Oldest associated notoungulate skeleton from South America

CARRILLO, J.; CARLINI, A. A.; LORENTE, M.; CIANCIO, M.; GELFO J. N.; GOIN F. J.; ASHER, R.

Berlin

Congreso; 74th ANNUAL MEETING OF the SOCIETY OF VERTEBRATE PALEONTOLOGY; 2014

Society of Vertebrate Paleontology

While fossil remains of endemic South American mammals are abundant in the South American Paleogene, relatively complete and associated remains of a single individual are extremely rare. Previous estimates of isotemnid anatomy have been based on multiple individuals, species, and/or genera to infer diversity in this cladistically basal notoungulate group. Here, we report the discovery of a single individual of Thomashuxleya externa (Isotemnidae) from the Vacan subage of the Casamayoran SALMA (likely within the Lutetian Stage and close to the Bridgerian NALMA in age), from Cañadón Vaca, a locality east of Lago Colhué Huapi in Chubut Province, Argentina. Parts of nearly all elements of the skeleton are represented, including the skull, jaws, vertebrae, fore- and hind limbs, shoulder and pelvic girdles, and pes. Associated skeletal remains of other individuals, found at the same locality, include a partial articulated manus and contribute additional cranial material with which variation in Thomashuxleya can be assessed. Data from our single individual skeleton are consistent with recent reconstructions of forelimb and pedal anatomy in Thomashuxleya, in particular regarding the previously uncertain association of an astragalus with other skeletal elements. The associated astragalus is consistent with a plantigrade posture without significant running adaptations and lacks a cotylar fossa. Forelimb anatomy is congruent with an erect posture for Thomashuxleya, as opposed to a crouching posture inferred for other Vacan isotemnids. This discovery provides an unusually complete anatomical basis for further studies of the long-ambiguous phylogenetic position of endemic South American Mammals.