Sharamynodon

Amynodon mongoliensis, Sharamynodon, and Lushiamynodon

The first Sharamynodon fossils were excavated at Ula Usu in the Shara Murun Formation of Inner Mongolia, China by an expedition from the American Museum of Natural History in June and September 1923.^[2] The fossils included a nearly complete skeleton with a sideways crushed skull (AMNH 20278), a left mandibular ramus (AMNH 20279), a left maxilla and left ramus (AMNH 20282), two associated mandibular rami (AMNH 20283), and the facial bones of a skull with some associated teeth (AMNH 20284). An additional specimen, a skeleton, skull, and lower jaws of an old individual (AMNH 21601) was also found in the Shara Murun Formation in 1925.^[2] The fossils were briefly described in a posthumous publication by Henry Fairfield Osborn in 1936, as the new species Amynodon mongoliensis.^[2]

Osborn referred A. mongoliensis to Amynodon due to its resemblance to the North American species A. intermedius^[2] (today considered a synonym of A. advenus^[7]), but did not elaborate significantly on the classification.^[2] In 1942, Miklós Kretzoi proposed that A. mongoliensis should be moved to its own genus due to several anatomical differences from the North American forms. Among these differences, Kretzoi mentioned that A. mongoliensis had a different shape and position of its nasal openings, a much reduced premolar region, and very small frontal teeth. Kretzoi proposed that the new genus should be named Sharamynodon.^[1]

Kretzoi's Sharamynodon was rejected as a distinct genus by Vera Gromova in 1954.^[8] Chinese paleontologists also ignored Sharamynodon, perhaps unaware of Kretzoi's suggestion for the new generic name, and continued to name several new Asian amynodonts as species of Amynodon.^[8][9]

In 1965, Zhou Mingzhen and Xu Yu-Xuan described the new amynodont genus Lushiamynodon, initially encompassing the type species L. menchiapuensis and a second species, L. obesus.^[4] L. menchiapuensis was based on a left maxilla with some teeth (IVPP V.3016) and another fragmentary left maxilla (IVPP V.3017), from the lower part of the Late Eocene Lushi Formation, found near the village of Menchiapu in Lushi, Henan, China.^[4] L. obesus was based on a partial skull and palate, with a few teeth preserved (IVPP V.3023), found in the Late Eocene red sandstone beds of Tungchang, a village in Xichuan County in northern Henan. These beds probably correspond in time to the Yuanchü Formation.^[4] Zhou and Xu described Lushiamynodon as a largely unspecialized amynodont, distinguished only by features in the teeth.^[4]

In 1966, Xu described a new maxilla (IVPP V.3216) of S. mongoliensis from Ula Usa, close to the original locality.^[10] Xu also described a new species of Lushiamynodon from Ula Usa, L. sharamurenensis, based on a partial skull from a juvenile individual (IVPP V.2892), a mandible from a juvenile individual (IVPP V.2892.1), another partial skull (IVPP V.2892.2), and some postcranial fossils. A partial mandible (IVPP V.3217) from "Ulan Shireh Obo", a site about 30 kilometers northeast of Ula Usa and presumably also from the Shara Murun Formation, was also referred to L. sharamurenensis by Xu.^[10] Wang J. described new isolated teeth of S. mongoliensis in 1978 from Inner Mongolia, as well as a new species of Lushiamynodon, L. wuchengensis, based on a fragmentary maxilla (IVPP V.5305) from Xixia in Henan, China.^[6]

Additional finds and reassessments

William P. Wall supported the validity of Sharamynodon as a distinct genus in 1989.^[9] Wall also proposed that many of the large number of Late Eocene amynodont species based on Asian fossils, placed in genera such as Amynodon, Lushiamynodon, and Sianodon, actually belonged to Sharamynodon, but that detailed reexaminations of all the Asian fossils was required to determine the taxonomy and true number of species.^[9] Many of the Late Eocene Asian species are difficult to include in phylogenetic analyses due to their fragmentary remains, which has complicated later studies.^[8] Some have been recovered in later analyses as outside of their supposed genus, but not in Sharamynodon either, such as "Amynodon" sinensis^[8] and "Sianodon" gaowangouensis.^[11] Lushiamynodon was designated by Wall as a probable synonym of Sharamynodon, since the description of its type species did not cite any derived features separating the two.^[9]

