Featherweight Relative of the T. Rex Is Found

Researchers working in the western Gobi Desert in Mongolia have discovered the almost complete skeleton of a tyrannosaurid dinosaur that was less than 3 years old when it died, younger and smaller than any previously known. The animal, Tarbosaurus bataar, is the closest relative of Tyrannosaurus rex, the predator that lived at the same time in North America.

In life, the specimen weighed less than 70 pounds, compared with the six-ton weight of a full-grown T. bataar, the researchers report in The Journal of Vertebrate Paleontology. The researchers were able to determine its age by microscopic examination of one of the leg bones, which reveal periodic pauses in growth, similar to the rings of a tree trunk.

The skulls of adult tyrannosaurids have extremely strong bones, especially those of the jaw, capable of tremendous twisting and bending forces. But the juvenile’s skull bones are more delicate, its teeth much thinner and its jaw much weaker. This suggests that a young T. bataar would be more likely to take its prey by stealth and speed rather than the overwhelming power its parents could use. In other words, T. bataar changed its diet as it matured, unlike some other predatory dinosaurs.

“This is one of the clearest pictures we have of these dinosaurs,” said Lawrence M. Witmer , a professor of paleontology at Ohio University and the senior author of the study. “It gives us the best glimpse into the changing lifestyles of these animals as they grew.”