The auditory region of the middle Eocene Litopterna Indalecia grandensis Bond & Vucetich, 1983: anatomical and phylogenetic approach

GARCÍA LÓPEZ D.A; BABOT M.J.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

International Palaeontological Association

The auditory region of Paleogene South American ungulates is poorly known, although efforts have been made recently in order to increase its knowledge. Most contributions have focused on notoungulates, given that this group is the most common and often best preserved in Paleogene levels.Nevertheless, limited information exists for Litopterna, Pyrotheria, and Astrapotheria. Indalecia grandensis is a small-sized litoptern recovered from the Lumbrera Formation in Salta Province, Argentina. Although some traits of the auditory region were mentioned in the original description of this taxon, this part of the skull has remained unstudied. Considering the increasing value of auditory traits in mammalian systematics and the fact that this is a well-preserved Eocene specimen,we present a detailed basicranial description and interpret the relevance of these characters in the context of South American ungulate phylogenies. Most comparative sources are representatives of Notoungulata, from which Indalecia diverges by the lack of a completely ossified bulla. Instead, an expanded ectotympanic covers the lateral part of the tympanic aspect of the petrosal and forms the floor of a short external auditory meatus. Another difference regarding known early-diverging notoungulates is the presence of a postpromontorial tympanic sinus that is well-separated from the stapedial fossa by a marked step and a poorly demarcated promontorium. On the other hand, several common traits are noted: presence of an expanded medial flange, large fossa for the tensor tympani muscle located laterally to the secondary facial foramen, and large tegmen tympani. The study of Indalecia also provides information about the pathway of some vessels, such as the internal carotid, the superior ramus of the stapedial artery, the arteria diploetica magna, and the occipital artery. The most important traits include evidence of a medial trajectory of the internal carotid and a passage between the squamosal and the tegmen tympani for the superior ramus of the stapedial artery. This latter character is interesting because it is found in early-diverging mammals, such as the non-placental eutherian Zalambdalestes. The inclusion of these and other craniodental features in a large data-matrix comprising notoungulates, astrapotheres, pyrotheres, litopterns, and basal mammalian taxa placed Indalecia as the sister-taxon of astrapotheres and notoungulates, not closely related to other Litopterna. Although basicranial characters are not among the synapomorphies that define the relationships of Indalecia, this result is concordant with the general similarities observed with at least Notoungulata, particularly for dental and petrosal features.