In 1971, Elizaveta Belyaeva [ru] named the new species Lushiamynodon? kirghisensis from the Alay Formation at Andarak, Kyrgyzstan, based on a fragmentary maxilla (PIN 1996/2) and some isolated upper teeth.^[12] The age of the fossils at Andarak were reassessed in 1993 as Ypresian (i.e. Early Eocene) based on chondrichtyan fossils, which would make L. kirghisensis the oldest known amynodont.^[12] In 1996, Alexander Averianov and Olga Potapova described new fossils of L. kirghisensis from Andarak, including both maxillae from a single individual (ZIN 34313) and a fragmentary premolar (ZIN 34021).^[12] Averianov and Potapova found the assignment of the species to Lushiamynodon to be dubious since Lushiamynodon is otherwise larger and purportedly differs in its dentition.^[12] They instead proposed that L. kirghisensis should be moved to the new genus Andarakodon.^[12] Averianov and Potapova further suggested that Andarakodon belonged to the derived amynodont tribe Metamynodontini, based on the forwards position of the infraorbital foramen and the more backwards beginning of the zygomatic arch.^[12]

In 2001, Spencer G. Lucas and Robert J. Emry described an incomplete skull (KAN-N2/872) and isolated teeth of S. mongoliensis from Kyzyl Murun in the Kyzulbulak Formation, near the Aktau Mountains in eastern Kazakhstan.^[13] Lucas and Emry rejected Averianov and Potapova's Andarakodon and instead found the original assignment of L. kirghisensis to Lushiamynodon to be correct. Since Lucas and Emry agreed with Wall's assessment that Lushiamynodon is a synonym of Sharamynodon, they treated L. kirghisensis as a species of Sharamynodon (S. kirghisensis).^[13] Lucas and Emry found all of the features used by Averianov and Potapova to diagnose Andarakodon to be dubious, as these features were either found in several amynodonts, or unsupported by the photographs of the fossils.^[13] Lucas and Emry also rejected the idea that Andarak was Ypresian in age, noting the presence of several groups otherwise known only from the Irdinmanhan at the site, and that reassigning its age based on the chondrichtyan fauna was dubious since it was based on correlations with stratigraphy in Europe.^[13] Lucas and Emry also supported Wall's suggestion that several fragmentary Asian amynodonts belong to Sharamynodon, treating it as a genus distributed across East Asia, including in China, Mongolia, and Japan.^[13] The Japanese occurrence follows Wall's suggestion that the poorly known "Amynodon" watanabei could belong to Sharamynodon.^[9]

In 2005, Averianov and Marc Godinot accepted the reassignment of Andarakodon kirghisensis to Sharamynodon, but maintained that the proposed earlier age of Andarak was a serious possibility.^[14] Lushiamynodon being a synonym of Sharamynodon was also accepted as likely in 2017 by Averianov, Igor Danilov, Jin Jinhua, and Wang Yingyong. Averianov et al. (2017) noted that the validity of Lushiamynodon species other than the type species (L. menchiapuensis) was problematic, since they are based on fragmentary remains.^[8] In 2018, Lushiamynodon was also designated a junior synonym of Sharamynodon by Jérémy Tissier et al.^[11] Bai et al. (2018) however used Lushiamynodon as a valid genus without elaboration,^[10] and Wang et al. (2020) considered Lushiamynodon's validity to be a matter in need of further investigation.^[15]

Skull and dentition

The skull of S. mongoliensis measured 49.5 centimeters (19.5 in) from nasal to occiput, and at least 22.5 centimeters (8.9 in) wide at the widest point of the zygomatic arches.^[2] In 1989, Wall diagnosed the skull of Sharamynodon as characterized by unfused and unthickened premaxillae with a reduced ascending process, preorbital fossae (grooves in front of the eye orbits) that extended slightly medial (to the center of) the eye orbits, and maxillae that were not part of the border of the external nares.^[9]

The most striking feature of the skull of Sharamynodon when compared to more basal amynodonts, such as Amynodon, are the modifications of the snout.^[16] The facial region of Sharamynodon is reduced in size when compared to less derived amynodonts, from encompassing about 40% of the length of the skull to 31–34%.^[16] This development has also resulted in a forward-shift of the position of the eyes; in Sharamynodon, the eye orbits were positioned right above the second molar.^[16] The premaxillae of Sharamynodon were relatively thin, and the diastema (in this case the gap between the canines and premolars) was short, evidence that the external nares had shifted backwards.^[16] The evolution of a shortened face in certain amynodonts is believed to have been driven by increasing use of the canines.^[16] The nasal incision of Sharamynodon extended back to about the half-way point of the diastema, further back than in more basal amynodonts. The lacrimal bones were more rugose than in more basal amynodonts, which suggests that Sharamynodon had well-developed musculature in its snout.^[16] These developments suggest that Sharamynodon had a well-developed and prehensile upper lip, similar to that of rhinoceroses today.^[16] Similar developments were even more extreme in some more derived amynodonts, such as Cadurcodon, which is believed to have had an elongated proboscis.^[16]

