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HARRY SEELEY looked like your typical Victorian gentleman: neatly trimmed beard, sharp side parting, smart suit. But he was a killer. In 1887, he destroyed the dinosaurs. The London intelligentsia were abuzz with excitement over the weird and wonderful ancient giants, but Seeley was having none of it. He looked at the fossil bones and reached a radical conclusion: technically, he said, there was no such thing as a dinosaur.

Seeley was eventually – mostly – overruled. But a study published last year is casting a fresh shadow over the awe-inspiring beasts of prehistoric Earth. It suggests we have completely misunderstood why they ruled the continents for tens of millions of years – and even what creatures qualify to be part of the dino-club in the first place.

The tale began in the early 1840s, a couple of kilometres east of London’s British Museum, when a leading scientific celebrity walked into a private collection on Aldersgate Street. Unlike many gentlemen scholars of the time, Richard Owen rose to the pinnacle of British science from the humblest of roots. His circle of friends included the royal family and Charles Dickens. Yet, for the most part, history remembers him in a different light.

“It is astonishing with what an intense feeling of hatred Owen is regarded by the majority of his contemporaries,” wrote biologist Thomas Huxley, whose intellectual fights with Owen gripped a nation. Owen is said to have been arrogant, spiteful and ruthlessly vengeful. He gained a reputation for stealing ideas and even specimens from his peers and, according to Julian Hume, who works at the Natural History Museum in London, was quite prepared to resort to blackmail to get his way.

For all his scoundrel qualities, Owen was instrumental in recognising dinosaurs as unique. In Aldersgate Street, he set eyes on an iguanodon fossil, one of three enormous prehistoric beasts – alongside megalosaurus and hylaeosaurus – that had been dug out of English soil. In a flash of brilliance, Owen recognised that the trio had several unusual features in common, particularly in the structure of a lower back bone called the sacrum. This convinced him that they were unlike anything alive today and closely related to one another. Owen called them dinosaurs.

In the years that followed, interest in dinosaurs exploded. More species were unearthed, showing that they had come in all shapes and sizes, from large, vicious-looking, two-legged predators to extraordinary armoured tank-like herbivores. Some scientists began to sketch out simple family trees showing how the different species might relate to one another.

That was when Seeley arrived on the scene. Dinosaurs evidently fascinated Seeley, and as he studied their anatomy he became increasingly convinced that his peers had missed something significant. Dinosaur hips, he noted, seemed to come in two flavours, with one bone in the pelvis that either pointed backwards, like in birds, or forwards like in lizards. “There is as marked a difference between these two pelvic types as can be found in any part of the animal kingdom,” he told the Royal Society in London in 1887. According to Seeley, this meant the two different groups of animals had been mistakenly lumped together under the same label. “Dinosauria has no existence as a natural group of animals,” he concluded, some 45 years after Owen had revealed dinosaurs to the world.

Astonishingly, Seeley’s dino-slaying became scientific orthodoxy. For much of the 20th century, the consensus was that dinosaurs weren’t really a thing. Sure, a number of giant creatures had once roamed the continents, but palaeontologists believed they didn’t all fit into one group like insects or mammals do. They continued to use the term “dinosaur” in an informal sense, but it lacked any real biological meaning.

Hips don’t lie

Instead, the ancient beasts were split into two groups, depending on the shape of their hips (see “Split at the hip”). They were either saurischians, meaning lizard-hipped – although these were unrelated to lizards – or ornithischians, meaning bird-hipped – but again with no relation to birds.

This split had profound consequences for the way 20th-century researchers thought about the rise of the ancient giants. As more and more fossils came to light, it gradually became clear that they had been incredibly diverse and ecologically successful for tens of millions of years. That sort of success can just boil down to luck, but if dinosaurs emerged from two independent rootstocks, it would imply that both groups got lucky at exactly the same time – which seemed unlikely.

Some researchers argued that this spectacular rise to dominance occurred because, despite being unrelated, saurischians and ornithischians shared a trait or traits that made them inherently superior. They all walked on vertical legs with their belly held off the ground, for instance. Perhaps they converged on this body plan independently, and it was so much better than anything else evolution had come up with that both groups then prospered.

It was only in the mid-1980s – a mere seven years before Jurassic Park roared onto the screen – that opinion swung the other way and dinosaurs were reinstated as a scientific fact.

According to palaeontologist Michael Benton at the University of Bristol, UK, the turning point came in 1984 at a conference in Tübingen, Germany. By then, a more rigorous method for analysing how different species related to each other had gained momentum. Cladistics took into account the full range of differences and similarities between animals to group them according to their shared evolutionary history. Although there were clearly two types of dinosaur hips, cladistics recognised that there were many other parts of the giant skeletons that looked very similar. For instance, within the hip socket of both “lizard-hipped” and “bird-hipped” animals, there was a distinctive gap where the bones of the pelvis didn’t quite meet. Cladistics allowed all of these differences and similarities to be balanced against each other.

