The Amynodontidae is an extinct family of Rhinocerotoidea (Mammalia, Perissodactyla), ranging from the Middle Eocene to the Late Oligocene in North America and Eurasia. The relationship between Amynodontids and other Rhinocerotoids remains obscure. Traditionally, Amynodontids were thought to be a separate family divergent from other Rhinocerotoids at early stage of Rhinocerotoid evolution. It has been proposed that Amynodontids are closely related to Paraceratheriids and Uintaceras. Recent phylogenetic work, however, has suggested that Amynodonts, Paraceratheriids and 'Pappaceras' meiomenus comprise monophyletic clade. Amynodontids are abundant in the Middle and Late Eocene of Asia and North America, but relatively rare in Europe. Twelve genera and 32 species of Amynodontidae have been named from Asia, however, some genera were considered to be invalid and their phylogenetic relationships remained controversial. The most primitive and oldest known Amynodontid is Rostriamynodon grangeri, which was discovered from the Middle Eocene Irdin Manha Formation of the Erlian Basin in China. Thus, Asia is the most probable place of the origin of Amynodontidae.
In a paper published in the journal Vertebrata PalAsiatica on 16 July 2020, Wang Xiao-Yang of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the College of Earth and Planetary Sciences at the University of Chinese Academy of Sciences, Wang Yuan-Qing, also of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the College of Earth and Planetary Sciences at the University of Chinese Academy of Sciences, and of the Center for Excellence in Life and Paleoenvironment, Zhang Rui of the Fourth Geological Exploration Institute of the Bureau of Geology and Mineral Resource of Henan Province, Zhang Zhong-Hui of the Henan Institute of Geological Survey, Liu Xiao-Ling of the Wangwushan-Daimeishan Global Geopark Administration, and Ren Li-Ping, also of the Henan Institute of Geological Survey, report a relatively well-preserved amynodontid skull collected in 2005 from the Middle Eocene Niezhuang Formation in the Jiyuan Basin, Henan Province, China.
The new specimen shows some primitive features and represents a new species of the genus Amynodontopsis, which improves our knowledge of the craniodental morphology and the geographical distribution of Amynodontids. In addition, it is also informative for understanding the origin of Amynodontopsis, as well as its phylogenetic relationship with other Amynodontids.
The new species is named Amynodontopsis jiyuanensis, where 'jiyuanensis' means 'from Jiyuan', in reference to the city where the specimen was found. The species is described from a single specimen, IVPP V 26305, a relatively well-preserved skull of an adult individual, housed in the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing. The right M2–3 are preserved in relatively good condition, and the left P4–M3 and the right P4–M1 are damaged to some extent. Only roots remain for C–P3 of both sides.
The skull was preserved in a relatively good condition, but it was slightly laterally compressed and the nasals, premaxillae, maxillae, left zygomatic arch and occiput were partially broken off at different extent. In lateral view, the dorsal profile of the skull is slightly convex, with the highest point located at the anterior portion of the frontal. The nasal slopes anteriorly to some extent and a shallow depression appears at the fronto-parietal suture. The premaxilla is partially damaged, and only the nasal process is preserved. It extends far back along the maxilla, and contacts the nasal dorsally, excluding the maxilla from the border of the external nares. The nasal notch is shallow, and probably ends above the post-canine diastema. The anterior tip of nasal is slightly damaged, but it can be determined that the nasals are over the external nares. The maxilla forms most of the facial portion of the skull with some nutrient foramina present anterior to the orbit. A well-developed preorbital fossa is slightly concave in its anterior part, and becomes deeper posteriorly, extending far back medial to the orbit and forms a deep recess medial to the anterior rim of the orbit. The maxilla contacts the jugal and lacrimal posteriorly. The orbit is relatively low on the skull, and its anterior border is above the M2. The orbit is moderately large (around 40 mm in diameter) and well-defined. The postorbital process of the frontal is distinctly developed, situated approximately above the posterior portion of the M3. The lacrimal forms the anterodorsal rim of the orbit and contacts with the frontal on the dorsal side. The orbital surface of the lacrimal extends posteriorly to the level of the postorbital process. The surface of the lacrimal is rugose, with a prominent lacrimal tuberosity present along the anterior margin of orbit. A lacrimal foramen is present below the tuberostiy. The jugal forms the ventral border of the orbit. The zygomatic arch is comparatively thin (particularly at the zygomatic process of the squamosal) with a flat lateral surface, and its anterior root is located above M2. The cranial foramina are poorly preserved, and only a strip-shaped fissure filled by matrix is discernable. This fissure, posterodorsal to the pterygoid crest, contains probably the foramen orbitale and the foramen rotundum. The squamosal bears a broad glenoid fossa and a blunt postglenoid process ventrally. The posttympanic process is separated from postglenoid process by the wide external auditory meatus. The paroccipital process is fused with the posttympanic process. The paroccipital or posttympanic process slightly extends anteroventrally and reaches approximately the same level as the postglenoid process. The ventral part of the paroccipital process is slightly damaged, and the paroccipital process tends to be longer than the posttympanic process. There is a shallow temporal fossa bounded by the nuchal crest and a high and thin sagittal crest. The top of supraoccipital extends posteriorly and overhangs the occipital condyles.
