Amphimoschus xishuiensis, Li, Yi-Kun, Mennecart, Bastien, Aiglstorfer, Manuela, Ni, Xi-Jun, Li, Qiang & Deng, Tao, 2021

Li, Yi-Kun, Mennecart, Bastien, Aiglstorfer, Manuela, Ni, Xi-Jun, Li, Qiang & Deng, Tao, 2021, The early evolution of cranial appendages in Bovoidea revealed by new species of Amphimoschus (Mammalia: Ruminantia) from China, Zoological Journal of the Linnean Society 196 (3), pp. 1039-1053 : 3-7

AMPHIMOSCHUS XISHUIENSIS SP. NOV.

v 2003 Amphimoschus cf. A. artenensis Wang et al. : 262.

v 2008 Amphimoschus cf. A. artenensis Wang et al. : 3, fig. 8.

Z o o b a n k r e g i s t r a t i o n: u r n:l s i d: z o o b a n k. org:act: 716C1F0F-1F6E-4112-8960-C0DB3AB67F9A

Holotype: IVPP V 25521.1 , a right hemimandible with tooth row p2–m3, discovered from the site DH199911 (39°29 ′ 24.0 ″ N, 94°43 ′ 52.8 ″ E) in 1999, equivalent to the localities of T. b. 312–313 ( Bohlin, 1946) and previously attributed to Amphimoschus cf. A. artenensis ( Wang et al., 2003, 2008).

Referred material: IVPP V 25521.2 , anterior part of a skull with right P4–M3 and left P3–M2, from the site in 2015 (39°29 ′ 25.0 ″ N, 94°43 ′ 52.6 ″ E) ; V 25521.3 , left maxilla with partial P3–M3, from the site in 2014 (39°29 ′ 24.7 ″ N, 94°43 ′ 53.0 ″ E) ; V 25521.4 , right maxilla with M2 and M3 from the site DH199911, previously attributed to Amphimoschus cf. A. artenensis ( Wang et al., 2003, 2008) .

Type locality: Xishuigou , East gully of the Tabenbuluk area, Subei Mongolian Autonomous County, Gansu Province, China.

Associated fauna and age: Xishuigou fauna, proposed by Wang et al. (2008) and comprising ‘ Kansupithecus ’, Platybelodon dangheensis Wang & Qiu, 2002 , Turcocerus halamagaiensis (Ye, 1989) , Kinometaxia guangpui Wang, 2004 and Heterosminthus intermedius Wang, 2003 ( Bohlin, 1946; Wang, 2002; Wang & Qiu, 2002; Wang et al., 2004; Li et al., 2021a); Tiejianggou Formation, ~17.0–19.7 Mya, late early Miocene ( Wang et al., 2013).

Etymology: Species name attributed to the fossil locality Xishuigou (Xishui gully).

Differential diagnosis: On average, larger in size than European Amphimoschus ( A. ponteleviensis ), A. xishuiensis possesses a smaller lower premolar/ molar ratio than any known specimen of A. ponteleviensis . Amphimoschus xishuiensis differs from A. ponteleviensis in having a relatively compressed p4 with narrow anterior valley and broad transverse cristid, a well-developed metastylid on m1–m3, an additional stylid at the lingual base on m1–m2, an anterolingual cingulum on P4, and the presence of cranial ornamentations, including a supraorbital bump, an antorbital protuberance and frontal thickening.

Description

Cranium: Only the right facial part is relatively well preserved in the skull V 25521.2 ( Fig. 2 ), which suffers from post-mortem deformation and possesses posterior–lateral to anterior–medial and transversal cracks at the dorsal and anterior parts. The nasal bones could be separated easily and seem to form a semicircular pipe, and they share a U-shaped suture line with the frontal. The antorbital vacuity is present posteriorly to the nasal and situated in a relatively anterior position, whereas its shape is unclear to define. The frontal bones are distinctly thickened, starting from the suture between the nasal and the frontal and ending with a rounded protuberance. The protuberance is located 10.9 mm anterior to the anterior orbital margin, and within this distance a ridge is present, connecting the protuberance to the orbit. The right orbit might not be circular in shape, given that its dorsal and posterior margins are less curved. The orbital length (32.3 mm) is slightly larger than its height (29.2 mm), but it is compressed dorsoventrally and lateromedially. A small supraorbital bump is situated centrally at the outer edge of the dorsal orbital margin, and it shows a bony fibrous internal structure consisting of thin, compact cortical and thick, spongy central regions, similar to the antorbital protuberance ( Fig. 2B ). Only the anterior part of the frontal midline is still in place, which is straight and simple. Its adjacent region is relatively flat and horizontal, whereas the posterior frontal is slightly elevated. A prominent ridge is present at the posterior part of the frontal, connecting the parietal to the orbit. The ventral orbital margin is robust, as is the preserved temporal process, indicating that the zygomatic is well developed. The facial region anterior to the orbit is long and flat; hence, the lacrimal fossa is absent. The bony surface of the maxilla is mostly exposed. The dorsal half of the maxilla, connected to the nasal, forms a smooth bulky relief, increasing the depression between the nasal and the maxilla.

