Rhinoceros sondaicus Desmarest, 1822

Suraprasit, Kantapon, Jaeger, Jean-Jacques, Chaimanee, Yaowalak, Chavasseau, Olivier, Yamee, Chotima, Tian, Pannipa & Panha, Somsak, 2016, The Middle Pleistocene vertebrate fauna from Khok Sung (Nakhon Ratchasima, Thailand): biochronological and paleobiogeographical implications, ZooKeys 613, pp. 1-157 : 17-20

Taxon classification Animalia Perissodactyla Rhinocerotidae

Rhinoceros sondaicus Desmarest, 1822

Referred material.

A left P2, DMR-KS-05-03-00-128; a left P3, DMR-KS-05-03-22-17; a left M1, DMR-KS-05-03-00-129; a left M3, DMR-KS-05-03-00-127; a mandible with right (i2 and p2-m3) and left (p3-m3) tooth rows, DMR-KS-05-03-00-126; a partial mandible, DMR-KS-05-03-31-28; a fragmentary nasal bone, DMR-KS-05-03-00-56; a left scapula, DMR-KS-05-03-00-58; a left humerus, DMR-KS-05-03-31-3; a right metacarpus II, DMR-KS-05-03-28-29; a metacarpus III, DMR-KS-05-03-22-49; a right metacarpus IV, DMR-KS-05-04-05-15; a left tibia, DMR-KS-05-03-00-52; a right calcaneus, DMR-KS-05-04-27-19; a left astragalus, DMR-KS-05-03-26-23.

Material description.

Upper dentition: P2 (DMR-KS-05-03-00-128: Fig. 10A), M1 (DMR-KS-05-03-00-129: Fig. 10C), and M3 (DMR-KS-05-03-00-127: Fig. 10D) are presumably from the same individual because they were found together at the same spot. The upper cheek teeth are lophodont (for measurements, see Tab. 9). Premolars are completely molarized (Fig. 10A, B) and molars exhibit well-preserved crochets. The M3 is triangular in occlusal outline and displays a well-developed parastyle, ectometaloph, medifossette, and hypocone, but a less developed parastyle fold (Fig. 10D).

Mandibles and lower dentition: a mandible (DMR-KS-05-03-00-126) preserves both sides of cheek tooth rows (right p2-m3 and left p3-m3), but most of its symphysis and entire ramus are broken off (Fig. 10 E–G) (for measurements, see Appendix 2). The posterior edge of the mandibular symphysis ends nearly at the middle part of p3. The ventral margin of the mandible is convex in lateral view (Fig. 10E). The mental foramen is situated below the p3. In ventral view, the small foramen is present at the central portion of the mandibular symphysis and the lingual mandibular outline is U-shaped (Fig. 10F, G). Only the basal part of a right tusk-like incisor is preserved in its socket. Another specimen DMR-KS-05-03-31-28 preserves a nearly complete mandibular symphysis and left p2 and p3 sockets (Fig. 10H, I). The left mandibular body behind the p3 is broken away. All lower cheek teeth are heavily worn and rectangular in occlusal outline (Fig. 10F) (for measurements, see Tab. 9).

Nasal: a nasal bone (DMR-KS-05-03-00-56) is short and robust, bending downward and narrowing anteriorly towards the tip (Fig. 10J). The anterior surface is nearly straight in lateral view (Fig. 10K), whereas its ventral surface is flattened at the central suture. This nasal bone is most similar to Rhinoceros sondaicus (e.g., specimen MNHN-ZMO-1985-159), because its anterior part is pointed rather than rounded ( Colbert 1942). In comparison, Rhinoceros unicornis displays a convex anterior surface in lateral view and a well-developed horn protuberance of the nasal region. The maximum length and width of the nasal are 131.1 mm and 88.8 mm, respectively.

Postcranial remains: postcranial elements include a scapula (Fig. 11A, B), a humerus (Fig. 11 C–E), three metacarpal bones (metacarpus II, III, and IV: Fig. 11 F–H), a tibia, a calcaneus (Fig. 11I), and an astragalus (Fig. 11J). All postcranial remains are comparable in size to the recent material ( Guérin 1980) (for measurements, see Appendix 1).

Taxonomic remarks and comparisons.

Four isolated cheek teeth (P2, P3, M1, and M3) assigned to Rhinoceros sondaicus are characterized by the following morphological features: a presence of the moderately developed crochet, sinuosity of the ectoloph, distinct parastyle fold, and deeper median valley compared to the posterior valley, and the absences of an antecrochet, protocone fold, and metacone bulge on M3. All of these characters coincide with the upper molars of Rhinoceros sondaicus ( Pocock 1945, Hooijer 1946, Zin-Maung-Maung-Thein et al. 2006, Groves and Leslie 2011).

Large tusk-like incisors (i2) are notably typical of Asian rhinoceroses. The two small alveoli corresponding to the lost central incisors are autapomorphic of Rhinoceros ( Groves and Leslie 2011). Our observations on the recent mandible iPHEP M05.5.001.B and MNHN-ZMO-1985-159 demonstrate that an alveolus extension of the lower incisors that reach posteriorly to the lingual side of the p2 is a characteristic of both living Javan ( Rhinoceros sondaicus ) and Indian ( Rhinoceros unicornis ) rhinoceroses ( Tong and Guérin 2009). This feature efficiently distinguishes Rhinoceros from the Sumatran rhinoceros, Dicerorhinus sumatrensis , where the alveoli of the lower incisors do not extend as far ( Tong and Guérin 2009). In the mandibles DMR-KS-05-03-00-126 and DMR-KS- 05-03-31-28, the lower incisor alveoli extend posteriorly into the mandibular symphysis, ventral to the lingual side of the p2 (Fig. 11H, I). The latter specimen also shares similar mandibular dimensions (Appendix 2) and morphology with the former specimen.

Isolated lower molars of rhinoceroses from Khok Sung are difficult to assign to either Rhinoceros unicornis or Rhinoceros sondaicus due to heavy wear. In addition, there is a significant size overlap between these two species ( Guérin 1980). The lengths of lower cheek teeth and molar rows provide a better distinction (little overlap in size) than those of isolated teeth. The lengths and widths of the cheek teeth on the mandible DMR-KS-05-03-00-126 fall almost within the range of Rhinoceros sondaicus , with the exception of some specimens (p3, p4, and m3) that fit well with the larger-sized Rhinoceros unicornis (Tab. 9). However, the lengths of the mandibular cheek tooth and molar rows of this specimen fall within the ranges of Rhinoceros sondaicus (211.5-257 mm and 126.5-147 mm, respectively) and outside of the ranges for Rhinoceros unicornis ( Guérin 1980: table. 6). The two mandibles, DMR-KS-05-03-00-126 and DMR-KS-05-03-31-28, are thus assigned to Rhinoceros sondaicus .