Anatomy, systematics, and funtional inferences of tarsal remains from Middle-Late Eocene of Northwest Argentina

ARMELLA MATÍAS; GARCÍA LÓPEZ D.A; BABOT M.J.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

International Palaeontological Association

In recent years, Paleogene outcrops in northwestern Argentina have provided abundant information about the evolution of South American mammals during the early Cenozoic. This region constituted an important area of early diversification for several groups of native ungulates during the Eocene, especially notoungulates. Nowadays, most works derived from these findings are focused on cranial and dental anatomy, although in many cases the specimens were found in association with postcranial elements. These elements can provide crucial information for phylogenetic analyses and supply important clues about the paleobiology of taxa that inhabited the region. This study presents a series of postcranial remains (proximal tarsals) from the Geste Formation that crops out at Antofagasta de la Sierra, Catamarca Province, Argentina. Three calcanei and two astragali were described, compared with extinct and extant ungulates, and systematically determined. Moreover, anatomical features were interpreted in a functional context. One calcaneus studied was assigned to Isotemnidae (Notoungulata), based on the oblique orientation of the line that runs along the distal portion of the sustentaculum, a markedly wide groove for the deep digital flexor tendon, and the extensive peroneal fossa, among other features. The other two calcanei and an incomplete astragalus were referred to Interatheriidae (Notoungulata) due to the presence of a fibular facet that is oriented obliquely with respect to the longitudinal axis of the calcaneus, the presence of two concavities in the posterior and lateral portion of the fibular facet, and a long astragalar neck with a markedly spherical head. Another element analyzed was an astragalus referred to Litopterna. This assignment is based on the presence of a pulley-like tibial trochlea, tibial and fibular crests approximately symmetric, and a markedly oblique astragalar neck with a cylindrical head. This last feature is less conspicuous than Miocene litopterns such as Lambdaconus and Tetramerorhinus, and resembles that of some didolodontids, such as Didolodus. On the other hand, the functional study allowed inferring the association of these bones with different types of stances. Based on the direction of different articulation planes (e.g., metatarsal, astragalar head, astragalar condyle planes, etc.) we suggest that the tarsal elements correspond to digitigrade animals. The study of these tarsals provided useful information about distinctive postcranial features of early forms of the most representative groups of native South American ungulates, and is of great importance for a more integrative view of the Paleogene forms in the region.