When you come for the king, you best not miss – particularly if the king in question is a 9-tonne dinosaur with the biggest teeth of any known land predator in history. “Tyrannosaurus rex: the tyrant lizard king. That name is so cool, and it’s just developed tremendous loyalty,” says Greg Paul, an independent dinosaur researcher based in Maryland. “There’s even a rock band named for the animal.”

Decades of settled science suggest the famous dinosaur was so formidable that it had a stranglehold on its ecosystem. It wasn’t just an apex predator; it was the apex predator of its time, having out-muscled everything else. Late last year, however, a small number of palaeontologists attempted to cut T. rex down to size. Based on incredible new fossils, they argued that the so-called king actually coexisted with at least two other tyrannosaurs – meaning it was just one hunter among several that stalked North America in the dying days of the Mesozoic Era.

When these scientists came for the king, they didn’t miss. Just months after they made their bold suggestion, the evidence has won over most dinosaur researchers. “Many of us have changed our minds,” says Steve Brusatte at the University of Edinburgh, UK.

It is rare that opinions on a high-profile dinosaur change so rapidly and so dramatically. The fact that they have comes with profound implications. Adding more tyrannosaurs into the mix suggests we may need to completely rethink the way that dinosaur ecosystems were organised – and perhaps even reassess how and why the dinosaur-dominated world came crashing down.

The king of the dinosaurs?

T. rex richly deserves its fearsome reputation. When it was named in 1905, it was by far the largest known predatory dinosaur. We now know that a few others grew as large, but none had quite the same majesty. “It’s got this really wide skull and enormous teeth,” says Scott Persons at the South Carolina State Museum. “T. rex is still king when it comes to biting power.”

For palaeontologists, though, the awe associated with this animal isn’t just about its physical prowess. Most have long suspected that it had a substantial influence over its ecosystem. T. rex lived across a large swathe of North America between about 69 million and 66 million years ago, with many fossils coming from a sequence of rocks with a suitably forbidding name: the Hell Creek Formation in Montana. These rocks contain fossils of a variety of plant-eating dinosaur species. But for years, it has been widely accepted that there were far fewer species of predatory dinosaur thanks to T. rex. The idea is that this killing machine was so effective that it outcompeted almost all other meat-eating species. Smaller predators were squeezed out by juvenile T. rex, medium-sized predators couldn’t cope with adolescent T. rex, and no gigantic predator could survive alongside the adults. “There was this assumption that in T. rex‘s ecosystem things were different,” says Lindsay Zanno at North Carolina State University.

This view became dominant despite evidence suggesting otherwise. In the 1940s, researchers found a skull in the Hell Creek Formation that came from a polar bear-sized carnivorous dinosaur. Most researchers assumed that the skull belonged to a young T. rex. But in the late 1980s, Robert Bakker – then at the University of Colorado – and his colleagues pointed out problems with that conclusion.

The skull – now known as the Cleveland skull because it is housed in a museum in the Ohio city – had 30 teeth in its upper jaw, whereas a fully grown T. rex typically had 24. What’s more, the Cleveland skull’s teeth were thin and blade-like. That doesn’t compare well to those of an adult T. rex, which are stouter – less steak knife, more killer banana. As such, Bakker’s team suggested the Cleveland skull belonged to a smaller predator that lived alongside T. rex, which they proposed naming Nanotyrannus lancensis.

Most researchers rejected this idea. Instead, they continued to see the Cleveland skull as a young T. rex, concluding that the species changed the number and shape of its teeth as it grew. Even when Nanotyrannus advocates pointed out that none of T. rex’s close relatives lost teeth as they grew, most researchers stuck with the young T. rex idea. “Part of me wonders if the fame surrounding Tyrannosaurus rex infiltrated our minds a little bit,” says Zanno. “Even as scientists, we thought: this is such an amazing animal that it does something completely different.”

The young T. rex hypothesis

The young T. rex idea received a major boost in a 2020 study. By then, fossil hunters had found additional remains of small tyrannosaurs in the Hell Creek Formation. Holly Woodward at Oklahoma State University, together with Zanno and other colleagues, analysed the remains of two partial skeletons nicknamed Jane and Petey.

