Cranial osteology of the late Oligocene Notohippidae Rhynchippus equinus Ameghino (Mammalia, Notoungulata) based on the most complete skull known for the species

MARTÍNEZ, GASTÓN; DOZO, MARÍA TERESA

Mendoza

Congreso; 4th International Paleontological Congress. Symposium 10: Evolution of South American endemic ungulates; 2014

IPA-CCT-Mendoza

Notohippidae is a family of medium-large notoungulates with complete dentition and early tendency to hypsodonty. They have been recorded from the Eocene to the early Miocene, being particularly diverse during the Deseadan SALMA (late Oligocene). Although Rhynchippus equinus is one of the most frequent Notohippidae in the fossil record, there are scarce data about cranial osteology other than the classical descriptions which date back to the early last century. In this context, the exceptionally preserved specimen MPEF-PV 695 (collected in outcrops of the Sarmiento Formation in Cabeza Blanca, Southeast of Chubut, Argentina) allowed us an exhaustive morphological review of the species facilitated by CT-scanning techniques. The specimen was imaged in its entirety and 3D reconstruction methods were carried out. Regarding general morphology, its narrow and elongated nasals and the moderate lateral expansion of the zygomatic arches resembles the slender rostral morphology of Mendozahippus fierensis and differs from that observed in Eurygenium pacegmun, Eurygenium latirostris and Pascualihippus boliviensis. In the basicraneal region, it is worth noting some relevant morphological data. At the level of the sphenoidal complex, a fossa interpreted as the origin of the tensor veli palatini muscle and the pterygoideus medialis is observed between the entopterygoid crest (pterygoid) and ectopterygoid crest (palatine and alisphenoid). The morphofunctional and osteological interpretation of this fossa represents an unexplored source of anatomical information among notoungulates. Internally, high resolution x-ray computed tomographic analysis revealed endocranial traits not previously observed such as the intracranial apertures of meatuses and foramina and their associated soft tissue. Regarding the ear region, petrosal morphology of MPEF-PV 695 is largely congruent with that described for the suborder Toxodontia (at least on the cerebellar aspect). However, some differences can be recognized when compared to Adinotherium ovinum, such as a wider but less pronounced subarcuate fossa (which houses the paraflocular lobe of the cerebellum) and a sharper crest between the subarcuate fossa and the internal auditory meatus. This crest is continuous with the strongly developed petrosal crest. In the occipital region, the petrosal is exposed as a strip of bone associated to the mastoid foramen. This description provides new anatomical data that extend the diagnosis of the species and should prove to be phylogenetically informative.