Pachyrukhinae (Mammalia, Notoungulata, Hegetotheriidae) from the early?middle Miocene of Patagonia, Argentina

LÓPEZ, GUILLERMO M.; BOND, MARIANO; PÉREZ, MARÍA ENCARNACIÓN

Villa de Leyva

Congreso; VI Congreso Latinoamericano de Paleontología de Vertebrados; 2018

Centro de Investigaciones Paleontológicas

The Pachyrukhinae are small to medium-sized notoungulates that are easily recognizedby their specialized anterior ??gliriform? dentition and ever-growing teeth. They are one ofthe most specialized groups of Notoungulata, not only in cranial and dental anatomy, butalso postcranially. They have been recorded in the southern half of South America(Argentina, Uruguay, Chile, and Bolivia). Presently, they are not known from the morenorthern tropical areas of South America, which could be related to specific adaptiverequirements of this group, perhaps a preference for more open areas than those probablypresent in northern of the continent. They are extremely abundant in the early Miocenebeds of the Santacrucian South American Land Mammal Age (SALMA) and in theChasicoan to Marplatan SALMAs (late Miocene to latest early Pleistocene). Up to now,the Pachyrukhinae recorded in the interval between the Santacrucian and ChasicoanSALMAs have been based on fragmentary remains and have not been reviewed since theiroriginal descriptions. Here we present a series of abundant and in some cases verycomplete remains coming from the locality ?El Petiso? in northwestern of ChubutProvince, central Patagonia, Argentina in sediments that bear a very diverse vertebrateassemblage. The mammals identified at this locality indicate a middle Miocene age (post-Colloncuran SALMA?), closer to the Laventan SALMA (middle Miocene) representingthe Serravallian Stage (13.82 to 11.63 Ma) and older than the Mayoan SALMA (latestmiddle Miocene). The materials collected at ?El Petiso? mentioned here are housed at theMuseo Paleontológico ?Egidio Feruglio? (MPEF) of Trelew, Chubut Province. In thislocality were recovered skulls, some of them very complete, numerous maxillary andmandibular fragments, as well as isolated teeth. The absence of P1/1 (and consequentpresence of a diastema), the hypertrophied incisors, and the presence of ever-growingupper molars that are curved lingually and labially convex with well-marked and distinctparacone and metacone allows these materials to be referred to the Pachyrukhinae,provisionally to the genus Pachyrukhos. On the basis of the present material, twodifferent taxa can be recognized. The first is 20% smaller and characterized by m1-2 witha straight lingual face and trigonids smaller than talonids. The other one, the larger form,has first lower molars with trigonids and talonids of similar size with an obliqueorientation of the lingual face of the trigonid in the anterolabial-posterolingual direction.Based on preliminary study, the smaller form is referred tentatively to the speciesPachyrukhos medianus, described from the Colloncuran beds in Neuquén Province. Thelarger species is not referred to any other described species and could represent a new taxon. As mentioned previously, Pachyrukhinae are well known in older Santacrucian(late early Miocene) faunal assemblages and in the younger Chasicoan (early lateMiocene). In this regard, the findings from ?El Petiso?, which records an intermediate age,are quite relevant because they will allow a better understanding of the evolutionaryhistory of this clade.