Sharamynodon had the dental formula 3.1.3.33.1.3.3,^[2][9] i.e. both the lower and upper jaws had three pairs of incisors, one pair of canines, three pairs of premolars, and three pairs of molars. Both the incisors and the second pair of premolars were similar to the same teeth in Amynodon. The lower molars of Sharamynodon were relatively narrower compared to Amynodon.^[9]

Size and postcranial skeleton

Manus of S. mongoliensis

Pes of S. mongoliensis

S. mongoliensis was a relatively large animal, similar in appearance and proportions to the North American Amynodon^[2] but nearly twice the size at about 1,000 kilograms (2,200 lb).^[8] S. mongoliensis stood about 1.4 meters (4.6 ft) tall at the withers, and its trunk was about 1.67 meters (5.5 ft) long.^[2] S. kirghisensis and S. menchiapuensis were similar in size to each other, but smaller than S. mongoliensis.^[13]

S. mongoliensis had seventeen thoracic vertebrae, four lumbar vertebrae, five pseudosacral vertebrae, and twenty-two or twenty-three caudal vertebrae.^[2]

Irdinmanhan and Sharamurunian Asia

Sharamynodon was largely restricted to the Irdinmanhan land mammal age; S. mongoliensis fossils from the Shara Murun Formation and the Kuzulbulak Formation have been considered to be Irdinmanhan in age,^[10][13] as are the S. menchiapuensis fossils from the Lushi Formation.^[18] Since the Shara Murun Formation is slightly later in age than the Irdin Manha Formation (from which the Irdinmanhan gets its name),^[19] recent research favors it as representing a new, slightly later land mammal age, the Sharamurunian.^[20] Lucas and Emry also suggest an Irdinmanhan age for S. krighisensis in the Alay Formation,^[13] but Averianov et al. maintain that it is from an earlier, Ypresian age, which would make it the earliest known amynodont.^[8][14] Depending on which other Asian amynodont fossils are proposed to belong to Sharamynodon, the genus may have survived into the Ergilian land mammal age.^[13]

The Arshantan (the land mammal age preceding the Irdinmanhan) and Irdinmanhan was a time of great change in the mammal fauna of East Asia. In earlier times, the local fauna had been dominated by various now extinct mammal orders, such as anagaloids, dinoceratans, mesonychians, pantodonts, and tillodonts. Some members of these groups were still present in the Arshantan, but new groups also appeared and grew very successful, such as artiodactyls, carnivorans, lagomorphs, perissodactyls, and rodents.^[20] There were some leftovers of the earlier groups during the Arshantan and Irdinmanhan, such as dinocerants Gobiatherium and Uintatherium, pantodont Eudinoceras, and tillodonts Chungchienia and Kuanchuanius.^[20]

Many of the new groups in East Asia are believed to have migrated to the region from Europe or North America, since they do not appear to have ancestral forms in Asia itself.^[20] The hyaenodonts were a new predatory group in the region, but they are known from older layers in North America.^[20] Additional predators that appeared in the region at this time were the oxyaenids, and the large artiodactyl Andrewsarchus.^[20] The Irdinmanhan also saw the appearance of the earliest known primates in China (Lushius), the appearance of leporoid lagomorphs (Shamolagus, Lushilagus), as well as amphicyonids (Cynodictis), nimravids (Eusmilus), and miacids (Miacis).^[20] Artiodactyls were diverse, and included anthracotheres (Anthracokeryx), entelodonts (Brachyhyops), dichobunids (Dichobune), helohyids (Gobiohyus), and archaeomerycids (Archaeomeryx).^[20]