Both skeletons include limb bones, which gave Woodward’s team a new way to test the Nanotyrannus idea. The crucial point here is that limb bones add rings as they grow, a little like tree trunks. By physically slicing through a limb bone and counting the rings under a microscope, researchers can estimate an animal’s age. Moreover, you can also assess its maturity, because growth rates slow as animals reach their adult size, and it is well established from extant animals that the growth rings then become more tightly spaced.

When the researchers looked at Jane and Petey’s bones, they discovered that both were between 13 and 15 years old and growing rapidly. This meant neither could be a fully grown Nanotyrannus. Instead, both seemed to be young T. rex, which we know didn’t reach full size until their 30s. For many researchers, the study was another nail in the coffin for Nanotyrannus. But waiting in the wings was another fossil that would change almost everyone’s minds.

In 2006, fossil hunters digging through the Hell Creek Formation unearthed the exceptionally well-preserved remains of a small tyrannosaur and a small Triceratops, which looked as if they were locked in combat. Now known as the Dueling Dinosaurs, the fossils were, by law, the property of the landowners, who put them up for auction in 2013. They didn’t sell, and commercial interest seemed to fade. A few years later, Zanno convinced the owners that the Dueling Dinosaurs should be in a public collection. The North Carolina Museum of Natural Sciences, where Zanno conducts some of her research, began a campaign to raise funds to purchase the fossils, reportedly about $6 million.

By 2022, the Dueling Dinosaurs had arrived at the museum. Zanno and her colleague James Napoli at Stony Brook University in New York state began analysing the small tyrannosaur, which has been nicknamed Manteo. Zanno was confident they would confirm that Manteo was a young T. rex. “That was the mainstream consensus,” she says. But the two researchers soon began to have grave doubts.

It wasn’t just that the teeth didn’t fit; neither did the rest of the skeleton. Manteo had an estimated weight of about 700 kilograms, less than a tenth the weight of a fully grown T. rex. Astonishingly, however, its arms were slightly longer than those of an 8-tonne adult T. rex. It also had more bones in its tail than adult T. rex. And, the clincher, the growth rings inside its leg bones indicated that Manteo was mature at the time of its death. “It was basically fully grown, and so that made it impossible for this to be a juvenile T. rex,” says Napoli.

There was one more surprise. Zanno and Napoli compared Manteo with other small tyrannosaurs to decide which it resembled. There were clear similarities with the Cleveland skull – but Manteo also looked a lot like Jane, even though the 2020 study had indicated Jane was still growing, and would have ended up substantially larger than Manteo. The easiest solution was to place the fossils in distinct species: Manteo and the Cleveland skull in Nanotyrannus lancensis, and Jane in a new species that the researchers named Nanotyrannus lethaeus (see diagram below). We had thought there was just one tyrannosaur species in the Hell Creek Formation – suddenly, there were three.

Zanno and Napoli published their results in October last year, and the palaeontological community almost instantly dropped its opposition to Nanotyrannus. “For years, many of us had been saying that we’re sceptical of Nanotyrannus, and we’re sceptical because nobody has found an adult,” says Brusatte. Manteo was that missing adult.

Incredibly, it was joined by another adult Nanotyrannus just weeks later, one that we have actually met already. The Nanotyrannus story had begun with the Cleveland skull and its strange teeth. But skulls, unlike leg bones, are difficult to age, so it was impossible to tell whether the Cleveland skull belonged to a juvenile or mature tyrannosaur. Or so we thought.

Then Caitlin Colleary at the Cleveland Museum of Natural History had a good idea. She realised that the skull preserved a tiny throat bone called the hyoid that, being tube-shaped like a leg bone, might contain growth rings. She encouraged her colleague Christopher Griffin at Princeton University to investigate. The work not only confirmed that growth rings are present, but also suggested that the Cleveland skull came from a small tyrannosaur that was mature when it died, a result that the pair and their colleagues published in December. “We were both surprised,” says Griffin. “Other people had said it was a juvenile, and we’d assumed it probably was.”

There are still some dissenters. Woodward says we know little about how the hyoid grows. “It makes me very cautious to use that bone as an indicator of maturity,” she says. Thomas Carr at Carthage College, Wisconsin, is even more sceptical. He is one of the strongest advocates for the idea that all the small tyrannosaurs from the Hell Creek Formation are young T. rex, and he thinks even Manteo is best viewed that way. Carr says T. rex and its closest relatives underwent distinctive changes as they became adults. For instance, the bones of the snout became rougher. “It looks like candle wax has dripped on them,” he says. Manteo and the other Nanotyrannus specimens lack those features. Carr says that “it doesn’t make sense” for the animals to be adults.

