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A team of paleontologists from the American Museum of Natural History and their colleagues have described the oldest, most complete fossil skeleton yet found of a primitive rabbit. The 55-million-year-old fossil animal, named Gomphos elkema is the oldest complete skeleton by about 20 million years and shows that some early lagomorphs, the group of animals that includes rabbits and hares, had a surprisingly modern, rabbit-like way of moving around. Its hindlimb was much longer than its forelimb, giving it a true "rabbit's foot" more than twice as long as its forepaw, which could be used for hopping. Other features of the skeleton, however, do not resemble modern rabbits. For instance,G. elkema, had a moderately sized tail, molar teeth with roots and distinct cusps, and a primitive jaw. Overall, this mix of features from modern rodents and lagomorphs supports the previously controversial but now generally agreed-upon idea that these two groups are closely related.
The G. elkema specimen was collected during one of the Museum's annual joint paleontological expeditions to the great fossil beds of Mongolia's Gobi Desert with the Mongolian Academy of Sciences. Co-led by Michael J. Novacek, Provost, Senior Vice President, and Curator in the Division of Paleontology, and Mark A. Norell, Chairman of the Division of Paleontology, Museum scientists will return this summer for the 16th consecutive year to explore this vast desert with their colleagues. The authors of the new research paper on G. elkema published in the journal Science include Dr. Novacek; Jin Meng, Associate Curator in the Museum's Division of Paleontology; Malcolm McKenna, Curator Emeritus in the Museum's Division of Paleontology; Robert J. Asher, Curator of Mammals at the Berlin Museum of Natural History; Guillermo W. Rougier, Associate Professor in the Department of Anatomical Sciences and Neurobiology at the University of Louisville; Demberlyn Dashzeveg of the Mongolian Academy of Sciences; and John Wible of the Carnegie Museum of Natural History.
"This research on G. elkema, along with the Museum's recently published research on fossils of a dinosaur-eating mammal, a tyrannosaur covered with protofeathers, and a sleeping troodontid, is representative of the unprecedented quality and quantity of paleontological discoveries and research projects currently undertaken by Museum scientists and their colleagues," said Dr. Novacek. "They also underscore the extraordinary quality and importance of fossils that have been recovered in recent years from Mongolia and northern China."
Dr. Asher, senior author of the new research paper, and Drs. Novacek and Meng were among the first to examine the fossil closely once its surrounding sediment was removed. "This animal's foot is huge," said Dr. Asher. "It's almost the size of its entire arm, like a modern rabbit's foot."
The Origins of Modern Mammals
The new G. elkema fossil also sheds light on a debate about the first appearance of modern placental mammals (mammals that develop for an extended period of time nourished by the placenta in the mother's uterus), a group that includes humans, deer, cows, rats, monkeys, whales, camels, horses, and bats. Paleontologists have wondered whether modern placentals existed earlier than 65 million years ago, a turning point in geologic history when the fossil record shows that many of Earth's species became extinct. This date is known as the "K-T boundary," referring to the break between the Cretaceous Period and the Tertiary Period that followed it.
Some paleontologists claim that ancient relatives of modern groups such as rabbits can be found in the fossil record tens of millions of years before the K-T boundary. An extinct Central Asian group of mammals called zalambdalestids are known to be more than 85 million years old, and they shared a close evolutionary relationship with modern rabbits, a hypothesis suggested by some paleontologists. Because G. elkema preserves so much information about the anatomy of ancient rabbits and their kin, the Museum team and their colleagues included it in a new analysis of the family relationships among mammals and found strong evidence against the point of view that zalambdalestids are evidence of modern mammals in the Cretaceous Period (spanning 145 to 65 million years ago). Instead, the team found that modern rabbits are more closely related to a group that includes rodents, primates, tree shrews, ungulates, and other modern placental mammals than to any mammal known before the K-T boundary.
"This extremely well-preserved fossil is providing a new contribution to the question of the divergence in the early evolution of placental mammals and lagomorphs, in particular," said Dr. Meng. "Our results basically say the divergence of the mammal group that includes lagomorphs occurred after the K-T boundary, 65 million years ago. This supports the conventional view that the timing of the divergence is not way back into the Cretaceous but is closer to the K-T boundary." Such a conclusion is at odds with estimates by some molecular biologists based on gene differences showing that lagomorphs and other placental mammals may have diverged at least 80 million years ago, well into the Cretaceous and long before the K-T extinction event.
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