The dinosaur family tree may need to be radically rewritten — and even uprooted and replanted elsewhere, a new analysis of about 75 different species shows.
The findings, published in the journal Nature, hint that dinosaurs may have originated in the northern hemisphere rather than the southern, and could upend an understanding of dinosaur evolution that has gone largely unchallenged for some 130 years.
"We might all have to rearrange our mental furniture," Kevin Padian, a UC Berkeley professor of integrative biology who was not involved in the study, wrote in a commentary.
To figure out the family tree of long-extinct animals with fossilized bones, scientists have to carefully study their shared features to see which ancient species were related, and what their common ancestors looked like. It's tricky and painstaking work, especially since similar physical features can sometimes emerge independently in two different species — a phenomenon known as convergent evolution. (Wings emerged separately in birds and bats, for example.)
Historically, dinosaurs have largely been categorized according to the shape of their pelvic bones — grouped either into "bird-hipped" dinosaurs such as Ornithischia, a group that includes stegosaurus, triceratops and duckbilled hadrosaurs, or into the "reptile-hipped" Saurischia. Saurischia is split further into theropods (including Tyrannosaurus rex and all birds) and Sauropodomorphs (which includes sauropods such as long-necked Apatosaurus).
But the fossil record hasn't been entirely supportive on that point, Padian pointed out. The black-sheep ornithischians, for example, have brought a bagful of uncomfortable issues to family get-togethers.
"In this dinosaur family-grouping game, the joker in the pack has always been Ornithischia," he wrote. "From their earliest appearance, they have been weird. They have a strange additional chin bone, their incisor teeth are smaller than those of other dinosaurs, their cheek teeth are regular and closely spaced like molars, they have beaks, and their hip bones are enigmatically organized. Also, unlike nearly all the other dinosaurs except Sauropoda, they are clearly herbivores, as their teeth and jaws show."
Stranger still, he pointed out, was that ornithischians didn't start diversifying in a major way until about 200 million years ago, around the Late Triassic or Early Jurassic period — even though they should have had plenty of time to do so.
That's not to mention that the "bird-hipped" ornithischians actually show an uncanny resemblance to "lizard-hipped" theropods (such as T. rex and velociraptor). This caught the eye of lead author Matthew Baron, a paleontology doctorate student at the University of Cambridge who wondered if that 19th century definition could be wrong.
"When I was looking at them, I noticed that many of the earliest ones had features that were very similar to those of theropods (meat eaters). This was odd to me as the old model for how we classify dinosaurs said that these two groups were distantly related," Baron said in an email.
Other papers classifying early dinosaur relationships also did not include "an adequate sample of early ornithischians" as a point of comparison, the study authors wrote.
"Because of their weird anatomy and rarity," Baron said, "people have often overlooked Ornithischia when asking questions about the earliest dinosaurs."
For this paper, Baron and his colleagues decided to take another look at the relationships between these dinosaur groups — one that threw out the assumptions that guided those previous groupings.
Other studies that looked at ornithischians, for example, tended to analyze what they already saw as shared traits of the group. But here, the scientists compared about 75 species using a whopping 457 different characteristics. Some of those traits would previously have been noted as points of comparison only in theropods or sauropodomorphs, but the scientists included ornithischians in their analysis of those traits, too.
"This is roughly 35,000 individual data points that had to be input by hand, based upon my observations," Baron said.
The researchers' analysis of the dinosaurs' skulls, teeth and skeletal structures appears to result in a significant reshuffling of dinosaur relationships. Rather than being an earlier, "basal" group, ornithischians are the sister group to theropods, together in a new clade the authors called Ornithoscelida. (The term itself is not new: It was coined by English biologist Thomas Henry Huxley around 1870.) The group Saurischia keeps sauropodomorphs but loses theropods in favor of herrerasaurids, which could never find an agreed-upon home in the traditional family tree.
"The results of this study challenge more than a century of dogma and recover an unexpected tree topology that necessitates fundamental reassessment of current hypotheses concerning early dinosaur evolution, palaeoecology and palaeobiology," the study authors wrote.
If other scientists test the authors' hypothesis and also find that the family tree is in for major pruning and grafting, the findings could have major implications for our understanding of dinosaur evolution.
For example, Padian pointed out, it's now possible that ornithischians may be descended from theropods (though he was quick to point out that the study authors didn't go so far as to say that).
"Also puzzling is Baron and colleagues' finding that the primitive-looking herrerasaurids, from the South American Triassic, are the sister group to the sauropods," Padian added. "This link is not strongly supported, but it is intriguing. Herrerasaurids were carnivores, and they are usually linked to or included with the carnivorous theropods."
This suggests that the meat-eating traits in herrerasaurids and theropods (such as sharp teeth and large skulls) actually arose independently in those two lineages, rather than being a shared characteristic. That means that dinosaurs' original ancestor may not have been purely carnivorous — an idea supported by recent research, which shows that the earliest dinosaurs were probably small, omnivorous and walked on two legs.
The new family tree also means that dinosaurs may have originated not in South America (on the ancient supercontinent of Gondwana), but instead somewhere in the northern hemisphere, on what was then supercontinent Laurasia.
"We know this will be controversial," Baron said. "We are disagreeing with 130 years of consensus. Some people have expressed a desire to test our hypothesis for themselves, and we really do welcome that. We want this to start a great new debate in our field, and we hope that people test our ideas and search for new specimens and new evidence that might support or refute our finds."
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