In dorsal view, the skull presents the dolichocephalic character. Nasals are narrow and long, their profile is straight and smooth without any rugosity. The posterior ends of the nasals are pointed and separated from each other by an intruded frontal, which makes the naso-frontal suture into a 'W' shape. The suture is located above the middle of the orbit. The frontals expand laterally, with the widest part of the cranium at the level of the postorbital process. The deep preorbital fossae extend medial to the orbit in dorsal view, forming a constriction at the preorbital portion of the skull roof. The fronto-parietal suture is faint, and situated at the level of the most constricting part of the braincase. The sagittal crest is prominent and bifurcates posteriorly into two branches being continuous with the nuchal crest.
The incisors are missing and canines are broken. The occlusal surfaces of P2–M1 are heavily worn. The right M2 and M3 are less worn, with both the protoloph and metaloph slightly broken. Moderate wear on molars shows that the skull was from a mature adult.
The canines are enlarged, inferred based on the vestige of the teeth, and situated anterior to the nasal notch. The heavily damaged right P4 is rectangular in outline with distinct and blunt paracone and metacone ribs on the labial wall. A well-developed crista on the lingual side of the ectoloph is preserved. A swollen antecrochet is present on the posterior side of the M1 protoloph. The M2 is trapezoidal in outline, and it is the longest in the upper tooth series. The labial length (38.2 mm) of the M2 is greater than the lingual length, and maximum width of the tooth is at the protoloph (37.2 mm). Thus, the ratio of the width and length of the M2 is nearly 0.98. The ectoloph of the M2 is elongate, extending posterolingually and slightly depressed lingually. The paracone rib is prominent and positioned relatively far anteriorly on the ectoloph, while the metacone rib is absent. Thus, the labial wall of ectoloph apparently straight and flat behind the protocone rib. Both the protoloph and metaloph are slightly inclined posterolingually and relatively long. The protoloph is strong with a faint antecrochet at base of the central valley, and there is no crochet or crista. The M3 is similar to the M2 in general morphology. The M3 is nearly quadrate and the protoloph and metaloph are well developed. The ectoloph is labially concave. The paracone rib is blunt and prominent. The metastyle is strongly deflected labially with a sharp posterior edge. The M3 metaloph and protoloph are nearly equal in length, relatively long, and widely separated; thus, the U-shaped central valley of the M3 is wider than that of the M2. The M3 lacks the crochet, antecrochet and crista. The upper molars have anterior and posterior cingula at base, while the labial cingula are absent and the weak lingual cingulum is only present at the central valley of the M1. The length of premolar series is estimated to be about half that of the molar series.
The new specimen has a well-developed preorbital fossa, a pronounced sagittal crest, a somewhat enlarged canine, a quadrangular M3 with deflected labially metastyle and simple upper molars without crista and crochet. The combination of these morphological features suggests the similarity of the new specimen to the Amynodontids. Thus, this animal is referred to the family Amynodontidae.
