December 3, 1998-A team of scientists announced today in the journal Nature the discovery of new specimens of an early relative of marsupials called Deltatheridium that provide unprecedented insight into the evolutionary split that eventually led to the rise of today's marsupials and placental mammals. The fossils were uncovered at Ukhaa Tolgod, one of the world's richest fossil sites, during the joint American Museum of Natural History/Mongolian Academy of Sciences expedition to the Gobi Desert of Mongolia, in Central Asia. The physical features observed in the new fossils allow scientists to define for the first time which characteristics are unique to the marsupial lineage, and allow them to draw a comprehensive family tree for both this group and our own progenitors, the placental mammals.
Marsupials, the group of mammals that includes opossums, kangaroos, and koalas, represent one of the three major branches of mammals living today. The other two branches are monotremes, which include the bizarre egg-laying duck-billed platypus, and placentals, which include most of the mammals familiar to us today, among them, humans. While scientists know that the fossil record for marsupials and their relatives stretches into the Mesozoic Era, known as the Age of the Dinosaurs, unraveling their evolutionary history has proved controversial because only very fragmentary fossils of this group have been found.
The two newly discovered Deltatheridium specimens, which are approximately 80 million years old, help fill critical gaps in the earliest stages of mammalian evolutionary history. The first of the new specimens is an adult with a nearly complete skull, jaws, and arm bones; the second is a juvenile with virtually complete jaws, various skull bones, and several additional bones from its body.
Deltatheridium was an opossum-like mammal with very sharp molars and long canines, suggesting it may have been a carnivore. With a skull almost two inches long, Deltath
Contact: Elizabeth Chapman, Karen de Seve
American Museum of Natural History