CONICET | Buscador de Institutos y Recursos Humanos

Hypsodonty represents a keyfeature in the evolutionary history of notoungulates that was early attainedand 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 was previouslyregarded as Hegetotheria or the Typotheria-Hegetotheria lineage. Eocene ?archaeohyracids?are among the first notoungulates to develop hypsodont cheek teeth, and the sixrecognized genera show different phylogenetic positions in the stem leading to mesotheriidsand hegetotheriids. ?Archaeohyracids? are recorded from the Barrancan (middleEocene) to the Deseadan (middle-late Oligocene), mainly in several localitiesof Argentinean Patagonia, central Chile, and Bolivia. Until recently, thesenotoungulates were unknown in northwestern Argentina, but in the last decade,the genus Punahyrax was described for the middle member of the Geste Formation(Catamarca and Salta provinces). Later, several other specimens were reportedfor this unit, including a mandible referred to Pseudhyrax eutrachytheroides.Here we re-evaluate these records and present new remains recovered from levelsof the Quebrada de Los Colorados Formation in Salta Province, which are latemiddle Eocene in age. The new materials include a partial mandible (PVL 4362),an almost complete mandible (IBIGEO-P 86), and cranial remains of a small form(IBIGEO-P 87). The restudy of the specimens of both units allows us torecognize at least five morphotypes: besides Punahyrax bondesioi and Pseudhyraxeutrachytheroides, a new morphotype referable to a small form of Pseudhyrax (significantlysmaller than P. strangulatus) was observed in the sample of Geste Formation. Inturn, the specimen IBIGEO-P 86 represents another gracile form of Pseudhyraxwith dental measurements comparable to P. eutrachytheroides but with a moreslender mandible, and IBIGEO-P 87 represents a new taxon, smaller thanProtarchaeohyrax and showing a particular combination of features (e.g.,vestigial parastyle, large talonid-trigonid fossettid, proportionally smallertrigonid). 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 be made.Several contributions of the last two decades have dealt with the record of ?archaeohyracids?.In this context, the Tinguirirican marks an important interval in the historyof these hypsodont typotherians, as this age marks the point of máximum diversityfor these forms and the first appearance of mesotheriids and hegetotheriids (thepeak of the radiation of ?rodent-like? notoungulates). Given that by theTinguirirican the main branches of this lineage were already present, it isclear that its diversification should have predated the lower Oligocene, anidea previously stated by some authors but not reflected in the fossil record,as only a limited number of ?archaeohyracids? was known for Eocene times. Ourfirst approach to the northwestern record brings to light a modest yet remarkablediversity of ?archaeohyracids? that will likely increase, given the extensivesampling efforts currently underway. Hypsodont notoungulates are unknown in thehighly fossiliferous and geographically and temporally close lower and upper Lumbreraformations. At least in part, some contributions state a temporal correlation betweenthe lower levels of the Quebrada de los Colorados Formation and upper Lumbrera(referred to the late middle Eocene), while the fossil-bearing levels of theGeste Formation have been extensively referred to the younger Mustersan age(late Eocene). The common presence of Pseudhyrax (a typical Mustersan genus) inthe Quebrada de los Colorados and Geste formations complicates this scenario,making taxonomic and biostratigraphic assessments much needed. Additionally,these records underscore the biogeographic peculiarities of the Eocene recordof early members of the typotherian ?rodent-like? lineage in the region, notcompletely understood nowadays.