Paul Sereno, world famous for his far-flung and well-publicized discoveries and descriptions, will be adding a new taxon name to his roster: Raptorex kriegsteini.
Sereno, P., Tan L., Brusatte, S. L., Kriegstein, H. J., Zhao X.-J., Cloward, K. 2009. Tyrannosaurid skeletal design first evolved at small body size. Science, in press — DOI: 10.1126/science.1177428
Sensationally, this new taxon is a small, and yet maturely-developed member of the dinosaur group that includes Tyrannosaurus. It features long legs, a long head, protportionately small vertebrae and a short spine, and yet has tiny arms. Representatives of the origin of the tyrannosaurs include other recent discoveries, such as Dilong, which apparenly had longer arms and a relatively shorter skull for its size. The paper is thoughrough in describing the juicy bits: Skeletal anatomy, proportions of the arms, etc. A large deal is spent attempting to compare it to some of the other recently discovered early tyrannosaurs like Xiongguanlong, Guanglong, and the aforementioned Dilong. For the most part, it seems relatively distinct. However, it is never adequately compared to Dilong, which according to the authors is from the same formation Raptorex is. This “according” is unfortunately due to an unfortunate circumstance in Raptorex‘s early history:
It was uncovered by a local prospector, and then sold to a private collector. That collector contacted Sereno, and we now have this fossil, in a museum, and available. The problem is is that we don’t actually know WHERE this fossil comes from. It may derived from the Yixian Formation, in the basal Cretaceous Jehol Group, where produced other fantastic discoveries like Caudipteryx, Sinosauropteryx, and the weird “quilled” Psittacosaurus, as well as possible congener Dilong. Problematically, it’s just a “may.”
Raptorex, however, possesses some features that have been attributed to another tyrannosaur taxon, Shanshanosaurus, as being juvenile in nature:
- The skull is inordinately large for the body, especially with a cranium as long as the pelvis.
- The orbit of the skull is very large, very rounded in aspect, and over 20% the length of the skull as a whole.
- The preorbital region of the skull makes up under 75% of the skull length, and the teeth are concordantly large for the animal’s size.
- Cervical vertebrae show incompletely fused neurocentral sutures in the dorsal series, as in Shanshanosaurus.
- In assessing rough development by studying a histological section of a long bone, the authors found two LAGs, while proposing several lose LAGs and unknown cortical ones were present. Without this information the proposed age may only have been 2 years, rather than the projected “subadult” age of 6 the authors apply to it.
These issues point out that Sereno et al may be too hasty in ascribing Raptorex 1. to the basal Lujiatun Member of the Yixian Formation (from whence Dilong comes from) and 2. That it is a subadult of relative maturity, despite showing hallmarks of immature status.