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 : 118-121

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 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.

Body mass

The body mass of UNISTRA.2015.0.1106 was estimated to be around 1.3 and 1.2 tons with the regression equations for the Rhinocerotidae and all Ungulates respectively ( Fortelius & Kappelman 1993). They are based on several measurements, including M2 and M3 lengths, basicondylar length, or zygomatic width ( Table 3 ; Appendix 3).

Our estimations are lower than those of Averianov et al. (2016) for Metamynodon planifrons (c. “ 1794 kg ”, see Averianov et al. (2016): supplemental table 2) based on the occlusal surface of m1, with the regression equation for generalized ungulates of Legendre (1989). This method can’t be applied to UNISTRA.2015.0.1106 because it is a skull, without mandible. As a comparison, similar results to ours were obtained estimating the body mass of VPM-9157 ( Scott & Osborn (1887); holotype of M. planifrons ), with the same equation, based on M3 length, basicondylar length, and zygomatic width ( Table 3 ; Appendix 3). The body mass of VPM-9157 was estimated to be around 1.5 and 1.3 tons with the regression equations for the Rhinocerotidae and all Ungulates respectively ( Fortelius & Kappelman 1993).

Despite this, our results are certainly over-estimated because the same problems with these methods explained earlier for Zaysanamynodon also apply to this genus (i.e., M2 and M3 much larger than all other cheek teeth in Amynodontidae , and retention of a metastyle on M3).

REMARKS

UNISTRA.2015.0.1106 can be referred to Rhinocerotoidea based on the π-shape form of upper molars ( Wall 1989). An attribution of UNISTRA.2015.0.1106 to Amynodontidae is reliable based on the reduced upper dental formula, with the absence of P1, which is a derived character typical of Amynodontidae ( Wall 1989; Tissier et al. 2018). UNISTRA.2015.0.1106 shares other diagnostic characters of the Amynodontidae : the absence of the crochet, antecrochet and crista; the reduced parastyle on the upper molars; the quadratic M3 with a large metastyle; the absence of horn; the presence of a preorbital fossa and a large sagittal crest ( Wall 1989; Tissier et al. 2018). Therefore, an attribution to Rhinocerotidae , Eggysodontidae , “ Hyracodontidae ” or Paraceratheriidae can be excluded.

UNISTRA.2015.0.1106 shares diagnostic characters of the Metamynodontini : the presence of the frontal-maxilla contact, a brachycephalic skull, an orbit positioned high on the skull, a reduced preorbital fossa and a wide, massive zygomatic arch ( Wall 1989).

Within Metamynodontini , UNISTRA.2015.0.1106 differs from Sellamynodon in having: a high zygomatic arch; a flat dorsal profile of the skull; the presence of a sagittal crest; a circular foramen magnum. UNISTRA.2015.0.1106 differs from Megalamynodon in having a well-developed paroccipital process. UNISTRA.2015.0.1106 differs from Paramynodon in having a protoloph on P2, an upper postcanine diastema shorter than the length of the upper premolars and the orbit positioned higher on the skull. UNISTRA.2015.0.1106 can be referred to Metamynodon by the presence of large and tusk-like canines, the premaxilla and the nasal reduced but still contact each other along border of external nares, a very large and massive zygomatic arch ( Wall 1989).

Three species of Metamynodon are considered as valid: Metamynodon chadronensis , Metamynodon mckinneyi and Metamynodon planifrons . Metamynodon chadronensis is known from fragmented skulls, mandibles, and isolated teeth ( Wood 1937; Wilson & Schiebout 1981). Metamynodon mckinneyi is only known from a mandible while our new material UNISTRA.2015.0.1106 to identify is a skull, so no direct comparison was possible. The holotype of M. chadronensis is also a lower jaw, but some incomplete skulls have been attributed to this species by Wilson & Schiebout (1981). These identifications are mostly based on dental measurements of the lower jaw, which are smaller than M. planifrons . However, the dimensions of the upper teeth do not really reflect this smaller dimension and sometimes fit within the observed range of measurements of M. planifrons (see Wilson & Schiebout 1981: table 14). Thus, in the absence of a lower jaw and of a clearer distinction between M. chadronensis and M. planifrons , we refer our specimen to M. planifrons , the type species of the genus, as primarily identified by Wood, in Gillet (1960). Furthermore, in our phylogenetic analysis UNISTRA.2015.0.1106 also shares with M. planifrons “a low anterior base of the zygomatic process of maxilla”; “an acute vomer”; and “a weak paracone fold on M3”.