Many know the Tyrannosaurus rex, a huge land apex predator that roamed during the twilight of the age of dinosaurs.
But it was not the only unfriendly “T. rex” alive in the Cretaceous period. There was also a marine reptile, the Tylosaurus rex, named this year. The T. rex of the sea, it rivalled its land counterpart in size and ferocity.
Scientists identified Tylosaurus rex as a distinct species after a wide-ranging examination of previously discovered fossils.
While the Tyrannosaurus rex roamed land that now forms western parts of North America roughly 66 million years ago, the Tylosaurus rex (see graphic) prowled an inland sea that once split North America around 80 million years ago.
The genus Tylosaurus has been known since its fossils were discovered during the 19th century. Its earliest fossils were likely unearthed by various Native American peoples long before the 1500s.
A genus is a grouping of closely related species that share similar characteristics. For example, lions and tigers are from the same genus but are different species.
The largest Tyrannosaurus rex specimen, nicknamed Sue and held at the Field Museum of Natural History in the US city of Chicago, is about 12.3 to 12.8 metres.
The largest Tylosaurus rex specimen is even longer, at 13.2 metres. It’s nicknamed Bunker and lives at the University of Kansas Natural History Museum.
“The skull alone is as long as I am tall – 5-foot-7 inches (1.7 metres),” said Amelia Zietlow, a palaeontologist from the History Museum at the Castle in the US state of Wisconsin. She is the lead author of the study published in the Bulletin of the American Museum of Natural History.
Tylosaurus is a member of a group of marine reptiles called mosasaurs. The group evolved from land-living lizards and became apex marine predators during the final 30 million years of the age of dinosaurs.
Today’s monitor lizards are perhaps among the Tylosaurus rex’s closest living relatives. That group includes the Komodo dragon, the largest living lizard, which can grow to three metres.
Study co-author Ron Tykoski is vice-president of science and curator of vertebrate palaeontology at the Perot Museum of Nature and Science in Dallas, Texas.
He said the Tylosaurus rex would have been twice the length of today’s largest great white sharks. Most of the fossils identified as Tylosaurus rex come from north and central Texas, though Bunker came from Kansas. The holotype specimen – the fossil used to formally name a species – is at the Perot Museum in Dallas.
The new species name, meaning “king of the Tylosaurs”, was a nod to Tyrannosaurus rex. These two T. rexes were not contemporaries – of the same age and living in the same era. However, forerunners of the giant dinosaur lived at the same time as the giant mosasaur.
And as Tyrannosaurus rex could target the largest prey on land, Tylosaurus rex could do the same in the sea. After all, it was among the largest of the mosasaurs.
“Strong muscles and serrated teeth suggest that Tylosaurus rex was preying on other large animals, likely other marine reptiles and large fish, and ripping them to pieces,” Zietlow from the History Museum at the Castle said.
“Tylosaurus species in general were not picky eaters – their teeth are not super specialised towards any one kind of prey besides ‘other animals.’”
What is a mosasaur?
Mosasaurs were large aquatic lizards that lived in oceans during the late Cretaceous period, between 94 and 66 million years ago. They were excellent swimmers and fed on fish and ammonoids, molluscs that were abundant in the Cretaceous seas.
They competed with other marine reptiles, such as plesiosaurs and ichthyosaurs, for food. These giant sea reptiles died out during a mass extinction caused by a giant asteroid that struck Earth, creating the famous Chicxulub crater in present-day Mexico.
