RE-DESCRIPTION OF THE AUDITORY REGION OF THE PUTATIVE BASAL ASTRAPOTHERE (MAMMALIA) EOASTRAPOSTYLOPS RIOLORENSE SORIA AND POWELL, 1981. SYSTEMATIC AND PHYLOGENETIC CONSIDERATIONS

KRAMARZ, A.; BOND, M.; ROUGIER, G.

ANNALS OF CARNEGIE MUSEUM

CARNEGIE MUSEUM NATURAL HISTORY

Lugar: Pittsburgh, PA; Año: 2017 vol. 84 p. 99 - 168

0097-4463

Eoastrapostylops riolorense is a primitive meridiungulate mammal known by two specimens from the earlyPaleogene Río Loro Formation, in NW Argentina. The holotype and most completespecimen is an almost complete skull, mandible, and a few associated postcranialelements. Eoastrapostylops is one of theoldest South American ungulates with a well-preserved skull; although itsoriginal description included some cranial characters, its attribution to the endemicSouth American Order Astrapotheria and the concomitant phylogenetic consequenceswere based on dental features. New preparation and examination of the holotype(PVL 4216) revealed features not mentioned and/or incorrectly interpreted in previousstudies; particularly important are those of the auditory region. The cranialanatomy is here re-described and compared with that of Astrapotherium, providing the first detailed description of a basalmeridiungulate auditory region. The general structure of the preserved elementsof Eoastrapostylops riolorense resemblesmore closely that of archaic ?ungulates? (i.e., condylarths) and litopternsthan that of Astrapotherium or otherastrapotheres with known skulls (e.g., Trigonostylops,Astraponotus); the more remarkabledifferences are: presence of distinct apertures (foramen ovale) for passage ofthe mandibular ramus of the trigeminal nerve, surfaces on the alisphenoid andsquamosal for attachment of the tympanic bone, low and crest-like postglenoidprocess, postglenoid foramen medial to the postglenoid process and not piercingits base, almond-shaped promontorium with a strong caudal tympanic process almostobliterating the post-promontorial tympanic sinus, and the mastoid processexposed laterally and posteriorly. We performed three sets of cladistic analysesbased on previously published matrices, including dental, cranial, andpostcranial features scored in a wide sample of South American ungulates and ?archaic?ungulates. The results suggest that Eoastrapostylopsrepresents a basal meridiungulate lineage that diverged before thedifferentiation among astrapotheres, pyrotheres, and notoungulates, and thus itcan be classified neither within Astrapotheria nor another clade of ordinalrank. Other groups of still uncertain status (e.g., Notopterna, Indalecidae) alsowould represent independent basal radiations, which would have characterizedthe early meridiungulate evolution, although this topic needs a more exhaustiveexploration.