Newly Discovered Beaked, Bird-like Dinosaur Tells Story of Finger Evolution

/ADVANCE FOR RELEASE AT 1 P.M. EDT WEDNESDAY, JUNE 17/

/ADVANCE/ WASHINGTON, June 17 /PRNewswire-USNewswire/ -- James Clark, The
George Washington University's Ronald B. Weintraub Professor of Biology, and
Xu Xing, of the Chinese Academy of Science's Institute of Vertebrate
Paleontology and Paleoanthropology in Beijing, have discovered a unique
beaked, plant-eating dinosaur in China. This finding demonstrates that
theropod, or bird-footed, dinosaurs were more ecologically diverse in the
Jurassic period than previously thought and offers important new evidence
about how the three-fingered hand of birds evolved from the hand of dinosaurs.
The discovery is featured in this week's edition of the journal Nature.

"This new animal is fascinating in and of itself, and when placed into an
evolutionary context it offers intriguing evidence about how the hand of birds
evolved," said Dr. Clark. Clark's graduate student, Jonah Choiniere, also was
involved in analyzing the new animal.

Dr. Xu, said, "This discovery is truly exciting, as it changes what we thought
we knew about the dinosaur hand. It also is amazing to bring conciliation
between the data from million-year-old bones and molecules of living birds."

Limusaurus inextricabilis (meaning "mire lizard who could not escape") was
found in 159 million-year-old deposits located in the Junggar Basin of
Xinjiang, northwestern China. The dinosaur earned its name from the way its
skeletons were preserved, stacked on top of each other in fossilized mire pits
that were the subject of a 2008 National Geographic film, "Dino Death Trap." A
close examination of the fossil shows that its upper and lower jaws were
toothless, demonstrating that the dinosaur possessed a fully developed beak.
Its lack of teeth, short arms without sharp claws and possession of gizzard
stones suggest that it was a plant-eater, though it is related to carnivorous
dinosaurs.

The newly discovered dinosaur's hand is unusual and provides surprising new
insights into a long-standing controversy over which fingers are present in
living birds, which are theropod dinosaur descendants. The hands of theropod
dinosaurs suggest that the outer two fingers were lost during the course of
evolution and the inner three remained. Conversely, embryos of living birds
suggest that birds have lost one finger from the outside and one from the
inside of the hand. Unlike all other theropods, the hand of Limusaurus
strongly reduced the first finger and increased the size of the second. Drs.
Clark and Xu and their co-authors argue that Limusaurus' hand represents a
transitional condition in which the inner finger was lost and the other
fingers took on the shape of the fingers next to them. The three fingers of
most advanced theropods are the second, third and fourth fingers -- the same
ones indicated by bird embryos -- contrary to the traditional interpretation
that they were the first, second and third.

Limusaurus is the first ceratosaur known from East Asia and one of the most
primitive members of the group. Ceratosaurs are a diverse group of theropods
that often bear crests or horns on their heads, and many have unusual, knobby
fingers lacking sharp claws.

The fossil beds in China that produced Limusaurus have previously yielded
skeletons of a variety of dinosaurs and contemporary animals described by Drs.
Clark and Xu and their colleagues. These include the oldest tyrannosaur,
Guanlong wucaii; the oldest horned dinosaur, Yinlong downsi; a new stegosaur,
Jiangjunosaurusjunggarensis; and the running crocodile relative, Junggarsuchus
sloani.

This research was funded in part by the National Geographic Society, the
National Science Foundation Earth Science Division, the Chinese National
Natural Science Foundation, the Jurassic Foundation and the Hilmar Sallee
bequest.

Dr. Clark has spent the last 18 years searching the Gobi Desert for dinosaurs.
In 1991, he helped organize the first American expedition to Mongolia with the
American Museum of National History. For the past seven years, his field work
with Dr. Xu has focused on dinosaurs from the middle of the Jurassic Period,
in the far western reaches of the Gobi.

The article, "A Jurassic ceratosaur from China and its significance for
theropod digit reduction and avian digital homologies" appears in the June 18
edition of Nature. Nature is a weekly international journal publishing
peer-reviewed research in all fields of science and technology on the basis of
its originality, importance, interdisciplinary interest, timeliness,
accessibility, elegance and surprising conclusions.

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SOURCE George Washington University

Michelle Sherrard of George Washington University, +1-202-994-1423,
mcs1@gwu.edu