IVPP V 26305 shares a number of characters with Amynodontopsis, such as a dolichocephalic skull, prominent sagittal crest, presence of premaxilla-nasal contact, orbit located low on skull and its anterior border positioned above the M2 protoloph, antecrochet presented on the M1, and the M3 metastyle is strongly deflected labially. Particularly, the large preorbital fossa extending behind the anterior borders of the orbits presents a noticeable difference from other genera. The combination of these morphological features indicates that V 26305 is referable to Amynodontopsis, which contains three previously named species: Amynodontopsis bodei, Amynodontopsis parvidens, Amynodontopsis tholos.
The new specimen is clearly distinguishable from the known species of Amynodontopsis in its craniodental morphology and smaller size. The type species Amynodontopsis bodei, from the Duchesnean North American Land Mammal Age, is clearly distinguishable from V 26305 in the following aspects: (1) the nasal notch extending back to a point of the anterior edge of the M1 in Amynodontopsis bodei, which is situated above the post-canine diastema in V 26305; (2) the nasals of Amynodontopsis bodei are shorter than thoseof V 26305; (3) the nasal process of the premaxilla is reduced and contacts with the nasal slightly by a downward extension of the nasals in Amynodontopsis bodei, whereas in V 26305, it is considerably longer and very broad laterally below the nasals, excluding the maxilla from the border of the external nares; (4) the zygomatic arch is relatively large in vertical dimension and angles sharply upward posteriorly in Amynodontopsis bodei, while in V 26305 the zygomatic arch is slender, passing upward gradually; (5) the postorbital process of jugal is absent in Amynodontopsis bodei, but present in V 26305; (6) the posttympanic process is further separated from the postglenoid process both in Amynodontopsis bodei and V 26305, but the external meatus of the former is more opening ventrally than that of the latter; (7) the profile is flat in Amynodontopsis bodei, while V 26305 has a convex dorsal profile, a lower occiput and the supraoccipital extending more posteriorly over the occipital condyles; (8) the protocone rib of the P4 is less prominent in Amynodontopsis bodei than in V 26305; (9) relative to M1, the M2 and M3 in Amynodontopsis bodei are proportionally larger than those in V 26305; (10) the metastyle is deflected labially on the M2 in Amynodontopsis bodei, while V 26305 has a metastyle of the M2 warped inward slightly; (11) the M2 of Amynodontopsis bodei lacks antecrochet, but V 26305 has a faint antecrochet on the M2; (12) the presence of wrinkles on inner side of the ectoloph of M2 and on the anterior side of the metaloph of the M3 is more specialized in Amynodontopsis bodei.
William Wall assigned a number of specimens, collected by the Central Asiatic Expeditions in the Erlian Basin, to the genus Amynodontopsis, and named two new species, Amynodontopsis parvidens and Amynodontopsis tholos. V 26305 obviously differs from Amynodontopsis parvidens in several craniodental features. The nasal of Amynodontopsis parvidens is more reduced than that of V 26305. The anterior tip of the nasal The nasal process of premaxilla in Amynodontopsis parvidens is greatly reduced laterally, causing the maxilla to become part of the border of the external nares in V 26305 is probably rounded, whereas it is generally squared off in Amynodontopsis parvidens, while in V 26305 the premaxilla is well developed in lateral extent, forming a distinct premaxilla-nasal contact in lateral view and restricting the maxilla from the border of the external nares. The nasal notch in Amynodontopsis parvidens extends to a point above the front of the P4, whereas it is situated above the post-canine diastema in V 26305.
Amynodontopsis tholos, from the Erlian Basin, is the largest species of Amynodontopsis and displays some derived characters. The facial region in Amynodontopsis tholos differs from that of V 26305 in having an extremely reduced nasal with its anterior border squared off, a premaxilla ending approximately two thirds the way up the external nares where it contacts the nasal and a nasal notch extending back to a point above the M1. The most distinctive difference between them is that the skull of Amynodontopsis tholos expands into a wide dome above the orbit. The antecrochet on the M1 of A. tholos is not as stout as in V 26305. The protoloph and metaloph of the M2 and M3 are oriented more posteriorly in Amynodontopsis tholos, whereas they are more developed and nearly perpendicular to the ectoloph in V 26305.
