A new bunodont South American native ungulate (Mammalia: Didolodontidae) from the Eocene of Lumbrera Formation, Argentina

GELFO J. N.; MADDEN, R.; ALONSO, RICARDO N.; CARLINI, A. A.

Congreso; VI Congreso Latinoamericano de Paleontología de Vertebrados; 2018

A new bunodont South American native ungulate assigned to Didolodontidae wasrecovered from Eocene sediments of the Lumbrera Formation (Santa Bárbara Subgroup)near the San Antonio de Los Cobres locality at Salta Province, Argentina. The specimen(MLP 98-XI-1-1) comes from the base of calcareous - fluvial beds where coarser gravelsoccurs, in a profile of pinkish-brown sediments. It is represented by a small left dentarybone with a canine alveolus and a brachydont dentition including p1-2, the roots of p3, and p4-m2. The jaw is broken mesial to the canine alveolus, which is circular and mesiallyprojected. There is only one root in p1 and p2. A single- rooted first premolar is wellknown for several archaic ungulate groups, particularly Kollpaniinae and Didolodontidae,but in contrast, their p2 is two-rooted. The p1 is transversally compressed, with a centralcusp that forms the higher part of a mesio-distally blade-like cristid. The p4 is rectangularin outline and molarized as in didolodontids, with the trigonid somewhat higher than thetalonid. The m1-2 are similar in morphology and cusp distribution; differences are due tothe greater wear of the m1, with a trigonid narrower than the talonid, while thesestructures are almost of the same width in m2. In m1-2, the trigonid is somewhat higherthan the talonid and has a short precingulid; the protoconid is the highest cusp, and astrong paracristid that is distally concave projects up to the paraconid. The metaconid issomewhat more distally placed than the protoconid and connected to the paraconid by ashort metacristid (deeply abraded by wear in m1). A v-shaped protocristid joins theprotoconid and the metaconid as part of the almost straight distal wall of the trigonid,which is not bending through the talonid. The trigonid basins are wide compared to otherPaleogene bunodont ungulates, but less so than the talonids. The hypoconid is theprincipal and most mesial cusp of the talonid, and a straight cristid obliqua projects up tothe base of the trigonid and ends below the vertex of the v-shaped protocristid. Thehypoconulid is the most distally placed cusp and is almost the same size as the entoconid.The three talonid cusps are connected by strong cristids. The specimen has somesimilarities with North American archaic ungulates (e.g. Periptychidae Anisonchinae) butmost resembles South American bunodont ungulates. In contrast to Kollpaniinae, thespecimen presents: more separate, stylized, and higher cusps rather than low cusps withbulbous bases; well-developed cristids; the talonid basin is wide and neither the base of thehypoconid nor the projection of the distal wall of the metaconid invade it. The specimenshares with Didolodontidae: more molarized p4 with a clear difference between trigonidand talonid; molars with a talonid equal to or even wider than the trigonid; and wellseparated hypoconid, hypoconulid, and entoconid. The specimen is preliminarily assigned to Didolodontidae and represents a new small (< 5 kg) morphotype among the bunodont ungulates known from lower latitudes of South America.