Here we present the first detailed morphological study of the auditory region of a tremarctine bear, the South American giant short-faced bear Arctotherium angustidens. We compared 19 specimens of A. angustidens with other tremarctines and ursines. Through the use of CT scans, we confirmed the presence of a recesus epitympanicus and an anterior incomplete septum of uncertain homology, not related with the septum bullae nor with the longitudinal septum formed by the ecto- and the entotympanic. A secondary crus formed by the lateral semicircular canal (LSC) with the posterior semicircular canal (PSC) of the inner ear was observed in A. angustidens, A. bonariense, Tremarctos ornatus, Ursus spelaeus, and Ursus arctos. This secondary crus was not previously reported for ursids. We also observed that the intraspecific variation in the auditory region of A. angustidens is related to 1) the position of the foramen postglenoideum, 2) the anterior projection of the bulla tympanica over the foramen lacerum and over the opening of the Eustachian tube (medial process of the bulla tympanica), and 3) the projection of the bulla tympanica over the posterior surface of the processus postglenoideus (tympanic process). In addition to this variation, we also identified the presence of interspecific variation in the external auditory region among Tremarctinae and Ursidae. These differences are related to the size of the processus mastoideus and the processus paraoccipitalis, the position of the foramen postglenoideum, and the presence/absence of contact between the bulla tympanica and the processus paraoccipitalis.

The Great American Biotic Interchange is considered to be a punctuated process, primarily occurring during four major pulses that began approximately 2.5 Ma. Central America and southeastern Mexico have a poor fossil record of this dynamic faunal history due to tropical climates. Exploration of submerged caves in the Yucatán, particularly the natural trap Hoyo Negro, is exposing a rich and remarkably well-preserved late Pleistocene fauna. Radiometric dates on megafauna range from approximately 38 400-12 850 cal BP, and extinct species include the ursid Arctotherium wingei and canid Protocyon troglodytes. Both genera were previously thought to be indigenous to and confined to South America and appear to represent an instance of large placental mammals, descended from North American progenitors, migrating back north across the Panama Isthmus. This discovery expands the distribution of these carnivorans greater than 2000 km outside South America. Their presence along with a diverse sloth assemblage suggests a more complex history of these organisms in Middle America. We suggest that landscape and ecological changes caused by latest Pleistocene glaciation supported an interchange pulse that included A. wingei, P. troglodytes and Homo sapiens.

Here we present the most detailed morphological study of the auditory region of a tremarctinae bear, Arctotherium tarijense Ameghino. In addition, we provide new anatomical information of the Tremarctinae inner ear, such as coplanarity and deviation from orthogonality of the semicircular canals, as an approach to infer the head movements which encountered the extinct forms in locomotion. Based on morphological comparisons, A. tarijense exhibits the following particular features: the cavum tympani presents the highest relative volume compared with other ursids; the processus paraoccipitalis has a foramen that is absent in other tremarctines; there is only one (ventral) recess in the anterior region of the cavum tympani; and the recessus epytimpanicus is the smallest for all ursids studied. In relation to the inner ear, A. tarijense shows the lowest values of orthogonality deviation and highest scores of locomotor agility. Based on this, is possible to make a preliminary proposal that this species had a relative high vestibular sensibility and therefore a better ability to explore different kind of habitats. However, this hypothesis might be contrasted among bears taking into account the orientation of each semicircular canal in a phylogenetic framework.