The new finding extends the fossil record of the bizarre group of bird-like dinosaurs with a large claw on the hand and very short, powerful arms – back 63 million years, further distancing them from birds on the evolutionary tree. Researchers say Haplocheirus sollers (meaning simple, skillful hand) represents the first direct evidence that dinosaurs of this type lived during the Late Jurassic, approximately 160 million years ago.
"Haplocheirus is a transitional fossil, because it shows an early evolutionary step in how the bizarre hands of later alvarezsaurs evolved from earlier predatory dinosaurs," said lead author Jonah Choiniere. "The fossil also confirms our predictions that Alvarezsauridae should have been evolving in the Late Jurassic time period."
The ten-foot long, nearly complete skeleton of Haplocheirus sollers specimen was found preserved in river-lain rock in the Xinjiang Autonomous Region of northwestern China, a region well-known for its Late Jurassic fossils. It was collected in 2004 during a series of expeditions to Xinjiang co-led by Dr. James Clark of GW and Dr. Xu Xing of the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology, co-authors on the report.
The fossil of the new species contains several distinguishing features that link it to Alvarezsauridae, the family of dinosaurs that includes species such as Mononykus (meaning one claw) and that was previously thought to be a flightless offshoot of ancient birds due to skeletal similarities.
Despite the similarity between the skeletons, Choiniere's research demonstrates that the family Alvarezsauridae evolved in parallel to birds and did not descend from them. The new species shows some of the earliest evolutionary stages in the development of a short, powerful arm with a single functional claw that may have been used for digging termites.
The Late Jurassic is an important time period for bird evolution, as evidence suggests that birds first evolved from theropod, or bird-footed, dinosaurs at that time. Paradoxically, fossils of dinosaurs closely related to birds from this time period are extremely rare, furthering the importance of Mr. Choiniere's work.
"The primary goal of our expeditions was to find evidence of the theropod dinosaurs closest to birds, and the discovery of Haplocheirus is one of our major discoveries," said Dr. Clark. "This spectacular skeleton shows how the strange arms of Mononykus and other alvarezsaurs evolved from a more typically theropod grasping hand."
Theropod dinosaurs include charismatic, meat-eaters like Tyrannosaurus rex but also modern birds. Alvarezsaurs are one of several groups of theropods closely related to birds, including well-known species like Velociraptor.
Citation: Jonah N. Choiniere et al., 'A Basal Alvarezsauroid Theropod from the Early Late Jurassic of Xinjiang, China', Science, January 2010, 327 (5965), 571 - 574; doi: 10.1126/science.1182143