Raptorex kriegsteini a new Tyrannosaurus relative

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More than you ever wanted to know about this new tiny T. rex, the newly named and described Raptorex kriegsteini!

Raptorex kriegsteini is the new dinosaur described by Paul Sereno and his team of scientists from the University of Chicago. It was named in the AAAS-run journal Science. 1

Raptorex is closely related to tyrannosaurid dinosaurs (like Tyrannosaurus rex or T. rex for short). What is interesting and special about Raptorex is that it shows many of the adaptations present in its larger cousins, the tyrannosaurids, for crush-type biting as well as legs which are proportioned for running. Many of the earlier known primitive tyrannosaurid relatives look much more like the early coelurosaurs from which the tyrannosaurids descended and not very much like the tyrannosaurids themselves except in a few skeletal features. Raptorex is the first discovered which closely looks much like its more advanced cousins.

In fact, Raptorex looks so much like tyrannosaurids, that at first, it was thought to be a young specimen of an already known tyrannosaurid, the much larger Tarbosaurus. According to the location it was found at though, it would be much too old geologically to be Tarbosaurus. But that's where the twist comes in, Raptorex was originally collected illegally and sold in the private market as a young specimen of Tarbosaurus. The buyer, Henry J. Kriegstein, suspected he had a specimen, and it turns out he may have been right. This isn't the first time it's been suspected that a primitive tyrannosaurid or tyrannosaurid relative is a juvenile of an already described species. For example, many young Tyrannosaurus and Tarbosaurus specimens were originally classified separately as different species known as 'aublysodontines'. As it turns out though, 'aublysodontines' simply were juvenile tyrannosaurids which looked different than their larger adult forms. Some people think that the tyrannosaurid Alioramus may represent a juvenile Tarbosaurus as well, but as this idea is only a rumor, I would be cautious of it. At any rate, Sereno et al. managed to push Raptorex through peer-review in a high ranking journal and it seems very likely that it may well be a separate species. 

The specimen is very interesting either way, even if it did turn out to be a juvenile. It is very complete, which is not very common for small tyrannosaurid fossils (separate species or not).

1Sereno, P.C., Tan, L., Brusatte, S. L., Kriegstein, H. J., Zhao, X., and Cloward, K. 2009. Tyrannosaurid skeletal design first evolved at small body size. Science Express, doi:10.1126/science.1177428

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