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Published online: 24 June 2009 | doi:10.1038/nchina.2009.134

Palaeontology: A firsthand look

Felix Cheung

Introduction

© (2009) Nature/James Clark

Xing Xu at the Chinese Academy of Sciences in Beijing and co-workers¹ have dug up the fossil of a small, herbivorous dinosaur from the Jurassic period in the Junggar Basin of western China. The dinosaur, dubbed Limusaurus, represents the first known Asian ceratosaur (a clade closely related to the tetanurans, which include birds) and the only known beaked, Jurassic herbivorous theropod (two-legged dinosaur). Most importantly, the limusaur features a shortened thumb (first digit, or 'finger') that could change our understanding of dinosaur hand-to-wing evolution.

Many palaeontologists believe that theropods are the ancestors of birds. There is one problem with this consensus though. The three digits of primitive theropods, including non-avian tetanurans, are thought to evolve from the first, second and third digits of an ancestral five-digit hand. The bird hand, however, which is now reduced and embedded in the wing, is thought to derive from the second, third and fourth digits.

The newly discovered limusaur has a greatly reduced first digit alongside more fully developed second, third and fourth digits (see image). In comparison, the wrist bones (not shown) of the limusaur are more like those associated with the second, third and fourth digits of tetanurans, but the finger bones themselves are more like the first, second and third digits.

The researchers argue that the hand-to-wing evolution involved complex developmental shifts in the wrist and fingers, and the limusaur allows a glimpse of this change as it happened.

The authors of this work are from:
Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; Department of Biological Sciences, George Washington University, Washington DC, USA; Natural History Museum of Guangxi, Nanning, China; Faculty of Earth Sciences, China University of Geosciences, Wuhan, China; Department of Biological Science, Florida State University, Tallahassee, Florida, USA; Royal Tyrrell Museum, Drumheller, Alberta, Canada; American Museum of Natural History, New York City, New York, USA; Instituto de Geologia, Universidad Nacional Autónoma de México, Ciudad Universitaria, Mexico City, Mexico; Research Institute of Exploration and Development, Xinjiang Oilfield Company, Karamay, China; Graduate University of Chinese Academy of Sciences, Beijing, China.

Reference

1. Xu, X. et al. A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature doi:10.1038/nature08124 (2009). | Article |