A dinosaur skull, overlooked for a century, belongs to a long-clawed cousin of T. rex

A tiger-sized Isle of Wight dinosaur hunted with long claws instead of a bite; it is the first of its kind ever found in Europe.

Tyrannosaurus rex is the dinosaur most people would least want to meet. Some of its cousins hunted with long arms and curved claws instead of a crushing bite. One of them has just turned up in a museum drawer in London.

The chunk of skull had been there since 1924, and it belongs to a new species the size of a large tiger.

The new dinosaur, Invictibellatrix amberina, is the first of these clawed hunters ever found in Europe. The fossil came from a beach on the Isle of Wight, off the south coast of England. Unfortunately, that one piece of braincase is all anyone has of the dinosaur.

Cassius Morrison of UCL Earth Sciences led the study with colleagues at the Natural History Museum in London and the University of Bristol. The species belongs to a group called megaraptorans, which recent family trees put close to the tyrannosaurs or inside that family.

"It was not the apex predator of its time, but for humans this dinosaur would have been more dangerous than a T. rex," Morrison said.

"We would have been perfectly sized prey, about a third of their body weight. They definitely would have chased us," he added. "A T. rex would not have bothered, as we would have been too small."

Megaraptoran fossil bought in 1924

The Natural History Museum bought the braincase in May 1924 from Reginald Walter Hooley, an amateur collector whose fossils came to the museum after his death.

It was picked up at the foot of a cliff at Sudmoor Point, on the island's southwest coast. Around 120 million years ago, flash floods on a nearby floodplain piled wood and bones into debris beds. This is where the rock formed that held the new dinosaur fossil.

Those rocks, the Wessex Formation, have produced many named dinosaurs, Morrison told Earth.com. The different species did not all live side by side; the cast changed as time went on.

The surf rolled the fossil around before someone found it, wearing down its surfaces and erasing most of the skull. "It is a real mystery that this fossil was overlooked for so long, sitting in a drawer for nearly 100 years," Morrison said.

Scanning the dinosaur's braincase

In an interview with Earth.com, Morrison said the fossil came up in a search for something else.

"It was part of my Ph.D. thesis to examine if any British theropods had semi aquatic or aquatic adaptations within their skulls, especially the braincase," he said.

Only around five theropod braincases are known from the U.K., the new one included, he added, so it had to be examined in detail.

Researchers find it difficult to place Megaraptorans because no complete skull of one has ever been found, and the best one belongs to a juvenile.

So the team scanned the fossil with X-rays at the museum and built a 3D model of the bone, plus one of the cavity that held the brain. Then they compared both with other two-legged meat-eaters.

Features of a new dinosaur species

Together, a few features set it apart from every named dinosaur on the island. An oval opening marks where the spinal cord entered the skull, and a hourglass-shaped crest runs across the back of the skull.

Furthermore, a pair of deep pits flanks the knob that joined skull to neck.

Two separate family trees, one built from 1,759 anatomical features and another from 356, both placed the fossil among the megaraptorans.

Its skull bones had fused together, which points to an adult or near-adult rather than a youngster of a known species.

Giant dinosaurs came later

Morrison said two things surprised him. The first was simply that the fossil was a megaraptoran at all. The next related to the fact that it was old, but had some relatively modern features.

"It is potentially one of the oldest, if not the oldest megaraptoran known, but had some relatively advanced features," he told Earth.com.

Later megaraptorans grew into giants with claws up to 14 inches (35 centimeters) long. One from Argentina reached an estimated 30 to 33 feet (9 to 10 meters) and weighed as much as an elephant.

Researchers described Joaquinraptor, a megaraptoran from Patagonia, only last year.

The smallest megaraptoran

Invictibellatrix was built on a different scale. Its braincase is about the size of one from a tyrannosaur relative that weighed 440 to 660 pounds (200 to 300 kilograms).

That makes it one of the smallest megaraptorans known, and perhaps the smallest. Judging from its closest relatives, Morrison put it at roughly 10 to 13 feet (3 to 4 meters) long and 5 feet (1.5 meters) tall.

Its claws were 4 to 8 inches long, and it would have hunted small dinosaurs, lizards and mammals, he said. "It would likely have had feathers and its claws might have been about 10cm to 20cm long," he added.

The route into Gondwana

Megaraptorans are thought to have started in Asia and ended up as top predators across the southern continents. Europe lay on the likely route, yet until now none had been found there.

"This new species is the first proof we have that megaraptorans were also present in Europe and that they reached Gondwana this way rather than island-hopping further south," said co-author Charlie Scherer, a Ph.D. student at UCL.

The team fed the new species into a model of how the group spread. It draws on the family trees, fossil ages, and maps of the continents as they lay at the time.

In the result the team prefers, Invictibellatrix descended from an Asian population that reached Europe about 130 million years ago, and the group then moved on through Africa.

Morrison said the study adds "further evidence of a potential migration of Megaraptorans from the northern continents via Africa to the southern continents."

By the same result, the giant southern megaraptorans arose in South America and Africa. The Australian species branched off from there. Earlier work had argued the opposite direction, starting from Australia, and Morrison's own earlier study traced tyrannosaurs from Asia to North America.

Extinction in the north

By the Late Cretaceous, tens of millions of years later, megaraptorans had vanished from the northern continents. In contrast, their southern relatives kept growing. The authors read that absence as a regional extinction rather than a gap in the fossil record.

The authors don't say why. Shifts in climate, changes in the environment, or competition from other predators such as tyrannosaurs are possibilities they raise.

Other animals moved between Europe and Africa at the time, spinosaurs among them.

Two caveats go with this. The two family trees disagree about where the species' ancestors came from. The team's preferred answer rests on the dataset with more megaraptorans in it. And the whole species is known from one worn piece of skull.

One place to look for answers is the skull itself. Braincase features make up about 4 percent of the traits in the biggest meat-eating dinosaur family trees, and the authors argue that more of them would help settle where megaraptorans belong.

"I will be greedy and say a baby megaraptoran or a relatively complete long necked sauropod," Morrison said when asked what he would most like to find next on the Isle of Wight.

The full study was published in the journal Royal Society Open Science.

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