A RECENTLY DISCOVERED PRE-OLIGOCENE DIVERSITY OF HYPSODONT TYPOTHERIANS (MAMMALIA, NOTOUNGULATA) IN NORTHWESTERN ARGENTINA

DANIEL GARCÍA-LÓPEZ; JUDITH BABOT; AGUSTÍN SCANFERLA

Villa de Leyva

Congreso; VI Congreso Latinoamericano de Paleontología de Vertebrados; 2018

Hypsodonty represents a key feature in the evolutionary history of notoungulates thatwas early attained and evolved in different lineages within the order, such as toxodontids,interatheriids, and “rodent-like” typotherians. The last group includes the polyphyletic“Archaeohyracidae” and the specialized Mesotheriidae and Hegetotheriidae and waspreviously regarded as Hegetotheria or the Typotheria-Hegetotheria lineage. Eocene“archaeohyracids” are among the first notoungulates to develop hypsodont cheek teeth,and the six recognized genera show different phylogenetic positions in the stem leading tomesotheriids and hegetotheriids. “Archaeohyracids” are recorded from the Barrancan(middle Eocene) to the Deseadan (middle-late Oligocene), mainly in several localities ofArgentinean Patagonia, central Chile, and Bolivia. Until recently, these notoungulateswere unknown in northwestern Argentina, but in the last decade, the genus Punahyraxwas described for the middle member of the Geste Formation (Catamarca and Saltaprovinces). Later, several other specimens were reported for this unit, including amandible referred to Pseudhyrax eutrachytheroides. Here we re-evaluate these records andpresent new remains recovered from levels of the Quebrada de Los Colorados Formationin Salta Province, which are late middle Eocene in age. The new materials include apartial mandible (PVL 4362), an almost complete mandible (IBIGEO-P 86), and cranialremains of a small form (IBIGEO-P 87). The restudy of the specimens of both units allows us to recognize at least five morphotypes: besides Punahyrax bondesioi andPseudhyrax eutrachytheroides, a new morphotype referable to a small form of Pseudhyrax(significantly smaller than P. strangulatus) was observed in the sample of Geste Formation.In turn, the specimen IBIGEO-P 86 represents another gracile form of Pseudhyrax withdental measurements comparable to P. eutrachytheroides but with a more slender mandible,and IBIGEO-P 87 represents a new taxon, smaller than Protarchaeohyrax and showing aparticular combination of features (e.g., vestigial parastyle, large talonid-trigonidfossettid, proportionally smaller trigonid). As for PVL 4362, this is a robust individual,larger than P. eutrachytheroides and with persistent fossettids on its premolars.Unfortunately, this is a very fragmentary specimen and no further observation can bemade. Several contributions of the last two decades have dealt with the record of“archaeohyracids”. In this context, the Tinguirirican marks an important interval in thehistory of these hypsodont typotherians, as this age marks the point of maximumdiversity for these forms and the first appearance of mesotheriids and hegetotheriids (thepeak of the radiation of “rodent-like” notoungulates). Given that by the Tinguirirican themain branches of this lineage were already present, it is clear that its diversificationshould have predated the lower Oligocene, an idea previously stated by some authors butnot reflected in the fossil record, as only a limited number of “archaeohyracids” wasknown for Eocene times. Our first approach to the northwestern record brings to light amodest yet remarkable diversity of “archaeohyracids” that will likely increase, given theextensive sampling efforts currently underway. Hypsodont notoungulates are unknown inthe highly fossiliferous and geographically and temporally close lower and upperLumbrera formations. At least in part, some contributions state a temporal correlationbetween the lower levels of the Quebrada de los Colorados Formation and upperLumbrera (referred to the late middle Eocene), while the fossil-bearing levels of the GesteFormation have been extensively referred to the younger Mustersan age (late Eocene).The common presence of Pseudhyrax (a typical Mustersan genus) in the Quebrada de losColorados and Geste formations complicates this scenario, making taxonomic andbiostratigraphic assessments much needed. Additionally, these records underscore thebiogeographic peculiarities of the Eocene record of early members of the typotherian“rodent-like” lineage in the region, not completely understood nowadays.