Scientists thought they had the dinosaur family tree figured out. But new evidence is casting doubt on exactly how dinosaurs evolved.
When Steve Brusatte heard that a 26-year-old PhD student had radically redrawn the dinosaur family tree – ripping up 130 years of scientific orthodoxy in the process – his first thought was that it must be a joke.
“I thought some crackpot has some new theory and everyone wants to know about it,” he says, recalling how journalists asking for his thoughts on the scientific paper clogged his inbox before he’d even had a chance to read it.
It wasn’t a joke. The paper, published in March 2017, took centre stageon the cover of Nature, one of the world’s most influential and widely-read scientific journals. For Brusatte, who has spent much of his career studying the evolution of early dinosaurs, it seemed to come from nowhere. “It was quite a shocking, iconoclastic article,” Brusatte says. “We were caught off-guard.”
The paper overturned one of the most fundamental things that we thought we knew about dinosaurs – that they split neatly into two groups. This is dinosaur 101. The first group, the Ornithischia, which means ‘bird-hipped’ and includes the Stegosaurus, Triceratops and Iguanodon. The second group is called the Saurischia, meaning ‘lizard-hipped’, and includes predatory dinosaurs (therapods) such as the Tyrannosaurus rex and Velociraptor as well as gigantic herbivorous dinosaurs (sauropodomorphs) including the Diplodocus and Argentinosaurus.
The classification of dinosaurs into either bird-hipped or lizard-hipped varieties goes way back to 1887, when British paleontologist Harry Seely came up with the idea as a way of categorising recent fossil finds. “That’s pretty much stood to the modern day – over the last 30 years or so that basic decision has been held up by actual analysis,” says Brusatte.
The Nature paper turned this categorisation on its head. It ripped the therapods away from the lizard-hipped category and lumped them in with the bird-hipped dinosaurs, creating a entirely new category called Ornithoscelida. The massive herbivorous dinosaurs now constituted a category all of their own, whereas before they were a sub-category of the lizard-hipped dinosaurs.
It’s hard to overstate how big a deal this is in the dinosaur world, says Paul Barrett, one of the paper’s co-authors. “It would be like looking at the evolutionary tree of mammals now and saying that we think dogs and cats aren’t that closely related, and dogs are more closely related to monkeys,” he says. “It effectively ripped in half one of the major groups that had been accepted for a long time and regrouped those.”
This split goes all the way back to the unknown common ancestor of all dinosaurs – thought to have existed around 247 million years ago – and rewrites the family tree of every subsequent dinosaur. Possibly.
N not every paleontologist is so sure that there’s enough evidence out there to really rewrite the dinosaur history books. After seeing the paper, Brusatte assembled an international team of early dinosaur experts to see if the data really supported the controversial claims.
“We were all very sceptical – there were some points in the paper that we were pretty dubious of,” he says. Now Brusatte and his team have written a reply – published in Nature – that argues against the findings of the earlier paper. Tweak a few of the results, Brusatte’s work demonstrates, and the new dinosaur categorisation almost completely falls apart.
Brusatte’s reanalysis doesn’t completely support the old categorisation of dinosaurs either – his results suggest it’s only slightly more likely than the new controversial theory. It even raised the possibility that there was a third way of reorganising the tree, that left the therapods, big predatory dinosaurs like T. Rex, all on their own. “All we’ve done is inject some more uncertainty into the thing,” he says. “This is the start of what I imagine will be a long period of debate among scientists.” The battle lines over the dinosaur family tree are finally starting to take shape.
Matthew Baron, the lead author of the original Nature paper, isn’t surprised that his theory is starting to draw flak from other researchers. “We kept it a really good secret. No one really saw it coming,” he says, “It was such a fundamental idea and we challenged it for the first time.” Brusatte disputes the notion that the theory has gone unchallenged for 130 years – he says there are around 50 to 60 modern analyses that have backed-up Seely’s original categorisation.
But Baron argues that some of the researchers who disagree with his theory are unwilling to accept that the long-established categorisation might not be correct. “[Brusatte’s group] assembled a group of people that were generally against the idea,” he says, and edited the original dataset based on their own interpretations of the original fossils.
For Max Langer, the lead author of the reply disputing Baron’s original findings, this dataset is absolutely central to his argument. “When you are going to make such a huge claim you have to have a solid dataset,” he says. There simply isn’t the evidence at the moment to overturn the well-established theory. The idea is just to say that this new hypothesis is not the final world, so let’s just think about this further.”
When Langer and Baron dispute evidence, what they’re really talking about are extremely early dinosaur fossils from the Triassic period, between 200 and 250 million years ago, tens of millions of years before dinosaurs really started to dominate the Earth. The evidence base for this period is tiny. Langer estimates that we only have fossils from around one or two per cent of creatures that existed at that time. “So every fossil has the potential to really change ideas,” he says.
Paleontologists look at very specific features on dinosaur fossils to decide how a particular animal should be categorised. This might mean counting the number of neck vertebrae, working out if it has an extra bone in its leg, or deciding whether a tiny part of the bone is big or not.
The problem is that a lot of this is ultimately subjective. First, paleontologists need to agree what features are important for categorising dinosaurs and then they need to rate them. Often, this means that a dataset is actually the product of a few paleontologists’ subjective take on fossils. “To get everyone to sit down and agree everything feature by feature would take decades,” says Brusatte.
For his study, Baron and his team visited fossil collections in South Africa, Argentina, China, America and elsewhere, analysing hundreds of features across dozens of animals. He re-analysed these fossils, looking for features that might provide clues about how they should be categorised. Running this analysis through an algorithm that looks for connections between species led Baron to the conclusion that the dinosaur family tree needed to be drastically rewritten.
Sometimes an entire theory can hinge on an analysis of just a handful of fossils from a single dinosaur. In his response to the criticisms of his study, Baron argues that tweaking the figures for a single dinosaur, Pisanosaurus, caused Langer and Brusatte’s counter-theory to completely unravel. As palaeontologists analyse and reanalyse existing fossils, it seems clear that if you’re looking for evidence to support your theory of choice, you’re in with a good chance of finding it.
Despite the ongoing academic tussle, early dinosaur experts agree on one thing: the debate will only be settled when we find enough early fossils to fill in the existing gaps in the dinosaur family tree. The hunt is on for what Brusatte described as the “Rosetta Stone” fossil, a common ancestor that blends features of two of the groups of dinosaurs, but not the other. The race to uncover the true origins of the dinosaurs is only just getting started.
Source: Wired
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