In 2018 Jérémy Tissier, Damien Becker, Vlad Codrea, Loïc Costeur, Cristina Fărcaş, Alexandru Solomon, Marton Venczel, and Olivier Maridet, referred three specimens, two maxillary fragments and a lower jaw fragment, to Amynodontopsis aff. Amynodontopsis bodei. The specimens were found from the Upper Eocene Valea Nadăşului Formation of Romania and the Middle Eocene Dorog Coal Formation of Hungary. They referred the specimens to Amynodontopsis mainly based on the moderate angle of the zygomatic process of the maxilla in ventral view, the absence of cement on upper molars and a weak paracone rib on the M3. However, the first two characters are not derived characters, and Amynodontopsis does not show the reduction of the paracone rib on the M3. In addition, the Romanian maxillary fragment with M1–3 differs from Amynodontopsis in the absence of an antecrochet on the M1, having a very small metastyle on the M2 and a metastyle directed more posterolingually on the M3. The Hungarian maxillary fragment with M2–3 differs from Amynodontopsis in having protocones constricted anteriorly and a continuous labial cingulum on the M1. The Romanian mandibular fragment with m1/2, differs from Amynodontopsis in having narrower cheek teeth with a better developed external groove. Therefore, the assignation of the European specimens needs reexamination. A mandible fragment with p4 and roots of m1–3, from southern Mexico, was identified as Amynodontopsis cf. bode, cannot be directly compared with V 26305.
Consequently, the skull of V 26305 is evidently characteristic of Amynodontopsis in its morphology. However, the new specimen differs from all three previously named species of Amynodontopsis and has unique characters. Therefore, Wang et al. name a new species, Amynodontopsis jiyuanensis, for V 26305.
Previous study has suggested some evolutionary trends in Amynodontid skull morphological features, such as the decrease of the nasal length, reduction of the nasal process of the premaxilla, the contact between nasal and premaxilla going from shortened to absent, and the nasal notch being located more posteriorly. In addition, the cheek teeth of Amynodontids are thought to have undergone evolutionary changes in being narrower, corresponding to the protoloph and metaloph being shorter and more obliquely orientated. Some characters of Amynodontopsis jiyuanensis are more primitive than those of the three other species. For instance, the P4 has nearly the same transverse width as the M1; the protoloph and metaloph on the upper molars are approximately equal in length, proportionally longer relative to the ectoloph, and closer to being perpendicular to the ectoloph; the nasal process of the premaxilla is long and very broad laterally below the nasals, excluding the maxilla from the border of the external nares; the nasal bone is not reduced; the nasal notch situates above the post-canine diastema. It is reliable to consider that Amynodontopsis jiyuanensis probably represents the most primitive taxon within the genus Amynodontopsis.
The Niezhuang Formation of the Jiyuan Basin is not very rich in Mammal fossils. Fossil Mammals reported from the formation include the Rodent Yuomys cavioides, the Amynodont Rhinocerotoids Lushiamynodon obesus, Sianodon chiyuanensis, Sianodon sinensis, and Sianodon sp. and an unidentified Hyracodontid Rhinocerotoid. Yuomys cavioides, one of the diagnostic taxa common in different sites and restricted to the Sharamurunian Asian Land Mammal Age, was found in Ula Usu, west side of the Shara Murun River. The Sharamurunian is currently correlated with the late Uintan North American Land Mammal Age that is earlier than the Duchesnean. Amynodontopsis jiyuanensis is thus the earliest fossil record of the genus.
The primitive characters and earlier occurrence of Amynodontopsis jiyuanensis suggest that Amynodontopsis originated in Asia no later than the Middle Eocene Sharamurunian Asian Land Mammal Age and migrated from Asia to North America later as ancestral form of Amynodontopsis bodei.
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