May 16, 2006
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GW PROFESSOR JAMES M. CLARK LEADS DISCOVERY OF THE OLDEST-KNOWN CERATOPSIAN, AN ANCESTOR OF TRICERATOPS AND OTHER HORNED DINOSAURS
New Find is Evolutionary Link Between Ceratopsians and Pachycephalosaurs, the "Bone-Headed" Dinosaurs
WASHINGTON -- James M. Clark, Ronald B. Weintraub Associate Professor of Biology at The George Washington University, and Xu Xing of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, have discovered the oldest-known ceratopsian, a finding that solidifies the close evolutionary evidence between ceratopsians and pachycephalosarians, the "bone-headed" dinosaurs. Roaming the earth 160 million years ago, the new basal ceratopsian dinosaur, Yinlong downsi, appeared 20 million years earlier than the previous oldest ceratopsian and 85 million years earlier than the best known ceratopsian, Triceratops.
Pachycephalosaurs and ceratopsians are two of the most specialized groups of dinosaurs, and Yinlong is a primitive transitional form with features that span the two groups. Yinlong is a ceratopsian, but it is very primitive and retains several features from the common ancestor ceratopsians shared with pachycephalosaurs, including features previously thought to be pachycephalosaur specializations. It sheds important light on the common ancestral condition for these two groups, from which both groups later diverged spectacularly.
The discovery is announced by Clark, Xu, and two colleagues in the May 17, 2006, online edition of the British science journal, Proceedings of the Royal Society B: Biological Sciences, in a paper titled, "A Basal Ceratopsian with Transitional Features from the Late Jurassic of Northwestern China."
"Ceratopsians were long thought to be related to the pachycephalosaurs, but the evidence has been weak," said Clark. "Yinlong combines features of pachycephalosaurs and ceratopsians, including some tubercles or low knobs on the back of the skull, which provide concrete evidence that the evolutionary relationship is indeed real. It shows that the common ancestor of the two groups had pachycephalosaur features that were then lost with ceratopsians."
The low knobs on the back of the skull of Yinlong are one of several features linking ceratopsians and pachycephalosaurs. Other features include a rough surface to bones in the upper and lower jaws, and the shape of the bones on the skull behind the eye. The horned ceratopsians and thick-skulled pachycephalosaurs make up the suborder Marginocephalia, part of the ornithischian dinosaurs. Both groups were herbivores, walking on two or four legs, and are characterized by a bony ridge or frill on the back of the skull. They evolved in the Jurassic Period, and became common in the Late Cretaceous Period.
"The discovery of Yinlong has shed important light on the evolution of ceratopsians and pachycephalosaurs," said Xu. "The evolutionary road of both these groups leads back to Yinlong, giving us greater insight to the development of ornithischian dinosaurs."
Yinlong downsi, an early relative of Triceratops, was much smaller than its ceratopsian descendant measuring a little over four feet once full grown. Triceratops were large animals approximately the size of a car weighing five tons with large horns and frills making up the skull. Unlike Triceratops, Yinlong downsi has no large horns or frills characteristic of larger ceratopsians. But Yinlong does possess a rostral bone, a distinct beak-like bone at the end of its snout, along with a raised and triangular-shaped skull, common to all ceratopsians.
The Yinlong specimen described in the paper was not a full-grown adult when it died, measuring 120 centimeters or about four feet. Its relatively short and slender forelimbs and very robust and long hindlimbs (the forelimbs are less than 40 percent the hindlimb length) suggest that Yinlong at times walked on its hind legs. While pachycephalosaurians and the common ancestor of the two groups walked on their hind legs, later ceratopsians walked on all fours. One of the features previously thought to tie the two groups together is the presence of a frill or shelf of bone at the back of their skull. However, the absence of a frill in Yinlong leads Clark and Xu to hypothesize that the frill "might have been independently developed in pachycephalosaurs and derived ceratopsians."
The previously known oldest ceratopsians appeared at the beginning of the Cretaceous Period, about 140 million years ago. The Shishugou Formation of the Junggar Basin, Xinjiang, China, where Yinlong was found, was deposited at the end of the Middle Jurassic Period and the beginning of the Late Jurassic Period, a time that is critical to the origins and early evolution of the major dinosaurian lineages, including birds. A group of paleontologists including Xu discovered two other possible Jurassic ceratopsians in the Tuchengzi and Houcheng formations of China. However, a recent radiometric sample from the Tuchengzhi Formation places its upper part in the Early Cretaceous Period. Consequently, the Shishugou ceratopsian represents the first unquestionable Jurassic ceratopsian species.
The new discovery's name Yinlong downsi combines both American and Chinese names. "Yinlong" means "hiding dragon" in Chinese, derived from the movie Crouching Tiger, Hidden Dragon, which was filmed in the area where the fossils were found. The latter part of the name is in memory of Will Downs, a life-long fossil hunter who joined many paleontological expeditions in China, including one with Clark and Xu's team in 2003, shortly before Downs' death. Discovered in 2004, the nearly complete skeleton was found with two other specimens of Yinlong on the edge of the Gobi Desert in northwestern China in the Junggar Basin, part of the Xinjiang Province.
Field work that led to the discovery of Yinlong was supported by the National Natural Science Foundation of China, the National Geographic Society, the National Science Foundation Division of Earth Sciences, and The George Washington University. Co-authors include Clark, Xu, Catherine A. Forster of Stony Brook University, and Jinyou Mo of China University of Geosciences.
Clark and Xu have led five separate cooperative expeditions into Xinjiang since 2001. This past February, Clark and Xu along with six other colleagues announced the discovery of a new genus and species of dinosaur that is the oldest known and most primitive tyrannosauroid - Guanlong wucaii, an early relative of Tyrannosaurs rex. Guanlong occurs in the same beds as Yinlong and was likely a predator on the smaller ceratopsian, foreshadowing the predation of Triceratops by T. rex 95 million years later. Clark and Xu published the Guanlong discovery in the February 9, 2006, edition of Nature with Forster, Mark Norell of the American Museum of Natural History, Gregory M. Erickson of Florida State University, David A. Eberth of the Royal Tyrrell Museum, Chengkai Jia of IVPP, and Qi Zhao of IVPP.
Clark is one of several GW faculty members who teach in the University's Robert Weintraub Program in Systematics and Evolution, a joint graduate program of GW and the Smithsonian's National Museum of Natural History. Through this program, named in honor of GW alumnus and former faculty member Robert L. Weintraub, the University and the Smithsonian have made a substantial commitment to education, research, and field work in systematics. The program's main goals are to discover and describe new species, quantify biological diversity, uncover similarities that indicate evolutionary history, and use this history to test models of how evolution occurs.
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For more information about the National Science Foundation and its involvement in this research, please contact Cheryl Dybas at (703) 292-7734 or firstname.lastname@example.org.
For more background about Jim Clark, visit www.gwu.edu/~clade/faculty/clark.
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