The most radical shakeup of the dinosaur family tree in a century has led scientists to propose an unlikely origin for the prehistoric beasts: an obscure cat-sized creature found in Scotland.
The analysis, which has already sparked controversy in the academic world, suggests that the two basic groups into which dinosaurs have been classified for more than a century need a fundamental rethink. If proved correct, the revised version of the family tree would overthrow some of the most basic assumptions about this chapter of evolutionary history, including what the common ancestor of all dinosaurs looked like and where it came from.
Until now, many scientists have backed the view that the first dinosaurs emerged around 237 million years ago on the ancient continent known as Gondwana, that would later become the southern hemisphere, based on a host of immaculately preserved fossils from South America and Tanzania.
However, the latest analysis identifies a Scottish specimen, called the Saltopus, as the closest thing in the fossil record to what the hypothetical common ancestor might look like.
Matt Baron, the graduate student who led the three-year project at the University of Cambridge, said that while it would never be possible to pinpoint the origin of dinosaurs with certainty his findings placed the northern hemisphere into contention. “It may just be that dinosaurs originated in Scotland,” he said.
“This is obviously going to be met with some hostility from Southern American researchers,” he added.
As anticipated, the conclusions have been met with robust criticism from some rival scientists, including Max Langer, a respected palaeontologist at the University of São Paulo in Brazil.
“There’s nothing special about this guy,” he said. “Saltopus is the right place in terms of evolution but you have much better fossils that would be better candidates for such a dinosaur precursor.”
The findings also support the possibility that dinosaurs such as Stegosaurus and Triceratops, traditionally portrayed as tank-like armoured beasts, may have been feathered.
The central thesis of the new work is that a classification system, used since the 1880s and a staple of palaeontology textbooks, is wrong.
The system places dinosaur species into two distinct categories, Ornithischia and Saurischia, based on an early observation that dinosaur hip bones displayed either a lizard-like pattern (Saurischia) or a bird-like one (Ornithischia).
The Ornithischia group includes the relatives of Stegosaurus and Triceratops, while the theropods (such as T. rex), sauropods (Diplodocus) and another meat-eating group called Herrerasauridae, all fell within the lizard-hipped Saurischia group. Counterintuitively, the dinosaur group that later gave rise to modern birds (theropods) falls on the lizard branch of the family tree.
While studying Stegosaurus-like dinosaurs, Baron noticed that, aside from the difference in hip bone configuration, there were a striking number of anatomical similarities between this group and the group T. rex belonged to, including the shape of the skull, hind limbs and ankle bones.
He and colleagues at the Natural History Museum meticulously recorded thousands of anatomical traits from 75 different specimens – around 35,000 data points in total – and performed a statistical analysis designed to group the specimens according to the most likely evolutionary scenario.
The results, published in Nature, suggest that theropods should be switched over to become an off-shoot of the same branch that led to Stegosaurus and Triceratops. The revised grouping of Ornithischia and Theropoda has been named the Ornithoscelida, reviving a name originally coined by the evolutionary biologist Thomas Henry Huxley in 1870.
Brian Switek, author of My Beloved Brontosaurus, described the proposed reshuffle as “definitely a shocker”. “Whether this new family tree sticks or not will be a matter of testing,” he added. “One group of scientists has come up with what is no doubt a controversial hypothesis, and now others will see if they get the same result, or if the idea is bolstered by additional evidence.”
The revision would imply that carnivores evolved on two separate occasions and, according to the authors, strengthens the case for the initial ancestor being omnivorous.
Baron said it also raises the possibility that feathers first appeared in ancestors common to both the T. rex branch and the Stegosaurus branch of the family tree, which would be consistent with a number of fossils showing hints of feathers.
“Maybe we did have fluffy Triceratops and fluffy Stegosaurus,” said Baron. “It could be that the feathers would have been poking out between the scales, it could have been a beautiful fluffy colourful plumage ... or scales covered in downy feathers. It’s possible.”
Langer argues that, while the Saltopus might be statistically a good candidate for a common ancestor, given the patchy nature of the fossil it is a poor choice. Rather than attempting to identify the true ancestor of all dinosaurs – which can never be known – scientists’ aim is to find an animal that is a decent approximation of the general form and traits displayed by that ancestor we know must have existed.
The fossil, found in a Lossiemouth quarry, comprises a pair of legs, some hip bones, and vertebrae, all of which have been badly squashed.
“It looks like a chicken carcass after a Sunday roast,” Baron acknowledges.
However, Jakob Vinther, a palaeobiologist at the University of Bristol who is not an author on the paper but who has carried out an independent analysis, said the conclusions, including the Saltopus finding, were “robust”. “When we do these other analyses we sometimes get very different results,” he said. “I get the same result.”
Vinther, whose background is in mollusc research, said that unlike most dinosaur scientists he was not invested in any particular result, but added: “I’ve heard a bit of murmuring already from people who are not too thrilled about this hypothesis.”
The revised tree, if accepted, would also require the formal definition of dinosaurs to be re-written. The group is defined as all the descendents of the last common ancestor of birds (Saurischia) and Triceratops (Ornithischia), which have been historically on far flung branches of the family tree. But under the new tree, these two branches are closer together, technically placing all the long-necked herbivores outside of the dinosaur family.
“We didn’t want to the those palaeontologists who told the world that Diplodocus and Brontosaurus weren’t dinosaurs,” said Baron. “We’d be like the guys who said Pluto wasn’t a planet.”