Neochorlakkia myaingensis, Ducrocq & Soe & Sein & Chaimanee & Chavasseau & Jaeger, 2022

Ducrocq, Stéphane, Soe, Aung Naing, Sein, Chit, Chaimanee, Yaowalak, Chavasseau, Olivier & Jaeger, Jean-Jacques, 2022, Neochorlakkia myaingensis n. gen., n. sp., a new Dichobunidae (Mammalia, Cetartiodactyla) from the middle Eocene Pondaung Formation, Myanmar, Comptes Rendus Palevol 21 (4), pp. 115-122 : 116-120

Neochorlakkia myaingensis n. gen., n. sp.

( Fig. 2 A-E)

urn:lsid:zoobank.org:act:D2E102A7-A89C-4A01-B6B6-7E27813C9B22

HOLOTYPE. — MFP-PK2-2019-003 , isolated right m3, deposited in the Pondaung Collections at the Myanmar Ministry of Culture, Nay Pyi Taw, Myanmar.

ETYMOLOGY. — Genus name derives from the Greek prefix neo, meaning more recent, and Chorlakkia Gingerich, Russell, Sigogneau-Russell & Hartenberger 1979 in reference of the strong morphological similarity of the lower molar from Pondaung with those of Chorlakkia hassani Gingerich, Russell, Sigogneau-Russell & Hartenberger, 1979 from the early middle Eocene of Pakistan. Species name refers to the Myaing township where the specimens were found.

REFERRED MATERIAL. — NMMP-KU 2000 ( Fig. 2D), fragmentary right m3 ( Tsubamoto et al. 2013: figs 2C, 5A).

LOCALITY AND HORIZON. — Paukkaung Kyitchaung 2 locality, Bahin area, Pondaung Formation, Myanmar; late middle Eocene.

DIAGNOSIS. — Middle-sized ungulate characterized by its very bunodont and short m3, lacking a paraconid and most of cristids. Differs from most of the primitive artiodactyls ( Diacodexeidae , Homacodontidae , European Dichobunidae , Raoellidae ) by its strongly bunodont m3 with poorly expressed crests, with a short trigonid lacking a paraconid, absence of a hypolophid, with a mesiodistal cristid obliqua and a reduced hypoconulid lobe. Differs from the Asian dichobunid Chorlakkia by its larger size, its hypoconid taller than the entoconid and hypoconulid, its entoconid in line with the hypoconid and by its hypoconulid lobe consisting in two small cusps.

DESCRIPTION

The tooth (MFP-PK2-2019-003) is a right m3 (L = 14.8 mm; W = 9.2 mm) without any contact facet on its distal wall. The lingual and buccal roots are fused and there is a fifth tiny root under the most posterior cusp that runs along the distal roots. The molar is bunodont with four main cusps and weakly expressed crests. The trigonid is somewhat taller than the talonid. The preprotocristid and premetacristid form a low and continuous blunt crest that mesially closes the trigonid. There is no other crest in the trigonid part and only a slight furrow separates both bulbous mesial cusps. The distal wall of the trigonid is flat and mesially slanted. The lingual part of the talonid basin seems to be lined by a very slight and low postmetacristid that connects with the preentocristid, and its buccal part is edged by the mesially directed cristid obliqua (prehypocristid) that ends low on the distal wall of the protoconid. It is not possible to distinguish whether the lingual end of the transverse valley was open because this area is cracked, but the occurrence of a low postmetacristid and preentocristid suggests that they likely constitute a weak lingual edge. There is no distinct crest (hypolophid) connecting the hypoconid and the smaller and lower entoconid, but a narrow mesiodistally oriented groove separates both cusps and curves buccally between the hypoconulid and the hypoconid. The short and slightly distally protruding hypoconulid lobe is composed of two small cusps separated by a very slight furrow, and a shallow slit can be observed between the mesial hypoconulid cusp and the entoconid, suggesting that both cusps might have been twinned. The hypoconulid is not connected to the hypoconid. A narrow cingulid is present along the mesial face of the molar and a short one can be observed at the buccal end of the transverse valley between the protoconid and the hypoconid ( Fig. 2 A-C, E).

