Shaking Up the Dinosaur Family Tree

The proposed new family tree of dinosaurs. The group to the left is for close relatives but not true dinosaurs. The old tree grouped the theropods, purple, with the Saurischia, green, and viewed the Saurischia and the Ornithischia as the two major branches of the tree. The scale to the left shows the placement of the tree in geological time. A is the branchpoint that includes all the dinosaurs, B represents the joint ancestor of Ornithischia and the theropods, and C is the joint ancestor of Saurischia and an early group known as herrerasaurs. (Baron et al./Nature)

For more than a century, the placement of dinosaurs on the branches of their family tree has been based on the shape of their hips.

This classification has now been radically challenged by proponents of a new tree which, if accepted, swaps large subfamilies around, sheds new light on dinosaurs’ evolution and suggests they may have originated not in South America, as widely assumed, but perhaps in some Northern Hemisphere locality such as Scotland.

A Victorian paleontologist, Harry Seeley, declared in 1888 that dinosaurs should be divided into the bird-hipped (Ornithischia) and the lizard-hipped (Saurischia) categories that have been accepted ever since.

Under this system, the heavily armored stegosaurs and ankylosaurs are placed on the Ornithischian branch of the family tree. The Saurischian branch includes both sauropods like the herbivorous diplodocus, and theropods like the meat-eating tyrannosaurs.

This longstanding classification has now been disputed by Matthew G. Baron of the University of Cambridge. Mr. Baron is a graduate student and his rewriting of the dinosaur family tree is a project to attain his Ph.D. But his ideas are supported by his two supervisors and co-authors, David B. Norman of the University of Cambridge, and Paul M. Barrett of the Natural History Museum in London, and by a prodigious database he has constructed of dinosaur anatomical features.

Mr. Baron started his work on the Ornithischian dinosaurs but came to feel they did not fit well in their place on the accepted family tree. With his supervisors’ encouragement, he set out to reconsider the entire dinosaur classification system. More than 1,000 species have already been identified, most of them dating from between 200 million and 66 million years ago. Dinosaurs became the dominant terrestrial species after the first date, and perished, all save the lineage leading to birds, at the second.

Mr. Baron spent three years visiting museums throughout the world and assessed important dinosaur fossils for the presence of 457 diagnostic anatomical features. Based on this information, a computer program called TNT arranged the dinosaur specimens in possible family trees. After analyzing 32 billion trees, the computer spat out the best possible arrangement of Mr. Baron’s three years’ worth of data collection. The run took just five minutes.

The new family tree of dinosaurs, published on Wednesday in the journal Nature, is quite unlike the old. “The results of this study challenge more than a century of dogma and recover an unexpected tree topology that necessitates fundamental reassessment of early dinosaur evolution,” Mr. Baron and his supervisors write.

Essentially they have found that the Ornithischian dinosaurs have many similarities with the theropods and so probably shared a common ancestor. As it happens, Thomas Huxley, the celebrated 19th century champion of Darwin’s theory of evolution, also thought Ornithischia and theropods belonged together in the same group, which he called Ornithoscelida. Mr. Baron says this name should be revived, with the two main branches of the new family tree being the Ornithoscelida and the Saurischia.

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SOURCE: NY Times, Nicholas Wade