Today's issue of Nature contains an article by Dr. James Clark (George Washington University) and Xu Xing (Chinese Academy of Science's Institute of Vertebrate Paleontology and Paleoanthropology in Beijing) and colleagues titled "A Jurassic ceratosaur from China and its significance fortheropod digit reduction and avian digital homologies". The paper describes a new beaked ceratosaur (yes, beaked ceratosaur) from the Jurassic of China. This new specimen also offers key information regarding the interpretation of digit homology between non- avian and avian dinosaurs.
As Tom Holtz noted to me in an earlier e-mail message....this is a "ceratosaur convergent on Effigia (a Triassic pseudosuchian): Truly weird!". I could not have summed it up any better!
Kudos also to my friend and colleague Sterling Nesbitt and to fellow blogger David Hone who are co-authors on this paper.
The following text is from the Reuters News Release . Photos are from here.
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, knobbyfingers 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, Jiangjunosaurus junggarensis; and the running crocodile relative, Junggarsuchus sloani.
Xing Xu et al, 2009. A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature 459:940. doi:10.1038/nature08124