The traditional view of dinosaurs' family tree has its branches all in the wrong places, a palaeontological study argues, calling for a radical shake-up of how we view dinosaurs' evolutionary past.
Dinosaurs have historically been divided into two groups: the Ornithischia, which have bird-like hips, and the Saurischia, which have reptile-like hips. The Saurischia was divided up into the carnivorous Theropoda – including Tyrannosaurus rex – and the herbivorous Sauropoda, including Brontosaurus.
Historic family tree
The groups have been divided up this way since 19th Century University of Cambridge palaeontologist Harry Govier Seeley first categorised them that way. But now Cambridge researchers are saying that he got it wrong, and the Theropoda are actually more closely related to the Ornithischia than the Saurischia.
This reanalysis of dinosaurs' family tree was sparked by research on the earliest Ornithischian dinosaurs had features very similar to the Theropoda. Further analysis of 457 anatomical characteristics of fossils – not just hips – of a huge range of dinosaur species suggested that the best way to group them by similarity was to throw out the old system and start again.
Rejigging the family tree rearranges some of the largest and most famous dinosaurs into a group all of their own.
"It leaves the Saurischia out in the cold in a sense. They tend to be the very giant dinosaurs – the brontosaurs, the ones with very long tails – which now become a group on their own," study author David Norman told IBTimes UK.
"And therefore the tree its very base – the fundamental relationships between three major groups of dinosaurs – has completely flipped."
Origin of birds
The new system also resolves one of the long-standing paradoxes in dinosaur palaeontology. Previously the bird-hipped Ornithischia were thought not to be very closely related to the Theropoda, which were the ancestors of modern-day birds. Reshuffling the branches of the family tree has brought these two bird-hipped groups closer together.
The new arrangement will have to be probed by other palaeontologists to test whether it does better reflect the ancient family history of the dinosaurs.
"We can't stand up now and say, 'this is the truth'. The fossil record inherently isn't solid enough for that," Norman said.
"But if people test it, challenge it and in the end agree with us, then it will become a truism that the Ornithischians, with their bird-like hips, were in fact involved in the ancestry of birds."
The research is published in a paper in the journal Nature.
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