Scientists have identified a new species of Archaeopteryx-like theropod dinosaur from lake deposits formed about 160 million years ago in northeastern China. According to findings published online today (July 28th, 2011) in the journal Nature, the new dinosaur species Xiaotingia zhengiis a close relative of Archaeopteryx, which is widely accepted as the most primitive bird. However, a phylogenetic analysis incorporating new information from this find suggests that Archaeopteryxis not a bird, which has significant implications for our understanding of bird origins.

Xiaotingia weighed about 800 grams, comparable to Archaeopteryx, and is one of the smallest non-avian theropod (carnivorous) dinosaurs ever identified by scientists. It has conical teeth, as well as long, robust arms that are similar to those of primitive birds. Its specialized feet have the highly extensible second toe characteristic of the Deinonychosauria, the group of bird-like dinosaurs that includes Velociraptor from the “Jurassic Park” films.

“The most exciting result from our research is that both Xiaotingia and Archaeopteryx are primitive deinonychosaurs rather than birds. In other words, Archaeopteryx is a kind of Velociraptor ancestor rather than a bird ancestor,” said Dr Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, the lead author of the scientific paper.

It is now widely accepted that birds are descended from dinosaurs, but scientists are still working hard to reconstruct how the dinosaur-bird transition occurred. One of the most important jobs is to reconstruct a reliable family tree, based on which scientists can find out which dinosaurs evolved into birds and what the most primitive birds were actually like. Xu’s team has made several significant discoveries related to this interesting issue.

“We named the four-winged dinosaur Anchiornis huxleyi in 2008," said Dr. Xu. "At that time we already noticed some striking similarities between Anchiornis huxleyi and Archaeopteryx, such as the presence of a long bony tail fully covered by large feathers. Xiaotingia is from the same site as Anchiornis. Like Anchiornis, it also bears long flight feathers on the feet, a feature that we suspected was also present in Archaeopteryx. The discovery of Xiaotingia provides further support for a close relationship between these species.”

The researchers noticed that Archaeopteryx, Xiaotingia, and Anchiornis are more similar to other deinonychosaurs than to birds in having a lightly built skull, in stark contrast to the more robust skull seen in other known primitive birds and also the oviraptorosaurian dinosaurs. A comprehensive analysis by the researchers indicates an interesting evolutionary divergence between a primarily herbivorous lineage leading to modern birds and a carnivorous lineage leading to Veloviraptor. This new pattern has significant implications for avian origins, and for related issues such as the early evolution of feathers and flight.

Fig.2 :  Holotype of Xiaotingia zhengi (Image by Dr Xing Xu)