Perissodactyls quickly grew to become dominant in East Asia; over a hundred perissodactyl species have been named from the Arshantan and Irdinmanhan in China alone, and fossils from the region are highly important for the study of perissodactyl evolution.^[20] Tapiroid fossils are the most common (families Deperetellidae, Helaletidae, and Lophialetidae), followed by the rhinocerotoids (families Amynodontidae, Hyracodontidae, and Paraceratheriidae).^[20] The amynodonts are believed to have originated in Asia,^[8][21] where S. mongoliensis is preceded by Rostriamynodon from the Irdin Manha Formation.^[21] Rhinocerotoids were diverse in size already at this early stage, from the small hyracodont Rhodopagus to the horse-sized Forstercooperia.^[20] Some palaeotheres were also present in Asia (Propalaeotherium), as were the earliest known chalicotherioids on the continent (Eomoropus, Grangeria, and Litolophus).^[20]

The mammal fauna of the Sharamurunian was similar to that of the Arshantan and Irdinmanhan, though some of the leftovers of earlier groups had gone extinct, such as the pantodonts and dinoceratans, and survivors were less diverse.^[20] Perissodactyls remained highly diverse.^[20]

Fauna and environment of specific localities

The Erlian Basin, in which the Shara Murun Formation is situated, experienced a subtropical and humid climate during the Eocene. The vegetation was highly diverse and dominated by deciduous trees and shrubs.^[22] S. mongoliensis was among the largest ungulates of Irdinmanhan Central Asia,^[23] alongside brontotheres such as Aktautitan,^[23] Parabrontops,^[24] and Protitan.^[23] The fauna known from the Shara Murun Formation was similar to the northern Irdin Manha Formation.^[19] In addition to Sharamynodon, the Shara Murun fauna includes predatory hyaenodonts ("Pterodon" hyaenoides),^[19][24] the brontotheres Parabrontops, Protitan, and Titanodectes,^[24] the early paracerathere Pappaceras,^[24] the enigmatic mammal Olsenia (described as a mesonychid^[19] but now considered to belong to its own family, Olseniidae),^[25] deperetellids (Deperetella), primitive ruminants (Archaeomeryx), various rodents,^[19] the fellow amynodont Cadurcodon,^[24] and the enigmatic rhinocerotoid Caenolophus,^[19] which is variously classified as a hyracodont or a primitive amynodont.^[8]

The Kyzylbulak Formation preserves fossils from a lacustrine (near a lake) environment, including various mammals, crocodilians, and trionychid turtles.^[26] Other than S. mongoliensis, the known mammals include the brontothere Aktautitan,^[27] hyracodonts Ardynia^[28] and Rhodopagus,^[27] and a tapiroid similar to Teleolophus.^[27]

The Andarak mammal fauna of the Alay Formation was deposited in a marine bed, and is found together with fossils of fishes and invertebrates.^[29] Land mammal fossils are thus remains of animals washed out into the Tethys Ocean. The mammals probably lived near the Tethys coast, in a forested and humid environment.^[29] The mammal fossils from Andarak are understudied, but include the several fellow perissodactyls: anthracobunids (Pilgrimella), chalicotherioids or equoids (perhaps Eomoropus or Propachynolophus), deperetellids (Teleolophus), hyracodonts (Rhodopagus and Pataecops), lophialetids (Eoletes, and possibly Lophialetes and Schlosseria), and Forstercooperia.^[29] Artiodactyls were represented by the diacodexeids (Diacodexis and Eolantianius).^[29] Also known from Andarak are the hyaenodont Neoparapterodon, indeterminate mesonychids, and various groups of small mammals, including proteutherians or lipotyphlans, micropternodontids, mixodontians, lagomorphs, and rodents.^[29]

The faunas of the Lushi Formation are divided into the Upper and Lower Lushi Faunas; S. menchiapuensis belongs to the Upper Lushi Fauna.^[30] This diverse fauna encompasses numerous other perissodactyls, including the fellow amynodont Sianodon honensis, the possible amynodont Caenolophus, the primitive rhinocerotoid Breviodon, the hyracodonts Prohyracodon and Rhodopagus, Forstercooperia, the brontotheres Protitan and Microtitan, the deperetellid Deperetella, the helaletid Colodon, and the chalicotherioids Lunania and Eomoropus.^[30] Several carnivorans are known, including the amphicyonid Cynodictis, miacid Miacis, and nimravid Eusmilus.^[30] Additional taxa recorded include the artiodactyls Andrewsarchus, Dichobune, Gobiohyus, and possibly Anthracotherium, the pantodont Eudinoceras, the mesonychids Honanodon and Lohoodon, the hyaenodonts Hyaenodon and Propterodon, the primate Lushius, the lagomorph Lushilagus, the rodent Tsinlingomys, and indeterminate trogosine tillodonts.^[30]