2017-03-22 14:17:03
Shaking Up the Dinosaur Family Tree

14:17, March 22 288 0

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.

Mr. Baron’s revolutionary new family tree may not be immediately accepted but experts seem likely to give it a serious hearing because of its database, the largest ever assembled, and its use of a standard tree-drawing program.

“It will be interesting to see how paleontologists receive this original and provocative reassessment of dinosaur origins and relationships,” Kevin Padian, a dinosaur expert at the University of California, Berkeley, wrote in an accompanying commentary in Nature.

“It’s a radical proposal with a reasonable basis but no one expects it will be the last word,” Dr. Padian said in an interview. Given that such a sudden shift in the dinosaur family tree might even be possible, people could wonder if dinosaur experts know what they’re doing, he said. His answer is that they do, but they have been faced with an unusual problem. There has been an explosion of new discoveries in the last 30 years, showing that new dinosaur groups evolved with a mix of old features inherited from their ancestors and new ones shaped by natural selection. But the new features are the same in many cases, an instance of what biologists call convergent evolution, making it very hard to assign each group to its right place on the dinosaur family tree.

Paul Sereno, a dinosaur specialist the University of Chicago who laid the basis for the modern version of Seeley’s classification, said the new paper would certainly stir the pot but he couldn’t see what new features or scoring system had contributed to the new result.

Mr. Baron said his work was not based on any new diagnostic features but on more data and the absence of any prior assumptions about what the tree should look like.

Having a correctly drawn family tree allows paleontologists to peer more deeply into the origins of the dinosaurs, because the species that lie close to the root of the major families may carry the same traits as the first dinosaur. Based on his tree, Mr. Baron believes that the original dinosaurs were small, two-footed animals with large grasping hands, as others have said before, but also omnivorous. Early dinosaurs had both knifelike teeth for eating meat and flatter teeth for chewing plants.

“In the very harsh climates of the late Triassic, being a generalist is probably a clever strategy,” Mr. Baron said. “The ability to run fast and eat anything and grasp with the hands is what gave dinosaurs their advantage.”

A critical stage in human evolution was walking upright, which freed the hands for grasping tools and weapons. “The parallels with human evolution are very noticeable and make you wonder what they could have achieved,” Mr. Baron said. “Toward the end, certain groups like the velociraptors were starting to get intelligent.”

The new tree implies that dinosaurs emerged some 247 million years ago, a little earlier than previous estimates, and that their origin may not have been in South America, where several very early dinosaurs have been found. Some species that could have shared a common ancestor with the first dinosaur have been found in places now part of the Northern Hemisphere, such as Saltopus elginensis, a small dinosaurlike creature found in Scotland. But Dr. Norman said present sampling did not allow any region to be identified as the dinosaurs’ place of origin, only that the Northern Hemisphere was just as likely as the Southern.

The proposed new family tree of dinosaurs has a lot of statistical support, Dr. Norman said. “That doesn’t mean it’s right, just that it’s the best we can do with the data we’ve got at the moment,” he said.