Paraceratherium linxiaense, Deng & Lu & Wang & Flynn & Sun & He & Chen, 2021

Deng, Tao, Lu, Xiaokang, Wang, Shiqi, Flynn, Lawrence J., Sun, Danhui, He, Wen & Chen, Shanqin, 2021, An Oligocene giant rhino provides insights into Paraceratherium evolution, Communications Biology 639 (1), pp. 1-10 : 2-3

Paraceratherium linxiaense sp. nov.

Type specimens. Acomplete skull and mandible with the associated atlas (holotype, HMV 2006 , Fig. 1 ), and an axis and two thoracic vertebrae of another individual (paratype, HMV 2007, Fig. 2 ), which are preserved at the Hezheng Paleozoological Museum in Hezheng County, Gansu Province, China. HMV 2006 represents a full adult individual. The specific name, linxiaense , refers to the geographical location of the discovery in the Linxia Basin ( Fig. 3 ).

Type locality and horizon. IVPP locality LX1808 (N35°35’05.16”, E103°18’51.02”; 1983 m above sea level, Fig. 4 ) is near the village of Wangjiachuan, 10.8 km southwest of the town of Dongxiang County, Linxia Hui Autonomous Prefecture, Gansu Province, China ( Fig. 3 ). HMV 2006 and 2007 are from the sandstones in the lower part of the Jiaozigou Formation ( Fig. 5 , Supplementary Note 1).

Age. IVPP locality LX1808 is faunally and paleomagnetically dated to the middle of chron C8r with an estimated age of 26.5 million years ago (Ma) in the late Oligocene ( Fig. 5 ).

Diagnosis. Paraceratherium linxiaense possesses features that characterize the genus, such as a giant body size, long premaxillae with anterior ends extending downward, separated parietal crests, high condyle compared to the height of nuchal surface, lower inferior border of the posttympanic process than the condyle, roughly horizontal anterior part of symphysis, and downward turning cone-shaped I1. It is more derived than other species within this genus in having a larger body size, deeper nasal notch above M2, much higher occipital part and posterior end of zygomatic arch, and smaller upper incisor I1. The lower margin of the horizontal mandibular ramus is concave under the diastema, and small i1 extends anteriorly and horizontally. The dental formula is 1.0.3.3/1.0.3.3. P2 is semimolarized, while P3 and P4 are submolarized. The metaconule connects with the ectoloph and the anterior point of the hypocone in moderate wear; the antecrochet is moderate; the lingual border of the protocone is rounded on molars; and the ecto-posterior corner of the protolophid is angular on p3 and p4. The atlas has an expanded transverse foramen and a dumb-bell shaped vertebral fossa.

Comparative description. The new species differs from other species of Paraceratherium (P. grangeri, P. huangheense, P. asiaticum, P. bugtiense, and P. lepidum) in having a deeper nasal notch whose bottom is located above the middle of M2, proportionally larger height of the condyle (43.9%) compared to height of the occipital surface (Supplementary Table 2), short muzzle bones and diastema anterior to cheek teeth, highly raised occiput, and high zygomatic arch with a prominent posterior end ( Fig. 1 ).

The nasals of P. linxiaense are flat and straight, and the nasal notch is very deep above the middle of M2 with a short distance from the orbit (15.3% of the basal cranial length) (Supplementary Table 2), much deeper than those of other species of Paraceratherium except P. lepidum, indicating a short prehensile nose trunk. The dorsal surface of the skull is shallowly depressed, different from the domed skull of P. grangeri 13 or the flat one of P. lepidum 7. The distance between the parietal crests is narrow and smaller than that of P. lepidum. The infraorbital foramen is situated above the P4/M1 boundary, and the anterior margin of the orbit is located above the M2/M3 boundary. Both characters are similar to those of P. lepidum and more posteriorly positioned than those of P. grangeri above the P3/P4 boundary and the middle of M2 respectively. The position of the zygomatic arch is high, posteriorly flush with the upper margin of the orbit like those of P. grangeri and P. lepidum, and much higher than that of P. bugtiense 14. The postorbital process is absent. The space between the posttympanic and postglenoid processes is moderate as in P. lepidum, wider than in P. grangeri and narrower than in P. bugtiense. The posttympanic process has no transverse expansion, and its lateral margin is almost flush with the postglenoid process. The posttympanic and paraoccipital processes fuse to become a wide and thick plate.

