Deltatheroidans (mentioned in the July 15, 1998 dispatch)

Deltatheroidans are early members of the Metatheria, group that includes the living marsupials (the group that contains oppossums and kangaroos and other pouched mammals) and all its fossil relatives. Delatatheroidans are known from the Late Cretaceous (about 80 million years) of Asia and possibly North America. A member of this group, Deltatheridium pretrituberculare is known from Ukhaa Tolgod, a Mongolian Late Cretaceous locality, that yields fossils about 80 million years old. The closest relatives of the metatherians are the eutherians, where the placentals (including us) belong. Placentals replace many teeth, that is we have milk teeth for all incisors, canines, and premolars when we are young, and these are later replaced by permanent dentition. In contrast, metatherians replace only the last premolar. It has been suggested that the suppresion of the replacement of most teeth is a marsupial adaptation to a long period of attachment to the nipples by the infants until they reach complete development.(Read more about the Deltatheridium in the Electronic Newspaper.)