In Tübingen, Benton and a few other groups presented the results of their cladistic analyses of dinosaur evolution. Collectively, they convinced the community that the giant beasts of the Jurassic and Cretaceous all descended from a common ancestor. Owen, they said, had been right all along. The results also showed that all modern birds are in fact living dinosaurs. Two years later, the first of those studies was published and dinosaurs were officially reborn as a genuine biological group. With that, researchers could get a new perspective on the question of why they had been so successful. Some said it was partly down to luck after all: for millions of years they seem to have been relatively minor players, only graduating to the big league in the aftermath of extinction events.

Then again, to flourish in challenging post-extinction times, they must have had some traits working in their favour. That probably didn’t include walking on vertical legs with bellies aloft because it was becoming clear that many contemporaries of the dinosaurs did that too. The cladistic studies came to the rescue. Although the consensus was that all dinosaurs had evolved from one species, the studies strongly suggested that those first dinosaurs soon divided into saurischians and ornithischians – Seeley’s idea. If a feature showed up in both groups, it might well have come from the common ancestor of all dinosaurs.

“Dinosaurs were reborn just seven years before Jurassic Park came out”

One feature fitting the bill was primitive, feather-like structures. Many researchers think these offered insulation long before they were used for flight. It is possible that the first dinosaurs benefitted from the warmth that proto-feathers provided, which gave them an evolutionary edge over other animals.

Such insights started to provide tentative answers to the central “where, when and why” questions of dinosaur evolution. Then, in March 2017, a trio of British palaeontologists once more rewrote all of dinosaur history. Armed with data on 450 anatomical features for 74 of the earliest known dinos and dino-like species, Matthew Baron and David Norman at the University of Cambridge, and Paul Barrett at the Natural History Museum, ran . What came out was a family tree unlike any seen before. And while it didn’t quite slay the dinosaurs, it had equally heartbreaking consequences for one much-loved giant. In the new analysis, the long-necked diplodocus and its sauropodomorph relatives fell off the dinosaur evolutionary tree entirely.

Their ejection was down to how we define what a dinosaur is. To qualify, animals must have evolved from the common ancestor of two dinosaurs: triceratops, and the modern house sparrow. According to Baron’s family tree, the sauropodomorphs didn’t fit the bill.

Mindful that some might find the eviction of diplodocus too sad to bear, Baron has suggested a solution: redefine dinosaurs as anything that evolved from the common ancestor that gave rise not just to triceratops and the house sparrow, but also diplodocus. “I didn’t think people would thank me for suggesting diplodocus wasn’t a dinosaur,” he says.

His reshuffle also has implications for what the earliest dinosaurs looked like, and potentially why they were so successful. We haven’t yet found a species of sauropodomorph with proto-feathers, meaning they could be a feature not present in the earliest dinosaurs. Instead, the new tree suggests that the earliest dinosaurs were omnivores that stood on two legs with “grasping” hands. If you think that sounds familiar, you’re not alone. “We’ve noticed the parallels with our human ancestors,” says Baron. “Obviously, there are huge differences in the way humans and dinosaurs achieved success. But for both, being upright with grasping hands was probably extremely useful.”

Being omnivorous may also have offered advantages for both groups. Hominins evolved from forest-dwelling primates, and some believe that being able to find food in the wide-open savannahs may have helped them spread to new environments. Could something similar have happened with dinosaurs? “All of this is basically arm-waving, but in the Triassic, when the environment was really harsh, there were huge inland deserts,” says Baron. “Not being too picky about what you ate was probably quite advantageous.”

Another way of seeing it is to say that the special je ne sais quoi that dinosaurs had might just have been a general adaptability rather than crucial specialisations that pigeonholed them to only being able to survive in specific ecosystems.

It is fair to say that the entire dinosaur research community sat up in their seats when Baron and his team published their study. Darren Naish at the University of Southampton, UK, immediately hailed the work as a game changer, destined to leave a lasting mark on the history of dinosaur research – perhaps one as significant as the one Seeley made back in 1887. Others think it is too early to abandon long-standing ideas about dinosaur evolution. Benton was part of a team, led by Max Langer at the University of Sao Paulo in Brazil, that spent a chunk of last year reassessing the results. They went through Baron’s data set looking for – and correcting – what they called errors that could explain why the exercise had supported such a drastically different dinosaur family tree.

. The headline message was broadly that the revised data set supported the family tree of the 1980s after all. Panic over. But a cladistic study on this scale rarely gives you just one possible answer. Among the options was a tree that was marginally less consistent with the data, but not in a statistically significant way – and it contained Baron’s new arrangement, with diplodocus and its kin back in exile. “That’s remarkable given they changed a lot of data,” says Baron. “I think they were really pushing to recover the old model.” He believes that confidence in that model has now been seriously dented.

If anything, the November analysis injected even more uncertainty into dino science. It suggested a third possible tree, one in which a group called the theropods – including the iconic T. rex – is isolated from the other dinosaurs. “That was a huge surprise,” says Baron. “Any one of the three arrangements between sauropodomorphs, theropods and ornithischians could be correct.”

With so much uncertainty, it is even harder to work out what the earliest dinosaurs looked like, what made them so successful – or indeed what species qualify as part of the club. Langer and his collaborators aren’t prepared to give up on the model of the 1980s. But even they concede that Baron’s research has shaken up the field, making us question assumptions we thought were rock solid.

This article appeared in print under the headline “When is a Dinosaur not a Dinosaur?”