Raptorex: When it Lived and Where it Lived

A little over a year ago, Paul Sereno made a splash when he published the description of Raptorex kreigsteini, the small tyrannosaurid that looks for all the world like a perfectly scaled down version of the giants of the late Cretaceous, complete with disproportionately small arms. But it was from the early Cretaceous, opening up interesting new paths of inquiry into the evolution of the iconic group.

Yesterday, NatureNews ran a story written by Zoë Corbyn focusing on the doubts of Peter Larson of the Black Hills Institute. His concern: Raptorex is a juvenile Tarbosaurus. He steps around the objection that Raptorex is far to old, geologically, to be a young specimen of Tarbosaurus by proposing that Sereno had the age wrong. The NatureNews story says:

On the basis of two other fossils — a fish vertebra and a freshwater clam — found alongside the dinosaur fossil, the paper says that the specimen of Raptorex is of Chinese origin and about 125 million years old. But the evidence is too vague, given that the fish and clam fossils are widespread in time and geographic area...

You may recall that Raptorex was given its specific name to honor the father of the fossil collector who purchased the fossils and donated them to science. When Henry Kreigstein bought the bones from a dealer operating out of Japan, he was told that they came from an "unspecified location in northern China."

Sereno et al's Science report of a year ago is more specific than that, saying that it came from:

Approximately 41°20′N and 119°40′E, collected privately in the border area between Liaoning Province and the Nei Mongol Autonomous Region of the People’s Republic of China.

The purported Raptorex locality, indicated by the marker. Image from Google Maps.

The rock it was found in is described like so:

Lujiatun Beds of the Yixian Formation, comprising a tuffaceous fluvial facies of the Jehol Group with its well-known Jehol Biota that includes the teleost Lycoptera and pelecypods, which were found in association with the holotypic skeleton. The matrix around the fossil is light green, massive, poorly sorted, tuffaceous, micaceous sandstone with fibrous gypsum. The light-colored, uncrushed bones were buried for the most part in articulation. The absence of laminated, fine-grained sediment or conchostracans characterizes the Lujiatun Beds of the Yixian Formation, dated to the late Early Cretaceous (Barremian-Aptian, ~125 Ma).

So, to translate: Sereno's paper puts forth that Raptorex lived at the earliest stage of the Jehol Biota, a famous Chinese ecosystem that has yielded a bunch of interesting animals. It was a river system that experienced periods of volcanic activity. The Raptorex fossils were discovered with fish and mollusks which are well-known part of that ecosystem. Raptorex was also articulated, meaning its bones were preserved in roughly in the same position they would have been when the animal died, and it was likely buried quickly. That Raptorex was buried in a river or stream, and not a calm body of water is evident in the structure of the rock surrounding the fossil, which isn't laminated, or finely layered, and lacks fossils of the ubiquitous crustaceans called conchostracans, which occur in still waters. This is consistent with the rock of the Lujiatun beds, our earliest window into the Jehol ecosystem.

Sereno's paper isn't wishy-washy on the subject of where Raptorex came from. (See comments below for more on this) Yet Corbyn writes, "Sereno says that the information he received from the various dealers stated that the fossil came from an unspecified location in northern China."

I have couple other problems with this story. First, it's not reporting a published research paper. It's reporting the fact that one guy, a tyrannosaur expert though he may be, doubts the identity of this dinosaur. Cool. Let's see a published paper about it. It also brings in University of Oslo paleontologist Jørn Hurum to back up Larson's claim, but his contribution amounts to "yeah, it looks like a baby Tarbosaurus skull to me." None of this addresses the fact that the skeleton of Raptorex exhibits signs of being that of a near-mature animal, and not a juvenile. A near-mature animal the size of a large dog. Tarbosaurus was just a smidge smaller than T. rex. If Raptorex is actually a young Tarbosaurus, it would have to go through the mother of all growth spurts right at the cusp of adulthood.

I don't really care if Raptorex isn't exactly what Sereno's paper purports it to be. It's an intriguing additon to the story of tyrannosaur evolution, but if it's been misinterpreted, so be it. No one's perfect. But we need a peer-reviewed paper to really evaluate Larson's ideas. We need a detailed analysis of the rock in which its fossils were preserved and another look at its osteology.

The fact that Kreigstein rescued Raptorex for science is commendable, but the fact that the fossil trade has muddied the origin of these fossils is, to put it mildly, yucky. I'll continue with this aspect of the story tomorrow. In the meantime, if I've gotten any part of this complicated story wrong here, be sure to correct me in the comments or by email.