The Littlest Big Rex

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.”

A comparison of <b>A</b>) a juvenile <i>Tarbosaurus</i>, <b>B</b>) a subadult <i>Tarbosaurus</i>, <b>C</b>) <i>Shanshanosaurus</i>, and <b>D</b>) <i>Raptorex</i>. Scale bar equals 5 centimeters for all images. <i>Tarbosaurus</i> and <i>Shanshanosaurus</i> after Currie and Dong (2002) and <i>Raptorex</i> after Sereno et al. (in press).

A comparison of A) a juvenile and B) a subadult Tarbosaurus, C) Shanshanosaurus, and D) Raptorex. Scale bar is 5 centimeters. A-C after Currie and Dong, 2002; D after Sereno et al. (in press).

Raptorex, however, possesses some features that have been attributed to another tyrannosaur taxon, Shanshanosaurus, as being juvenile in nature:

  1. The skull is inordinately large for the body, especially with a cranium as long as the pelvis.
  2. The orbit of the skull is very large, very rounded in aspect, and over 20% the length of the skull as a whole.
  3. 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.
  4. Cervical vertebrae show incompletely fused neurocentral sutures in the dorsal series, as in Shanshanosaurus.
  5. 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.

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6 Responses to The Littlest Big Rex

  1. Mark Wildman says:

    Sereno? Hasty? Never! Interestingly, Sereno et al also introduced Alioramus altai at Bristol. Nice material – interesting animal but after Phil Currie had introduced his new phylogeny of Tyrannosauroidea (no great surprises by the way), he then proceeded to briefly take apart the Alioramus work that Brusatte had presented in a couple of brief sentences. All good fun of course but Currie was deadly serious and, I thought, slightly annoyed. Still, it was fun!

  2. Ummm… It is the AMNH team and not Sereno that announced the new Alioramus species (note that we shouldn’t be using the complete unpublished species name yet…)

    Also, the matrix and other fossils associated with the Raptorex fossils would not be consistent with a later Late Cretaceous deposit.

  3. Mark Wildman says:

    Thanks – I stand corrected (I’ll remember for the future). It was great to meet you by the way! Thanks for your time.

  4. Steve Brusatte says:

    I think it’s quite misrepresentative to say that Philip “took apart” our work on Alioramus! I surely didn’t take his comments that way, and his comments were quite benign. Pete Makovicky and I had just given talks on our respective longirostrine tyrannosauroids, Xiongguanlong and Alioramus. At the end of his talk, Philip made the point that, compared to other theropods, all tyrannosarids have quite elongate skulls. That is true. But it’s also true that Xiongguanlong and Alioramus have much longer, lower skulls than other tyrannosauroids. Check out the cranial reconstruction at:

  5. qilong says:

    I appreciate the commentary so far! Many ore interesting specimens to come to light allowing us to view potential autapomorphic specimens despite ontogenetic changes known for skeletons. When it comes to the recent Alioramus remotus and concordantly Shanshanosaurus honshanensis (both of which have at one point been suggested as juvenile Tarbosaurus) and now A. altai, the question remains whether the features present that are diagnostic to these taxa are not themaselves merely juvenile features not present in an adult form. How diagnostic, precisely, are these? And how is Shanshanosaurus compared to Alioramus precisely?

  6. Steve Brusatte says:

    Both Alioramus specimens, the types of A. remotus and A. altai, are of about the same size (and presumably growth stage), so it is difficult to know whether the various autapomorphies would have remained in the adult form. However, there are LOTS of autapomorphies of Alioramus (not just a few), and in ceratopsians the ornamental features become more pronounced in the adult (which is important since some of the Alioramus autapomorphies relate to the jugal and nasal horns).

    One important thing to consider is that we can compare A. altai directly with similarly aged and sized juveniles of Tyrannosaurus and Tarbosaurus. We lay out a number of explicit comparisons with exemplar specimens of juvenile Tarbosaurus in our online supplement. These Tarbosaurus specimens do NOT possess the various Alioramus autapomorphies, and also have many features of the adult Tarbosaurus that are not present in Alioramus.

    As for Shanshanosaurus, it is much, much, much smaller than the two Alioramus specimens. It was likely a very young individual. It lacks the Alioramus autapomorphies on the maxilla and cervical vertebrae (and probably others, but I’m working from memory here), but does share with Alioramus a single pneumatic feature of the cervicals that is otherwise unknown in other tyrannosauroids. Perhaps some feature of the pneumatic system were ephemeral, and more pronounced in juveniles, in tyrannosauroids.

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