EPIPUBIC BONES IN EUTHERIAN MAMMALS FROM THE LATE CRETACEOUS OF MONGOLIA

An important transformation in the evolution of mammals was the loss o f the epipubic bones. These are elements projecting anteriorly from th e pelvic girdle into the abdominal region in a variety of Mesozoic mam mals, related tritylodonts, marsupials and monotremes but not in livin g eutherian (placental) mammals(1-3). Here we describe a new eutherian from the Late Cretaceous period of Mongolia, and report the first rec ord of epipubic bones in two distinct eutherian lineages. The presence of epipubic bones and other primitive features suggests that these gr oups occupy a basal position in the Eutheria. It has been argued that the epipubic bones support the pouch in living mammals(1,3,4), but epi pubic bones have since been related to locomotion and suspension of th e litter mass of several attached, lactating offspring(5). The loss of the epipubic bones in eutherians can be related to the evolution of p rolonged gestation, which would not require prolonged external attachm ent of altricial young. Thus the occurrence of epipubic bones in two C retaceous eutherians suggests that the dramatic modifications connecte d with typical placental reproduction(3,6,7) may have been later event s in the evolution of the Eutheria.