A new juvenile Tarbosaurus

Poor Tarbosaurus. Even though it was a top predator during the Cretaceous most people have never heard about it, the theropod from Asia being a poor man's Tyrannosaurus. (Some people think that Tarbosaurus = Tyrannosaurus, but I side with those who hold that they are distinct.) Still, even though it is not as famous as it's North American cousin it is still pretty cool that the recovery of a nearly complete skeleton of a juvenile Tarbosaurus has just been announced. Discovered two years ago in the Gobi Desert the fossils have now been prepped, revealing a young dinosaur only about 7 feet long.

This skeleton, head thrown back and tail arched up in the classic opisthotonic posture, is going to be important in the current debates over dinosaur growth rates and tyrannosaurid diversity. In terms of the latter there has been a resurgence in debate over Nanotyrannus and this specimen will likely be marshaled as evidence for and against the existence of such a "pygmy tyrant."

For those unfamiliar with the current argument some paleontologists support the idea, based upon skulls and teeth, that there was a second, "pygmy" genus of tyrannosaurid that lived alongside the larger predator. In his book Rex Appeal, for instance, Peter Larson argued that the smaller predator was a hypercarnivorous specialist, more analogous to a flesh-eating cat than the bone-crunching hyena analog in Tyrannosaurus. Larson cites a number of what he identifies as Nanotyrannus teeth found around the skeletons of well-preserved prey animals like Triceratops as evidence, yet this correlation does not necessarily mean there was another kind of tyrannosaurid around, especially if young Tyrannosaurus had somewhat different habits than their parents.

Although the current argument was initiated with the description of Nanotyrannus in 1988 the debate really took off with the discovery of the juvenile tyrannosaurid "Jane." Some argued that this dinosaur was a Nanotyrannus and others that it represents a young Tyrannosaurus (which, I must admit, I am more in agreement with), the arguments having been carried over into the recently published symposium book Tyrannosaurus rex: The Tyrant King. Why is the new Tarbosaurus juvenile important? A major factor in the Nanotyrannus debate is development; what exactly did Tyrannosaurus look like when young? What were it's body proportions? Did it have more teeth? What explanation is there for these differences?

Such questions can only be answered with the discovery of well-preserved young specimens, the new skeleton being such an example. Yet the young Tarbosaurus will not seal the debate. If Tarbosaurus is indeed distinct from Tyrannosaurus then we shouldn't expect their ontogeny to be exactly the same; even if the new specimen helps us understand the development of Tarbosaurus we can't say that it would have been exactly the same for Tyrannosaurus (although general trends might be able to be identified and compared to see how the group in general grew up). It is a fascinating find, most certainly, but given the state of things I can see a new round of arguments by those who would like to see Tarbosaurus ascend to the Tyrannosaurus throne and those who want Nanotyrannus to be removed from it.

[Hat-tip to Greg Laden]