WASHINGTON, March 12 (Reuters) - A new analysis of a large shinbone unearthed in a remote spot in northwestern New Mexico in the 1970s shows it belongs to a close relative of Tyrannosaurus rex that predated that huge meat-eating dinosaur by several million years and potentially was a direct ancestor.
The shinbone, or tibia, dates to about 74 million years ago during the Cretaceous Period. It is about 3.2 feet (96 cm) long and five inches (13 cm) wide, approximately 80% of those dimensions in the same
bone of the largest-known T. rex, the specimen called Sue displayed at the Field Museum in Chicago.
The bone's size and shape indicate it belongs to the group of meat-eating dinosaurs called tyrannosaurs. This group reached its apogee in T. rex, which first appeared roughly 67 million years ago in the twilight of the age of dinosaurs.
No other bones from this dinosaur were discovered. A single bone offers insufficient information to merit assigning a scientific name. The researchers are calling this dinosaur the Hunter Wash tyrannosaur, referencing the rock formation where the tibia was discovered by University of New Mexico students in the Bisti-De-na-zin Wilderness in the state's San Juan County.
The tibia's dimensions, according to the researchers, indicate the dinosaur weighed about 4.7 tons, approximately half as massive as Sue but about twice as massive as Daspletosaurus and Gorgosaurus, other large North American tyrannosaurs roughly contemporaneous to the Hunter Wash tyrannosaur. It would have been the largest tyrannosaur that had ever lived to that point in time.
"T. rex's enormous size is a specialization that emerges relatively late in tyrannosaur evolution. As you go back in time you'd expect to find animals that aren't quite as big, so this kind of tracks," said paleontologist Nick Longrich of the University of Bath in England, lead author of the research published on Thursday in the journal Scientific Reports.
This dinosaur may have been 35 feet (10.5 meters) long, compared to Sue's length of 40-1/2 feet (12.3 meters).
"The fossil is consistent with this species being directly ancestral to T. rex, but we'd really need a complete skull to know if it's directly ancestral or a close cousin of T. rex. It's possible it might be an early species of the genus Tyrannosaurus, but again we need more complete material," Longrich said.
A genus is a group of closely related species sharing similar traits. For example, lions and tigers are from the same genus but represent different species. There is a debate among paleontologists as to whether various Tyrannosaurus fossils deserve to be recognized as species distinct from T. rex.
"This tibia, or shin bone, matches very, very well with the shin bone of Tyrannosaurus," said paleontologist and study co-author Anthony Fiorillo, executive director of the New Mexico Museum of Natural History & Science.
"Given the remarkable similarity of the tibia in our study to that of Tyrannosaurus rex, yes, we feel they are very closely related," Fiorillo said. "We'll know more when we find more."
There were various lineages of meat-eating dinosaurs, several achieving great size. T. rex is the most massive one known. With knowledge of tyrannosaurs growing based on new fossil discoveries, the researchers took a fresh look at the tibia, stored at Fiorillo's museum.
The Hunter Wash tyrannosaur roamed a coastal floodplain alongside numerous plant-eating horned, armored and duckbilled dinosaurs.
North America at the time was split in two by a vast seaway. T. rex inhabited the landmass called Laramidia that comprised the continent's western portion. It was the apex predator, known from fossils from Laramidia's northern and southern parts.
The researchers said evidence is mounting that the genus Tyrannosaurus evolved in the southern part of Laramidia, perhaps in New Mexico, Texas or Mexico. Longrich noted, for instance, that a single tail bone dating to 69 million years ago found in Texas appears to have come from a T. rex-sized tyrannosaur.
For now, the Hunter Wash tyrannosaur is represented by one lonely bone.
"We are developing our plans to expand to search for more material," Fiorillo said.
(Reporting by Will Dunham, Editing by Rosalba O'Brien)
