Wednesday, June 17,
2009
Scientists have discovered a unique beaked, plant-eating dinosaur
in China. The finding, they say, demonstrates that theropod, or
bird-footed, dinosaurs were more ecologically diverse in the
Jurassic period than previously thought, and offers important
evidence about how the three-fingered hand of birds evolved from
the hand of dinosaurs.
The discovery is reported in a paper published in this week's
edition of the journal Nature.
"This work on dinosaurs provides a whole new perspective on the
evolution of bird manual digits," said H. Richard Lane, program
director in the National Science Foundation (NSF)'s Division of
Earth Sciences, which funded the research.
"This new animal is fascinating, and when placed into an
evolutionary context it offers intriguing evidence about how the
hand of birds evolved," said scientist James Clark of George
Washington University.
Clark, along with Xu Xing of the Chinese Academy of Science's
Institute of Vertebrate Paleontology and Paleoanthropology in
Beijing, made the discovery. Clark's graduate student, Jonah
Choiniere, also was involved in analyzing the new animal.
"This finding is truly exciting, as it changes what we thought we
knew about the dinosaur hand," said Xu. "It also brings
conciliation between the data from million-year-old bones and
molecules of living birds."
Limusaurus inextricabilis ("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.
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. Clark and Xu
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 Clark and Xu.
These include the oldest tyrannosaur, Guanlong wucaii; the oldest
horned dinosaur, Yinlong downsi; a new stegosaur, Jiangjunosaurus
junggarensis; and the running crocodile relative, Junggarsuchus
sloani.
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National Science Foundation
Thanks to National Science
Foundation for this article.