For P. linxiaense , the posterior border of the mandibular symphysis is situated at the p4/m1 boundary, and the posterior margin of the ascending ramus is anteriorly inclined, different from vertical situation in P. bugtiense and P. lepidum. The vascular notch of lower margin of the mandible is deep but more anterior than that of P. lepidum. The mandibular diastema has a straight and slowly declining upper margin like in P. grangeri, while strongly declined in P. asiaticum and P. lepidum, and convex in P. huangheense and P. bugtiense. The mental foramen is situated under the p3/p4 boundary, more posteriorly than in P. asiaticum, P. bugtiense, and P. lepidum where is under p2.

The distance between the anterior margins of I1 and P2 is 164 mm. DP1 is absent like in most species of Paraceratherium , but present in P. grangeri. The paracone rib is absent from P2 to M1, weak in M2, and marked in M3, which is the common character of the genus Paraceratherium , much different from the strong paracone rib of the primitive giant rhino Forstercooperia 15. The occlusal surface of P2 is triangular in P. linxiaense , different from the trapezium outline in P. grangeri and P. asiaticum. There is an obvious separating groove between the protocone and hypocone of P3 and P4. The hypocone of P4 is situated behind the metaconule as in P. grangeri and P. lepidum, and it is expanded and rounded, while that of P. lepidum is square. The antecrochet on upper molars is larger than those of P. grangeri and P. asiaticum. M3 is triangular in occlusal outline, similar to those of P. bugtiense and P. lepidum, but different from the rectangular outline in P. grangeri and P. asiaticum.

P. linxiaense has more reduced i1. The p2 is small and singlerooted, but large and double-rooted in P. grangeri. The p3 and p4 have a rudimentary entolophid, while the entoconid of P. asiaticum is an isolated cusp. The entolophid of m3 is nearly transverse.

Additional descriptions, measurements and comparisons, such as information of the postcranial bones, are provided in Supplementary Note 2 and Supplementary Tables 1–6.

Figures

Fig. 1 Holotype (HMV 2006) of Paraceratherium linxiaense sp. nov. Skull: a lateral view; b ventral view; c dorsal view; d anterior view; e occipital view. Mandible: f, h lateral view and medial view of left ramus, respectively; g occlusal view. Skull and mandible share the scale bar, but both anterior and nuchal views have an independent scale bar.

Fig. 2 Vertebrae of Paraceratherium linxiaense. a atlas (HMV 2006); b axis (HMV 2007); c 4th–5th articulated thoracic vertebrae (HMV 2007). 1, right view; 2, posterior view; 3, left view; 4, dorsal view; 5, anterior view; 6, ventral view; 7, postero-ventral view. The articular facets on 4th-5th thoracic vertebrae are marked with the red semitransparent outline whatever it is intact or has been damaged.

Fig. 3 Type locality of Paraceratherium linxiaense. Map showing the giant rhino fossil locality of the Linxia Basin in Wangjiachuan Village, Dongxiang County, Gansu Province, China.

Fig. 4 Type horizon of Paraceratherium linxiaense. Exposures of fluvial and lacustrine sediments of the Linxia Basin (LX 1808) at the type locality of this new species (HMV 2006, yellow square) and where the axis and thoracic vertebrae were also found in Wangjiachuan Village, Dongxiang County, Gansu Province, China.

Fig. 5 Cenozoic section of the Linxia Basin. Deposits with correlation to ATNTS 201249 (left) for the magnetostratigraphic results50 (middle) and the Wangjiachuan section bearing the fossils of Paraceratherium linxiaense sp. nov. (right).