Femur of a monotreme (Mammalia, Monotremata) from the Early Paleocene Salamanca Formation of Patagonia, Argentina

FORASIEPI, A. M.; MARTINELLI, A. G.

AMEGHINIANA

Asociacion Paleontologica Argentina

Lugar: Buenos Aires; Año: 2003 vol. 40 p. 625 - 630

0002-7014

A complete distal part of a left femur and a medial distal part of a right femur from the Early Paleocene Salamanca Formation at Punta Peligro, Chubut, Argrntina, are described. Comparisons with monotremes, morganucodontids, Vincelestes, multituberculates, dryolestoids, metatherians and eutherians suggest that they correspond to a monotreme. The Punta Peligro specimens share with the latter the following features: 1) flat distal femoral end; 2) rectangular trochlea; 3) lack of patellar groove and 4) both condyles located at same level. As in Ornithorhynchidae and differing from Tachyglossidae, the shaft is considerably narrow respect to the epiphysis width and the main axis of the medial condyle is inclined. Because the size of the new femur and the estimated body size for the only non-australian monotreme Monotrematum sudamericanum are quite similar and both belong to the same formational and stratigraphic level, the femora are tentatively referred to this species. The diagnostic features of the femur of Monotrematum are as follow: 1) sharp distal ridge connecting both condyles; 2) intercondylar fossa slightly narrower than in the other monotremes and possibly 3) the presence of a divided supratrochlear facet.