Upper teeth: The enamel of the teeth is wrinkled. The M1 of IVPP V 25521.2 and 25521.3 is heavily worn, indicating that those specimens belong to adult individuals. V 25521.4 had a similar age at death, because it possesses a similar stage of wear to V 25521.3 ( Fig. 3 ). The length of the upper molar row of V 25521.3 is estimated at 45 mm. The length of M1–M 2 in V 25521.3 is 29.8 mm, and that of M2–M 3 in V 25521.4 is 30.2 mm. The P3 is fragmentary in IVPP V 25521.2 and 25521.3. The lingual cone is located almost centrally, slightly shifted posteriorly, with a slightly convex posterolingual crista. The labial cone is broken, whereas it possesses a developed labial rib. The posterolabial crista is shorter than the posterolingual one. The labiolingually oriented central fold separates the fossa into anterior and posterior parts. The outline of the P4 is triangular, with a rounded lingual portion. The labial cone is large, and the anterolabial and posterolabial cristae are short and parallel to the anteroposterior direction. The labial cone rib is moderately developed and located centrally. The labial cone is more slender but higher than the lingual one. The lingual cone is located almost centrally and shifted only slightly to the posterior. The anterior and posterior styles are moderately developed. There is a central fold that forms an enamel bulge on the posterior part of the fossa. The anterolingual cingulum is present and constricted anterior to the lingual cone in V 25521.3, whereas in V 25521.2, the cingulum surrounds the lingual cone. The outline of the upper molars is relatively square. The M2 and M3 are of similar length. Owing to heavy wear, the main cusps of the M1 are flat. On M2 and M3 of IVPP V 25521.3 and 25521.4, the labial end of the straight postprotocrista is not bifurcated but instead turns anteriorly; however, it is bifurcated on M3 of V 25521.2. The labial end of the postprotocrista reaches the posterolingual base of the paracone that forms a small internal enamel fold into the anterior fossa. The metaconule is similar in size to the protocone on M1 and M2, whereas it is smaller than the protocone on M3. The preprotocrista, similar to the premetaconulecrista, is anterolabially oriented. The preprotocrista joins the parastyle on its lingual side, enclosing the anterior fossa, and the postmetaconulecrista joins the small metastyle, enclosing the posterior fossa posteriorly. The postmetaconulecrista bears a metaconule fold. The straight premetaconulecrista is not connected to any crista on its labial end, where a small bifurcation is present. On heavily worn teeth, the lingual branch of that bifurcation fuses with the postprotocrista, forming an enamel island at mid-length. The paracone rib is moderately developed and the metacone rib less developed. The metacone wall is inclined posteriorly relative to the paracone wall on M1 and M2 and slightly less inclined on M3. The labial cristae are short. On M1 and M2, the postparacrista reaches the labial end of the premetacrista, and there is no fusion on M2. The parastyle and metastyle are more developed from M1 to M3. The mesostyle is well developed and anteriorly directed on M1, anterolabially directed on M2, and slightly smaller and labially directed on M3. On M1 and M2, the mesostyle meets the premetacrista only, but it seems to combine the postparacrista and premetacrista on M3. The entostyle is large and trapezoidal on M1 and M2 but is smaller and ovoid on M3. The entostyle is fully isolated on M3, whereas its margin fuses with the premetaconulecrista entirely on M1 and slightly on M2. A faint and small posterior cingulum is present on M1 of V 25521.2. Anterior and lingual cingula are present on M2 and M3, separated by the protocone on M2 but connected and surrounding the protocone on M3.

Hemimandible: The hemimandible (V 25521.1) is relatively stocky, with a high corpus mandibulae ( Fig. 3I, J ). The height of the corpus mandibulae anterior to the m3 is 27.9 mm, and it gradually decreases in height to only 19.6 mm anterior to the p2. A small crest is present anterior to the p2. The distance between the symphysis and p2 is about as long as the premolar row.

