There is little, if any, argument as to which dinosaur is the favorite among children in the U.S. – western North America’s own Tyrannosaurus rex wins hands down. But children in Asia have their own home-grown favorite in T. rex’s very close cousin, Tarbosaurus bataar.
In the next issue of the Journal of Vertebrate Paleontology, Takanobu Tsuihiji and several other Japanese, Mongolian and U.S. paleontologists describe an exquisitely preserved skull of a juvenile T. bataar determined to be only 2 to 3 years old at the time of its death, about 70 million years ago, in what is now the western Gobi Desert of Mongolia. Although less than a foot long, this skull is anything but short on the information it is revealing, particularly with respect to the changes that took place as these top predators grew from juveniles to adults.
Found in 2006 during the Hayashibara Museum of Natural Sciences–Mongolian Paleontological Center Joint Expedition, the skull and nearly entire skeleton were collected at the Bugin Tsav locality, where larger, adult specimens of the same species have also been recovered.
As the authors of the paper report, finding such a complete juvenile skull of Tarbosaurus is highly important for several reasons. Chief among these is the information it provides on the skeletal changes that occurred as juveniles grew into adults. Using X-Ray CT scanning, they found that this skull lacks almost all the adaptations for powerful biting and twisting seen in adult Tarbosaurus.
“We knew that adult Tarbosaurus were a lot like T. rex,” said Tsuihiji, a post-doctoral researcher at the National Museum of Nature and Science in Tokyo and lead author of the study. “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.” Tarbosaurus, therefore, must have changed dietary niches as it got older, “hunting only smaller prey that it could subdue without damage to its skull as a juvenile, but taking larger, more dangerous prey as its skull became progressively stronger with age.” According to Mahito Watabe of the Hayashibara Museum of Natural Sciences in Okayama, who led the 2006 expedition that discovered the new skull, these smaller prey may have included the bony-headed dinosaur Prenocephale, which is found in nearby rocks of the same age.
An additional important benefit to studying this new, very young individual of T. bataar will be to help clarify whether previously recovered juvenile and adult specimens of tyrannosaurs represent the same or different species. It has long been known that juvenile tyrannosaurids have features that make them appear more primitive than adults of the same species. “The beauty of our new young skull,” says study co-author Lawrence Witmer, the Chang Professor of Paleontology at the Ohio University College of Osteopathic Medicine, “is that we absolutely know for many good reasons that our specimen is Tarbosaurus. We can use this known growth series to get a better sense of whether some of the more controversial juvenile finds grew up to be Tarbosaurus, Tyrannosaurus, or some other species.” This, in turn, will result in a greater overall understanding of the evolutionary history of the group.
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