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

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

ANNALS OF CARNEGIE MUSEUM

CARNEGIE MUSEUM NATURAL HISTORY

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

0097-4463

ABSTRACTEoastrapostylops riolorense Soria and Powell, 1981, is a primitive meridiungulate mammal known by two specimens from the early PaleogeneRío Loro Formation, in NW Argentina. The holotype and most complete specimen is an almost complete skull, mandible, and a few associatedpostcranial elements. Eoastrapostylops is one of the oldest South American ungulates with a well-preserved skull; although its original descriptionincluded some cranial characters, its attribution to the endemic South American Order Astrapotheria and the concomitant phylogeneticconsequences were based on dental features. New preparation and examination of the holotype (PVL 4216) revealed features not mentioned and/or incorrectly interpreted in previous studies; particularly important are those of the auditory region. The cranial anatomy is here re-described andcompared with that of Astrapotherium Burmeister, 1879, providing the first detailed description of a basal meridiungulate auditory region. Thegeneral structure of the preserved elements of E. riolorense resembles more closely that of archaic ?ungulates? (i.e., condylarths) and litopterns thanthat of Astrapotherium or other astrapotheres with known skulls (e.g., Trigonostylops Ameghino, 1897, Astraponotus Ameghino, 1901); the moreremarkable differences are: presence of distinct apertures (foramen ovale) for passage of the mandibular ramus of the trigeminal nerve, surfaceson the alisphenoid and squamosal for attachment of the ectotympanic bone, low and crest-like postglenoid process, postglenoid foramen medialto the postglenoid process and not piercing its base, almond-shaped promontorium with a strong caudal tympanic process almost obliterating thepost-promontorial tympanic sinus, and the mastoid process exposed laterally and posteriorly. We performed three sets of cladistic analyses basedon previously published matrices, including dental, cranial, and postcranial features scored in a wide sample of South American ungulates and?archaic? ungulates. The results suggest that Eoastrapostylops represents a basal meridiungulate lineage that diverged before the differentiationamong astrapotheres, pyrotheres, and notoungulates, and thus it can be classified neither within Astrapotheria nor another clade of ordinal rank.Other groups of still uncertain status (e.g., Notopterna, Indalecidae) also would represent independent basal radiations, which would have characterizedthe early meridiungulate evolution, although this topic needs a more exhaustive exploration.