Rearranging the dinosaur family tree

by Mary Caperton Morton
Friday, June 16, 2017

The old (left) and new (right) dinosaur family trees. Credit: K. Cantner, AGI, after Baron et al., Nature, March 2017.

Dinosaurs have long been grouped into two major clades — Ornithischia and Saurischia — largely based on the shapes of their hips. But new phylogenetic research is shaking up the dinosaur family tree, suggesting the traditional two-branch system needs reorganizing.

Under the two-branch system, originally proposed in 1888, dinosaurs are either “bird-hipped” ornithischians or “reptile-hipped” saurischians. Ornithischians include mainly herbivores, such as stegosaurs and duckbilled hadrosaurs; the saurischians are divided into carnivorous theropods (which, despite their reptilian hips, would later evolve into birds) such as Tyrannosaurus rex, and the long-necked, long-tailed herbivorous sauropods, such as Diplodocus.

However, a new phylogenetic analysis, led by Matthew Baron of the University of Cambridge in England and London’s Natural History Museum, found that the ornithischians should be grouped with the theropods, creating a new group called the Ornithoscelida that excludes the sauropods. The regrouping helps explain why birds evolved from supposedly reptile-hipped dinosaurs: Both ornithischians and theropods eventually evolved a bird-like hip arrangement, but they did so at different times in their evolutionary history, with the theropods becoming bird-hipped later, as they transitioned into birds, the team wrote in Nature.

“The repercussions of this research are both surprising and profound,” said co-author David Norman, also of the University of Cambridge, in a statement. “The bird-hipped dinosaurs, so often considered paradoxically named because they appeared to have nothing to do with bird origins, are now firmly attached to the ancestry of living birds.”

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