COMPARISONS

Among basal cetartiodactyls, the Diacodexeidae can be distinguished from Neochorlakkia n. gen. by their much less bunodont lower molars with better expressed crests that often exhibit a paraconid, a talonid basin usually lingually open with a more oblique cristid obliqua, a m3 hypoconulid better developed and connected to the hypoconid and the entoconid. The Asian Homacodontidae Limeryx Métais, Qi, Guo & Beard, 2005 , Asiohomacodon Tsubamoto, Tun, Egi, Takai, Shigehara, Soe, Aung & Thein, 2003 and Tsaganohyus Kondrashov, Lopatin & Lucas, 2004 are also more selenodont with a paraconid ( Limeryx and Tsaganohyus ), a hypolophid and a Zhailimeryx fold on the mesial face of the entoconid ( Limeryx ; see Guo et al. 2000), and their cristid obliqua is more obliquely oriented ( Fig. 3C, D). The Asian Dichobunidae are mostly represented by fragmentary material that complicates their precise systematic affinities. For example, the Lantianiinae for which lower molars are known ( Elaschitotherium Métais, Guo & Beard, 2004 and Eolantianus Averianov, 1996 ) display less bunodont crowns, with a more obliquely oriented cristid obliqua, a distinct paraconid ( Elaschitotherium ), a hypolophid and a stronger hypoconulid lobe ( Fig. 3E, F). Haqueina Dehm & Oettingen-Spielberg, 1958 from the middle Eocene of Pakistan is slightly smaller than Neochorlakkia n. gen., but it is also less bunodont with better expressed crests (postcristids on mesial cusps), more oblique cristid obliqua, with a discrete hypolophid connecting the hypoconid and the entoconid, and a narrower and more distally protruding hypoconulid ( Fig. 3G). Pakibune Thewissen, Gingerich & Russell, 1987 from the early-middle Eocene of Pakistan is known only from an isolated m3 that is much smaller and more elongated than the Pondaung molar, with less bunodont cusps, a paraconid, a more oblique cristid obliqua, an entoconid distinctly more distal than the hypoconid, and better developed hypoconulid and buccal cingulid ( Fig. 3H).

Although Wutuhyus Tong & Wang, 2006 from the early Eocene of eastern China shares bunodont cusps with the Pondaung form, it is also much smaller, with an enlarged trigonid that exhibits a prominent paraconid and a metaconid posterior to the protoconid, and an unreduced hypoconulid with two cusps. Tsubamoto et al. (2013) noted that the general morphology of the partial m3 NMMP-KU 2000 ( Fig. 2D) somewhat reminds that of some raoellid forms: its strong bunodonty, blunt crests, absence of paraconid, reduced hypoconulid, mesially oriented cristid obliqua and well developed talonid basin (“crushing basin”) are features that can also be observed in raoellids. These are features that can also be observed in Neochorlakkia n. gen. ( Figs 2A, E; 3A). However, the lack of crests that enclose the central basin that are characteristic of raoellids ( Orliac & Ducrocq 2012), especially the hypolophid that borders the basin distally and the almost absent postcristids of the mesial cusps on both m3 points to different affinities ( Fig. 3I).

Interestingly enough, although the Dichobunidae Chorlakkia hassani from the early-middle Eocene of Pakistan is much smaller than the Pondaung specimen, the occlusal outline and morphology of the m3 of both taxa are strikingly similar.Indeed, the bunodont protoconid and metaconid not connected distally by crests, the lack of a paraconid, the deep talonid basin, the almost mesiodistally oriented cristid obliqua, the entoconid lingually protruding, and the reduced hypoconulid lobe are features shared by the Pakistani and Pondaung forms ( Fig. 2E, F). However, MFP- PK2-2019-003 exhibits a hypoconid slightly taller than the entoconid-hypoconulid complex, an entoconid in line with the hypoconid, an hypoconulid consisting of two very small cusps and smaller than the entoconid ( Fig. 3A, B). Vislobokova (2004) identified a new species of Chorlakkia ( C. valerii Vislobokova, 2004 ) from the middle Eocene of Mongolia. This taxon displays several differences with C. hassani such as a lower molar crown more elongated, a trigonid much higher than the talonid, a hypoconulid lobe single cusped and much more developed distally, an entoconid more lingually protruding and in line with the hypoconid. It also differs from Neochorlakkia n. gen. by its much smaller size, more elongated crown with the trigonid much taller than the talonid, more oblique cristid obliqua, entoconid lower than hypoconid, presence of a posthypocristid, single-cusped hypoconulid separated from the entoconid by a depression, more elongated hypoconulid lobe, and weaker mesial cingulid. Nevertheless, the structural features that can be observed in the ungulate from Mongolia but not in Chorlakkia cast doubts on the generic attribution of C. valerii .