Lower teeth: The complete cheek tooth row is preserved in V 25521.1, with 73.6 mm length ( Fig. 3I, J ). The ratio between lower premolar and molar rows is 0.65 (lower premolar row 29.5 mm; lower molar row 45.5 mm). The enamel is wrinkled on all teeth. The m1 is heavily worn, indicating that the specimen belongs to an adult individual. The lower premolars share the basic structure, with a more complex pattern from p2 to p4. The p3, the p4 and the posterior portion of p2 are strongly worn. The premolars become longer from p2 to p4. The p2 has an elongated triangular outline in the occlusal view, whereas the p3 and p4 are more enlarged in the anterior portion. The p4 is stocky. The anterior conid is small and anterolingually directed on p2 and is larger and more lingually directed on p3 and p4. It protrudes less lingually than the mesolingual conid on p2 and p3, but more-or-less at the same axis as the mesolingual conid on p4. The anterior valley forms a more acute angle from p2 to p4. Only the p3 and p4 possess an anterior stylid, which is small and directed lingually. The anterolabial cristid is short and anterolingually oriented. The mesolabial conid is positioned anteriorly on p2 to centrally on p4, and it is the highest cuspid of the lower premolars. On p2, the mesolabial conid is situated slightly more lingually than the posterolabial conid, and the groove anterior to the posterolabial conid is shallow. On p3 and p4, the mesolabial conid is situated more labially than the posterolabial conid, and that groove on p4 is more profound than on p3. There is no distinct mesolingual conid. The transverse cristid is more prolonged and broader from p2 to p4 and is of similar posterolingual orientation. The posterolingual conid on p2 seems short, ending in the middle part of the tooth, whereas it forms the posterolingual end on p3 and p4. The posterolingual conid protrudes more lingually than the transverse cristid on p3 and p4. The posterior stylid is long on p2 and reaches the lingualmost side of the tooth. On p3 and p4, it is shorter and joins the posterolingual conid on its posterior half. There is no cingulid in any premolar. The molars are longer from m1 to m3. The lingual wall on m1 is relatively flat. The metaconid rib, stronger from m1 to m3, is more developed than the corresponding entoconid rib. In the occlusal view, the entoconid is lingually flattened, with straight and short cristids. The metastylid, starting from the middle of the postmetacristid, is well developed and more posteriorly elongated from m1 to m3. On m3, it even forms a fold covering the anterior portion of the pre-entocristid. The postmetacristid, pre-entocristid and internal postprotocristid are fused together on the lingual half of the teeth. There is no external postprotocristid in any molar. The preprotocristid is elongated, curved and fused with the premetacristid, which is short and straight. Owing to heavy wear, the prehypocristid fuses a little with the lingual part of the internal postprotocristid on m2, whereas on m3 it is lingually isolated. The posthypocristid is straighter from m1 to m3 because the posterior tooth is less protruding than the anterior one. The posthypocristid reaches the lingual side of m2 (not observed on m1 owing to wear), enclosing the posterior fossa; however, there is no direct fusion between the postentocristid and posthypocristid. The third basin of m3 is well structured. The entoconulid is well developed, elongated and globular and forms a bulgy lingual part of the back fossa. The entoconulid connects with the postentocristid on its anterolingual part and the posthypocristid on its anterolabial part. The short postentoconulidcristid does not join the posthypoconulidcristid, which leaves the posterior part of the back fossa open. The hypoconulid is large, and the prehypoconulidcristid fuses with the posthypocristid in the lingual mid-length. The back fossa of m3 is narrow, anteroposteriorly oriented and posteriorly open. An additional stylid occurs at the lingual base of the junction between the metaconid and entoconid, and that stylid is well developed on m1 and m2, forming a small column, and is smaller on m3. The ectostylid is well developed, high, elliptic and oblique in shape on m2, and smaller and rounded on m 3 in the occlusal view. The posterior ectostylid is vestigial. The anterior cingulid is not observed on m1 owing to wear but is weakly present on m2 and m3.

Figures

Figure 2. Skull of Amphimoschus xishuiensis from Tabenbuluk area, China. A, right lateral view of computed tomography (CT) reconstruction of IVPP V 25521.2 (3D models seen in Li et al., 2021b). B, CT scanning screenshots of sagittal view at slice 1115.1, axial view at slice 8158.5, and axial view at slice 6961.5.C, right lateral view.D, dorsal view.Scale bar applies to C and D.

Figure 3. Dental nomenclature and cheek teeth of Amphimoschus xishuiensis from Tabenbuluk area, China. A, upper premolar: 1, anterior style; 2, anterolabial crista; 3, fossa; 4, anterolingual crista; 5, anterolingual cingulum; 6, labial cone; 7, posterior style; 8, posterolabial crista; 9, central fold; 10, posterolingual crista; 11, lingual cone. B, upper molar: 12, parastyle; 13, anterior fossa; 14, postprotocrista; 15, preprotocrista; 16, protocone; 17, anterior and lingual cingula; 18, preparacrista; 19, paracone; 20, postparacrista; 21, mesostyle; 22, premetacrista; 23, metacone; 24, postmetacrista; 25, metastyle; 26, premetaconulecrista; 27, posterior fossa; 28, postmetaconulecrista; 29, metaconule; 30, entostyle. C, lower premolar: 31, anterior stylid; 32, anterior conid; 33, anterior valley; 34, transverse cristid; 35, posterior valley; 36, posterolingual conid; 37, back valley; 38, posterior stylid; 39, posterolabial conid; 40, mesolabial conid.D, lower molar:41, mesostylid; 42, premetacristid; 43, metaconid; 44, postmetacristid; 45, metastylid; 46, pre-entocristid; 47, entoconid; 48, postentocristid; 49, posterior fossa; 50, entoconulid; 51, back fossa of m3; 52, hypoconulid; 53, posthypocristid; 54, hypoconid; 55, prehypocristid; 56, ectostylid; 57, protoconid; 58, preprotocristid; 59, postprotocristid; 60, anterior fossa. E, occlusal view of IVPP V 25521.2. F, occlusal view of V 25521.3. G, occlusal view of V 25521.4. H, labial view of V 25521.4. I, occlusal view of V 25521.1. J, lingual view of computed tomography reconstruction of V 25521.1 (3D models seen in Li et al., 2021b). Scale bar applies to E–J.