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Raptorex kriegsteini is a species (possibly) with quite a controversial history. It is based on a well-preserved fossil excavated from somewhere in Mongolia and then brought to the US, where it was put on sale at a gem and mineral show, labelled (ominously) as a "juvenile Tarbosaurus." The specimen was bought by a private collector, who then donated it to science for study by Paul Sereno. Sereno descided that rather than a juvenile Tarbosaurus, he had the adult form of a new species on his hands. Sereno, in his description, noted that the specimen was in fact a juvenile. Sereno did not compare the specimen to known juvenile tyrannosaurs, and while he included Tarbosaurus in a phylogenetic analysis, he only used an adult specimen. In a letter to Nature News, paleontologist Peter Larson publically doubted the identity of Raptorex as a unique species, claiming that it was indistinguishable from a baby Tarbosaurus (though, as an informal letter, his reply didn't include rigorous data and was not peer reviewed, so Sereno basically dismissed it without further comment).
The crux of the debate rests on where Raptorex came from. The slab the skeleton was preserved in, Sereno claimed, bore the hallmarks of the early Cretaceous Yixian Formation (specifically the Lujiatun beds), many millions of years older than Tarbosaurus. Sereno identified the fish genus Lycoptera, which is a key index fossil of the Jehol biota. However, the "Lycoptera" is represented only by a single vertebra, and Sereno didn't describe th bone or compare it to Lycoptera. Later study by Larson, Denver Fowler and others showed that the bone is actually nothing like Lycoptera and must have come from a completely differnet kind of fish.
So, was there a miniature "tyrannosaurid"-like species running around hnting Caudipteryx? Probably not. Then what us Raptorex? For now, we can't be sure. It's similar to Tarbosaurus juveniles of the same age (about 3 years old), but not identical. It may be a juvenile specimen of a similar Late Cretaceous Monglolian tyrannosaur, like Zhuchengtyrannus or "Tyrannosaurus" zhuchengensis. Without knowing where and when the fossil came from, we may never know, so for now Raptorex is simply a dubious name for an indeterminate juvenile tyrannosaurid.
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