First Description of the Auditory Region of a Tremarctinae (Ursidae, Mammalia) Bear: The Case of Arctotherium angustidens

M E ARNAUDO; L H SOIBELZON; P BONA; B SCHUBERT

JOURNAL OF MAMMALIAN EVOLUTION

SPRINGER

Lugar: Berlin; Año: 2013 p. 1 - 10

1064-7554

Here we present the first detailed morphologicalstudy of the auditory region of a tremarctine bear, the SouthAmerican giant short-faced bear Arctotherium angustidens.We compared 19 specimens of A. angustidens with othertremarctines and ursines. Through the use of CT scans, weconfirmed the presence of a recesus epitympanicus and ananterior incomplete septum of uncertain homology, not relatedwith the septum bullae nor with the longitudinal septumformed by the ecto- and the entotympanic. A secondary crusformed by the lateral semicircular canal (LSC) with theposterior semicircular canal (PSC) of the inner ear wasobserved in A. angustidens, A. bonariense, Tremarctosornatus, Ursus spelaeus, and Ursus arctos. This secondarycrus was not previously reported for ursids.We also observedthat the intraspecific variation in the auditory region of A.angustidens is related to 1) the position of the foramenpostglenoideum, 2) the anterior projection of the bullatympanica over the foramen lacerum and over the openingof the Eustachian tube (medial process of the bullatympanica), and 3) the projection of the bulla tympanica overthe posterior surface of the processus postglenoideus(tympanic process). In addition to this variation, we alsoidentified the presence of interspecific variation in the externalauditory region among Tremarctinae and Ursidae. Thesedifferences are related to the size of the processus mastoideusand the processus paraoccipitalis, the position of the foramenpostglenoideum, and the presence/absence of contact betweenthe bulla tympanica and the processus paraoccipitalis.