 ovules/seeds in Sinocarpus versus ∼3-5 in Gansufructus) [38,39]. Hyrcantha karatscheensis is distinguished by its apocarpous gynoecium, and the terminal fruiting units that consist of three or five carpels [40]. Among many extant families of eudicots, the combined characteristics of Gansufructus, such as simple, lanceolate and alternately arranged leaves with entire margins and pinnate-reticulate venation, as well as subglobose and polycarpous fruits, suggest a systematic position among the basal grade of eudicots or the basal core eudicots as also suggested for Sinocarpus [38,39]. In particular, there are similarities to extant Ranunculaceae, Myrothamnaceae and Buxaceae, but Gansufructus differs from all three of these families. Ranunculaceae is characterized by spiral phyllotaxis, simple to compound leaves and apocarpous gynoecium [41]. Myrothamnaceae is distinguished by decussate phyllotaxis, sessile leaves and catkin-like inflorescences [42]. Buxaceae is distinct in having fruits with two to three carpels, each of which always carries only two ovules [43].
Palynological preparations made from the fossil specimens failed to provide pollen associated with Gansufructus. However, poorly preserved tricolpate pollen grains, typical of eudicots, do occur in the fossil-bearing strata, and the pollen assemblage is mainly dominated by grains assignable to the extinct pollen genus Retitricolpites (Fig. 5F and G). Previous palynological analyses have suggested a relatively temperate and humid climate in the study area during the early Albian, with an increase of xerophytic vegetation in palynological flora indicating obvious later aridification [31], which is also supported by the discovery of Cheirolepidiaceae macro-fossils from the uppermost Zhonggou Formation [27].
The slender, flexible and upright stems of Gansufructus are not lignified, and the longitudinal grooves or ribs on the stem surface probably represent vascular bundles of a herbaceous plant [40,44]. The numerous narrow-lanceolate and alternate leaves attached to the axes and panicle-like infructescence with numerous fruits terminally at the leafy axes would have required stable support and a sufficient vascular system. Gansufructus saligna was probably a herbaceous or scarcely woody plant growing in a terrestrial environment. The association with riparian Equisetum [28] and fishes suggest a locally wet and lowland environment. Therefore, Gansufructus is supposed to be a terrestrial herb colonizing lowland areas, probably growing in the mud or floodplains along lakeshores in a humid environment. The current fossil specimens, together with other taxa recorded from the Jehol Biota and other regions [5,17,[35][36][37][38][39][40]44], indicate the presence of diverse early eudicots of low stature colonizing areas during the middle-late Early Cretaceous.

MATERIALS AND METHODS
All the fossil specimens were collected from the uppermost Zhonggou Formation of Hanxia Section (39 • 50 N, 97 • 15 E), about ∼40 km west of Laojunmiao County, Jiuquan City, Gansu Province (Fig. 1). The fossils were photographed using a SONY Alpha 7 II EOS digital camera coupled with a SONY 50 mm FE macro lens and a stereo microscope (Zeiss, Oberkochen, Germany). The cuticle remains that were removed from the fossil specimens were firstly treated with 10% HCl (hydrogen chloride), and then 50% HF (hydrogen fluoride) subsequently. Some of them were treated with saturated NaClO (sodium hypochlorite), stained in a safranine solution, mounted on slides, embedded in a glycerine jelly, sealed with Canada balsam and then observed under an Axio Scope A1 light microscope and a Stemi508 fluorescence microscope (Zeiss, Oberkochen, Germany) at the Key Laboratory of Petroleum Resources, Gansu Province, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China.