But Napoli thinks he can explain this. When he and Zanno ran an analysis to reconstruct an evolutionary tree of T. rex and a few dozen of its relatives, they found that Nanotyrannus isn’t a close relative of T. rex after all. Although it lived alongside T. rex, Nanotyrannus was actually a far more primitive tyrannosaur. As such, it might not have undergone the changes typical of T. rex and its ilk.

Meanwhile, although Woodward is sceptical of some of the new evidence, her latest research arguably strengthens the Nanotyrannus hypothesis. A few weeks ago, her team published a study detailing their latest findings about T. rex growth rates. Of the 17 individuals the researchers analysed, 15 of them seemed to have grown at a similar rate. The final two were the small tyrannosaurs Jane and Petey, which both seem to have grown more slowly. Woodward’s team said this might indicate that neither tyrannosaur belonged in T. rex after all.

Remarkably, the tyrant lizard king may yet be broken up into even more species – because the Nanotyrannus debate isn’t the only one that has been bubbling away for the past 20 years. A few researchers have argued that we also need to rethink whether all of the large T. rex skeletons truly belong to a single species. In 2022, this debate went public when Greg Paul, Scott Persons and their colleague Jay Van Raalte, then at the College of Charleston in South Carolina, published a study arguing there were in fact three species of large tyrannosaur in the Hell Creek Formation: an ancestral form that they named Tyrannosaurus imperator, which gave rise to two species that lived side by side – a relatively sleek form that they named Tyrannosaurus regina and a bulkier predator, which retained the name T. rex.

The proposal didn’t go down well. Within months, a number of researchers – including Carr, Zanno, Napoli and Brusatte – wrote a response arguing the fossil evidence simply didn’t support the idea of three large tyrannosaurs. But in their Manteo study, Zanno and Napoli reassessed that position. If Nanotyrannus had been hiding in plain sight for so many years, might not additional species of giant tyrannosaur be hiding, too? “We are thinking of collecting data to look at this,” says Zanno – although it will be years before they reach any conclusions.

All of this has implications that could be as huge as T. rex itself. It is now understood that large dinosaurs were just as energetic and colourful as modern animals, but palaeontologists have been reluctant to imagine that dinosaur ecosystems were as diverse as today’s. This is because most large dinosaur species were thought to follow the envisioned T. rex pattern to some extent: they laid large numbers of eggs, creating armies of hungry hatchlings that should have provided tough competition for smaller species. But if T. rex, the poster child for this model, coexisted with other tyrannosaurs, a reassessment is overdue. “Maybe ecosystems weren’t that different from today,” says Napoli. “But there’s a beauty in that because it means we should expect a taxonomically diverse array of species, like modern ecosystems.”

Ancient ecosystems

Just as significant is that the Hell Creek Formation represents one of the very last dinosaur-dominated ecosystems, one that was still in full swing on the day an asteroid triggered a mass extinction. Some researchers argue that the number of dinosaur species had begun declining millions of years earlier, contributing to the extinction. Confirmation that Nanotyrannus was real adds to evidence that dinosaurs were, instead, thriving on impact day – which, wrote Zanno and Napoli in their Manteo study, suggests we need to critically re-evaluate decades of research on the mass extinction. “Predators are really important signals of ecosystem health and stability,” says Zanno. “So even though Nanotyrannus is just one genus, its position in the food chain tells us a lot about the broader stability of the ecosystem before the asteroid struck.”

Most exciting of all, the reassessment of T. rex is only just getting started. Breaking the iconic dinosaur into a series of different species raises new questions. Exactly how, for instance, did the various tyrannosaurs carve up the ancient landscape between themselves? We may get some answers when Zanno and Napoli examine Manteo’s teeth, because the chemical composition of tooth enamel is known to reflect diet.

“Did Nanotyrannus and T. rex diverge in their behaviours to survive together? It’s a fascinating question,” says Zanno. “But until a few months ago, nobody even realised it was one we had to tackle.”

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