Dec. 3, 1998 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, Deltatheridium was a relative giant in the Lilliputian world of Mesozoic mammals. Its diet probably included lizards and other early mammals. One of the specimens was preserved with a crushed fragment of a mammal skull ( since broken remains are very rare at Uhkaa Tolgod, it is possible that the mammal was the Deltatheridium's last meal.
The location of Deltatheridium in Central Asia suggests that modern marsupials, which today are most diverse in South America and Australia, may have arisen from a lineage that actually originated in Asia.
Before the current discovery, scientists debated whether Deltatheridium should be placed in the placental lineage or the marsupial lineage. Study of the new fossils reveals unequivocally that Deltatheridium is an early relative of marsupials, and in fact is one of the earilest marsupial relatives ever discovered.
Some of the features Deltatheridium shares with modern-day marsupials include a large bony process at the back of the jaw for the attachment of chewing muscles and a distinct pattern of openings in the skull to accommodate blood circulation, but it lacked many other features that arose in modern marsupials.
The juvenile specimen found at Ukhaa Tolgod is particularly important because it died when its adult teeth were just beginning to emerge, so the sequence in which the teeth grew can be observed. Analysis of the teeth of the juvenile Deltatheridium provides one of the strongest links between this animal and modern marsupials ó unlike any other group of mammals, marsupials replace only one tooth after birth, the last premolar. The juvenile specimen shows this same highly distinctive tooth replacement pattern.
Authors on the December 3 Nature paper are: Guillermo W. Rougier, assistant professor at the Department of Anatomical Sciences and Neurobiology at the University of Louisville, who is also a research associate in the Department of Vertebrate Paleontology at the American Museum of Natural History; John R. Wible, Section of Mammals at the Carnegie Museum of Natural History in Pittsburgh, who is also a research associate in the American Museum of Natural History's Department of Mammalogy; and Michael J. Novacek, Curator in the Department of Vertebrate Paleontology at the American Museum of Natural History.
This research was supported by the National Science Foundation, the Jaffe Foundation, the National Geographic Society, the Eppley Foundation, the Mercedes-Benz Corporation, the James Carter Memorial Fund, and the Frick Laboratory Endowment of the American Museum of Natural History.
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