Oct 13 : The partial skull of a tiger-sized predator that prowled what is now the Isle of Wight off England's southern coast about 120 million years ago is giving scientists a better understanding of an enigmatic group of meat-eating dinosaurs known for running down prey and ripping them apart with massive hand claws.

Researchers said the fossil belongs to a species they have named Invictibellatrix amberina, an early member of the lineage of Cretaceous Period carnivorous dinosaurs called megaraptorans. Invictibellatrix adds to the knowledge of skull anatomy among early megaraptorans and, being the first one known from Europe, helps show how the group spread globally.

Invictibellatrix is one of the smallest megaraptorans. Based on fossils of closely related megaraptorans, the researchers estimated that Invictibellatrix was about 16 feet (5 meters) long, with the weight of a large tiger. Its body may have been covered in filament-like feathers, they said. Megaraptorans were part of the group called theropods that spans all the meat-eating dinosaurs.

"Megaraptorans are very unique among theropods because they have relatively small and slender heads but had long and strong arms with massive hand claws as well as very lightly built skeletons," said Cassius Morrison, a postdoctoral researcher at the University of Leeds in England and lead author of the study published in the journal Royal Society Open Science.

"These characteristics imply that megaraptorans were agile hunters capable of chasing down prey and using their large arms and claws to catch them and disembowel them using their hand claws," Morrison said.

The largest-known megaraptoran, named Maip, was about 33 feet (10 meters) long, inhabiting Patagonia during the twilight of the dinosaur age, about 50 million years after Invictibellatrix lived.

Littler megaraptorans like Invictibellatrix may have hunted smaller prey like mammals, smaller reptiles and baby dinosaurs. Larger megaraptorans may have targeted bigger plant-eating dinosaurs like duckbilled hadrosaurs and long-necked sauropods, though probably not the biggest ones.

Megaraptorans hunted in the opposite way from massive predators like Tyrannosaurus rex. Tyrannosaurus employed its large and stoutly built head, enormous teeth and incredible bite strength to kill prey, but had puny arms of little use in hunting.

Invictibellatrix was not the Isle of Wight's top predator, with larger theropods around, alongside various herbivorous dinosaurs, and may have filled a role similar to a hyena today.

There is a debate among dinosaur paleontologists as to where megaraptorans fit on the theropod family tree. Despite differences in how they were built and hunted, one hypothesis is that megaraptorans were closely related to tyrannosaurs, the group that included Tyrannosaurus and others that dominated the landscapes in North America and Asia later in the Cretaceous. 'UNCONQUERED FEMALE WARRIOR' Invictibellatrix means "unconquered female warrior." Morrison noted that after humble beginnings megaraptorans developed into apex predators in South America, Australia and Antarctica, becoming "the ruling queens," while the tyrannosaurs became the "tyrant kings" in North America and Asia.

Scientists are trying to unravel the evolutionary history of megaraptorans. A leading hypothesis has megaraptorans originating in Asia before migrating through Europe into Africa and then on into South America, thriving until the dinosaurs were doomed by an asteroid strike 66 million years ago. This hypothesis was lacking fossil evidence because no megaraptorans had been known from Europe until now.

"These findings help to uncover the evolution of one of the most mysterious theropod groups," University College London doctoral student and study co-author Charlie Scherer said.

The Invictibellatrix fossil, a fragmented braincase, was discovered about a century ago and acquired by the Natural History Museum in London in 1924.

"It was left in one of the collection's drawers ever since, and no one noticed it until a couple of years ago," Scherer said.

While Tyrannosaurus may be more imposing than Invictibellatrix, the researchers noted that if a person were somehow transported back in time — like in the recent film "The End of Oak Street" — Invictibellatrix might present more of a danger.

"A smaller theropod like Invictibellatrix would likely have been a much bigger threat to a human than something like T. rex because we would likely be too small for T. rex to see as a worthwhile meal because of how massive it was and the amount of energy it would be required to catch us," Morrison said.

"But we would be the perfect size for something smaller like Invictibellatrix, as we are about a third to a quarter of its weight, making us a good hearty meal and more than just a snack. Additionally, Invictibellatrix would have the agility and maneuverability to pursue and catch a human."