May 10, 2011

Young Tyrannosaurs Relied On Agility To Catch Prey

A new study from an international team of scientists suggests juvenile and adult predatory dinosaurs had a range of hunting and feeding strategies.

While adult tyrannosaurs wielded size and power to take down large prey, juveniles relied on their agility to hunt smaller game.

"It's one of the secrets of success for tyrannosaurs"”the different age groups weren't competing with each other for food because their diets shifted as they grew," said Ohio University paleontologist Lawrence Witmer.

The scientists -- from Japan, Mongolia, and the United States -- analyzed the youngest and most-complete known skull from the Tyrannosaur family. Their research offers a new view of the growth and feeding strategies of these predators. The 70-million-year-old skull comes from a juvenile Mongolian species known as Tarbosaurus bataar, the closest known relative of Tyrannosaurus rex.

Analysis of the skull revealed changes in skull structure that suggest that young tyrannosaurs had a much different lifestyle than their adult counterparts.

"We knew that adult Tarbosaurus were a lot like T. rex," said lead author Takanobu Tsuihiji, a former Ohio University postdoctoral fellow who is now a postdoctoral researcher at the National Museum of Nature and Science in Tokyo, Japan.

"Adults show features throughout the skull associated with a powerful bite"¦large muscle attachments, bony buttresses, specialized teeth. The juvenile is so young that it doesn't really have any of these features yet, and so it must have been feeding quite differently from its parents," Tsuihiji said.

Based on further thorough analysis of the microstructure of the leg bones of the same dinosaur the skull came from, Andrew Lee of Ohio University, co-author of the study, estimated that the juvenile was only 2 to 3 years old when it died. It was 9 feet long, 3 feet tall at the hip and weighed about 70 pounds. In comparison, Tarbosaurus adults were 35 to 40 feet long, 15 feet tall and weighed as much as 6 tons.

"This little guy may have been only 2 or 3, but it was no toddler"¦although it does give new meaning to the phrase 'terrible twos," said Witmer. "We don't know to what extent its parents were bringing it food, and so it was probably already a pretty capable hunter. Its skull wasn't as strong as the adult's, and would have had to have been a more careful hunter, using quickness and agility rather than raw power."

The different hunting and feeding strategies of juveniles and adults may have reduced competition among Tarbosaurus and strengthened their role as an apex predator in their environment.

"The juvenile skull shows that there must have a change in dietary niches as the animals got older," Tsuihiji said. "The younger animals would have taken smaller prey that they could subdue without risking damage to their skulls, whereas the older animals and adults had progressively stronger skulls that would have allowed taking larger, more dangerous prey."

The juvenile skull is important because it helps scientists clarify the identity of small, potentially juvenile specimens of other tyrannosaurs previously discovered.

"The beauty of our new young skull is that we absolutely know for many good reasons that it's Tarbosaurus," said Witmer. "We can use this known growth series to get a better sense of whether some of these more controversial finds grew up to be Tarbosaurus, Tyrannosaurus or some other species."

The study results are published in the Journal of Vertebrate Paleontology.


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