Metamynodon planifrons Scott & Osborn, 1887

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 : 118-120

Metamynodon planifrons Scott & Osborn, 1887

( Figs 5; 6; Table 3)

Metamynodon rex Troxell, 1921: 24 .

Metamynodon planifrons – Scott & Osborn 1887: 165.

TYPE MATERIAL. — Holotype. VPM-9157, skull and anterior part of the left mandible ( Scott & Osborn 1887; see online the database of the Zoological Collections of the Museum of Comparative Zoology - Harvard University).

REFERRED MATERIAL. — UNISTRA.2015.0.1106, skull with right C1 and P2-M3.

TYPE LOCALITY AND HORIZON. — Big Badlands, South Dakota, United States. White River Group, Brule Formation, early Oligocene ( Scott & Osborn 1887; Scott 1941; Benton et al. 2015).

DESCRIPTION

Skull

The skull UNISTRA.2015.0.1106 is incomplete ( Fig. 5). The anterior surface of the nasal is damaged and its extremity lacking. The right orbit is filled with matrix. The skull is brachycephalic (sensu Antoine 2002), with a maximum zygomatic width/nasal-occipital length ratio>0.50. The preorbital region constitutes less than 30% of the skull. There are no rugosities suggesting the presence of a horn.

In lateral view ( Fig. 5A, B), the dorsal profile of the skull is flat. The tooth row extends beyond the middle of the skull. The premaxilla and the nasal contact each over along the border of external nares. The nasal incision is in front of the P2. The nasal, frontal and maxilla contact each other. There is a reduced and deep preorbital fossa. The anterior border of the orbit is above P4-M1. The orbit is relatively high on the skull. The anterior base of the zygomatic process of the maxilla is low. The position of the zygomatic arch on the skull is high. On the squamosal, a posterior groove on the zygomatic process is present. Both squamosal and frontal have a postorbital process. The squamosal area between the temporal and nuchal crests is flat. The external acoustic pseudomeatus is closed. The posterior margin of the pterygoid is nearly horizontal. The occipital side inclines posteriorly. A poorly developed nuchal tubercle (or occipital protuberance) on the occipital is present.

In dorsal view ( Fig. 5C, D), the zygomatic arches are wide and massive. The zygomatic index (maximum width at the zygomatic/maximum width at the frontals) is 1.55 (sensu Antoine 2002). The sagittal crest is strong. The external occipital protuberance is straight.

In ventral view ( Fig. 5E, F), the zygomatic arches are complete and diverge abruptly from the maxilla. It starts above the M2 and separates above the M3. The vomer is acute. The post-glenoid apophysis (= process) is flat. The posttympanic process is fused to the postglenoid process. The posttympanic

A

B

process is poorly developed but the paroccipital process is developed and fused together.

In occipital view ( Fig.5G, H),the foramen magnum is circular.

The dental formula is I 3/?, C 1/?, P 3/?, M 3/?. The alveoli of the incisors and the left canine can be observed in occlusal view. The right canine is in place, strong and cylindrical. The diastema between the canines and premolars is short.The series of premolars and molars are complete but worn. The premolar series are very short compared to the molar series (sensu Antoine 2002), with a LP3-4/LM1-3 ratio ≤0.42. There is no crochet and no antecrochet on the upper cheek teeth.

Upper cheek teeth ( Fig. 6)

The P3 is submolariform and the P4 is semimolariform (sensu Heissig 1969). The protocone and hypocone are connected by a lingual bridge. The premolars have no labial cingulum, and their lingual cingulum is continuous. The postfossette is wide. The metaloph is transverse. On P2, the protoloph is present and does not join to the ectoloph. The hypocone and protocone of P2 are fused. On P3-4, the protoloph is fused to the ectoloph. The hypocone and the metacone are linked. The P4 has a crista.

The upper molars have a π form ( Fig. 6A). They have neither lingual nor labial cingula. There is no constriction of the protocone. The parastyle is reduced. There is no crista and cristella. On M1-2, the protocone and hypocone are fused. There is no paracone fold. The metaloph is continuous and long, the metacone fold is absent. The metastyle is short. The posterior part of the ectoloph is straight. The M3 has quadrangular shape. The protoloph is transverse, the ectoloph and metaloph are distinct. The paracone fold is weak. The metastyle is directed posterolingually.