Zaisanamynodon borisovi Belyaeva, 1971

Veine-Tonizzo, Léa, Tissier, Jérémy, Bukhsianidze, Maia, Vasilyan, Davit & Becker, Damien, 2023, Cranial morphology and phylogenetic relationships of Amynodontidae Scott & Osborn, 1883 (Perissodactyla, Rhinocerotoidea), Comptes Rendus Palevol 22 (8), pp. 109-142 : 113-116

Zaisanamynodon borisovi Belyaeva, 1971

( Figs 3; 4; Table 2)

Zaisanamynodon borisovi Belyaeva, 1971: 43 .

TYPE MATERIAL. — Holotype. ANPIN 2761 /1-22, incomplete skull, lower jaw, and part of the postcranial skeleton, including most of the cervical vertebrae and forelimbs ( Belyaeva 1971).

REFERRED MATERIAL. — ZSN-KKS-28- IPB, fragmentary skull with P2-M 3 in connection with a fragmentary mandible with p4-m3. MJSN.2020.008.01 and MJSN.2020.008.02 are 3D-printed copies of this skull and mandible, respectively.

TYPE LOCALITY AND HORIZON. — Kiin Kerish locality (48°07’52.2”N, 84°28’52.3”E), north-west of Kiin Kerish Mountain, Kazakhstan. Lower Aksyir Svita, Ergilian ALMA (36.5-33.9 Ma) (i.e., late Eocene) ( Borisov 1963; Belyaeva 1971; Russell & Zhai 1987; Lucas & Emry 1996; Lucas et al. 1996; Emry et al. 1998; Vandenberghe et al. 2012).

EMENDED DIAGNOSIS. — Differs from Zaisanamynodon protheroi in having a relatively short rostrum (about 12% of skull length), an anterior margin of the orbit above M1, incisors relatively large (especially I3/i3), a P2 less complex without anterior and posterior crests connected to the metaloph, a weak paracone fold on M3, lower canine relatively slender and curved, p3 more molariform, and a slight labial groove (cleft) between trigonid and talonid on lower molars.

DIAGNOSIS DISCUSSION. — In the revised diagnosis of Z. borisovi in Lucas (2006), the “lack of a complete cingulum” on P2-4 is considered as a character to distinguish Z. borisovi from Zaisanamynodon protheroi . However, in the original diagnosis of Zaisanamynodon borisovi, Belyaeva (1971) mentioned the presence of a well-developed anteroposterior and lingual collar (i.e., cingulum) and a weak labial collar (i.e., cingulum) on the upper premolars. In the description of Chinese specimens of Z. borisovi, Lucas et al. (1996) indicated the presence of a “complete lingual cingulum” on P2-3, and a “postero-lingual cingulum” on P4, which has been confirmed after direct observation of the specimen AMNH.26034. This feature is also observed on the studied material (ZSN-KKS-28-IPB). Therefore, the lingual cingulum on P2-4 can no longer be considered as a diagnostic character to distinguish Z. protheroi from Z. borisovi .

DESCRIPTION

Skull and mandible

The skull ZSN-KKS-28-IPB ( Fig. 3A, B) is incomplete, deformed and some parts are covered by matrix. The maxilla is incomplete, and only the left anterior part of the jugal is still preserved. The anterior margin of the orbit is above M1. The zygomatic process progressively diverges from the maxilla. It starts above the M2 and separates from it above the M3. The mandible ZSN-KKS-28-IPB ( Fig. 4A, B) is incomplete; the most anterior part is absent. The corpus mandibulae has a straight ventral profile. Only the anterior part of the ramus is present, and it is vertical. The dental formula of the specimen is I?/?, C?/?, P 3/2? M 3/3. The upper and lower anterior dentition are not preserved. The dental wear of the cheek teeth is moderate, the crown height is low, and the enamel is wrinkled and corrugated. The right P2 is very worn. The left P2 is not preserved and the left M3 is covered by matrix. There is no cement, crochet or antecrochet. The left p3 is not preserved but the roots of the right p3 can be observed in occlusal view.

Upper cheek teeth ( Fig. 3 C-G)

The upper premolars are not molarized and are very short compared to the molars (sensu Antoine 2002), with a LP3-4/ LM1-3 ratio ≤0.42. Premolars have a labial and lingual cingulum. The lingual cingulum is strong and continuous. The postfossette is wide. The P2 is very small. It bears a single labial cusp and probably had a single lingual one. The labial cingulum is continuous. On P3-4, the protocone and the hypocone are fused into a single lingual cusp. The protocone is not constricted. The metaloph is directed posterolingually. The paracone fold is very strong, forming a well-developed vertical ridge. On P3, the protoloph is not joined to the ectoloph, whereas they are joined on P4. The hypocone and the metacone of P4 are separated, whereas they are joined for P3. There is a crista on P3 and on the left P4. There is a shallow groove separating the paracone and the metacone on the ectoloph of P4. The metaloph of P4 is very short and weak. P4 has a third loph ( Fig. 3F), posterior to the metaloph and closing the postfossette. This third loph is thin, low, and discontinuous.

The upper molars have a π shape. They have a weak labial cingulum; a weak lingual cingulum is present in anterior and/ or posterior parts of M2 and posterior part of M3 whereas lingual cingulum is lacking on M1. Therefore, character 109 (1) and character 114 (2) in the matrix are scored as “usually present” and “usually absent”, respectively (sensu Antoine 2002). There are no crista and cristella. The protocone is widened. The metaloph is short, continuous, and its width decreases between M1-3. The ectoloph is very long, straight, and smooth. The fold of the paracone is weak, and the parastyle is reduced. There is a postfossette on each molar. The metastyle on M1-2 is long. The M1-2 have a continuous posterior cingulum. The M3 has a quadrangular shape. Its protoloph and metaloph are oblique to its ectoloph. Its metastyle is directed posterolingually. The ectoloph is concave labially.

Lower cheek teeth ( Fig. 4 C-H)

The p4-m3 series have a continuous labial cingulum. The lingual cingulum is only interrupted below the hypoconid. The trigonid is obtuse and is smaller than the talonid. The trigonid and talonid are open on the lingual side. The talonid is equal to the trigonid on the p4-m1, whereas the talonid is longer than the trigonid on the m2-3. The metaconid and entoconid are not constricted. The ectolophid is completely smooth (no labial groove), except on the p4, which has very shallow ectolophid and paralophid grooves. The lingual opening of the posterior valley on the p4 is U-shaped. The lower molar series are long. They bear an oblique hypolophid.