Has a Tiny Tyrant Been Dethroned?

In late 2009, paleontologist Paul Sereno and colleagues announced the discovery of a tiny Cretaceous tyrant. They called it Raptorex, and in a paper titled “Tyrannosaurid Skeletal Design First Evolved at Small Body Size,” the researchers who described the animal interpreted its anatomy as an indication that the big-headed, small-armed body plan of immense predators such as Tyrannosaurus and Albertosaurus first evolved in small bodies. But not everyone agreed that Raptorex truly was a unique, tiny tyrant dinosaur. After all, the specimen was originally sold at a fossil show as a juvenile Tarbosaurus, and a brief Nature News article published last autumn said that a different team of researchers were preparing a paper which identified Raptorex as a young Tarbosaurus. That paper, written by paleontologist Denver Fowler and colleagues, has now been published in PLoS One.

Fowler and co-authors point out that the status of Raptorex as a unique, small tyrannosaur depends upon two lines of ambiguous evidence. The first concerns the geologic age of the animal. The Raptorex skeleton—given the designation LH PV18—was purchased at a fossil show and did not come with detailed information about where it was found. Judging where the fossil was found and the age of the surrounding rock depended on tiny fossils included in bits of rock still stuck to the skeleton. Sereno and colleagues attributed the skeleton to the Yixian Formation, making it about 125 million years old, but Fowler and co-authors contend that the dinosaur probably came from geologically younger rock layers of the Late Cretaceous. If this is correct, and the Raptorex skeleton is not as old as had been hypothesized, then the dinosaur might not be an indication that trademark tyrannosaur traits evolved early and in small animals.

Raptorex may not have been a unique species of dinosaur, either. The original analysis presented the animal’s skeleton as that of a subadult or young adult, meaning that the dinosaur probably would not have grown too much bigger. Fowler and colleagues, however, argue that Raptorex was probably younger. LH PV18 may be the skeleton of a juvenile animal, which opens the possibility that the dinosaur called “Raptorex” is actually an immature growth stage of Tarbosaurus.

It may turn out that both sides of this debate are partially correct. Two months ago yet another team of scientists, led by Takanobu Tsuihiji, published a detailed description of a nearly-complete juvenile Tarbosaurus. The discovery of this individual allowed for a detailed comparison with other young tyrannosaurs, and the researchers included a section on Raptorex. After noting that juvenile tyrannosaurids often exhibit archaic traits—which may lead paleontologists to confuse immature animals for small, primitive species—Tsuihiji and colleagues pointed out that the Raptorex skeleton and their juvenile Tarbosaurus skeleton differed in some significant ways. In addition to a few minute skull features, the Raptorex skeleton is set apart from all other known tyrannosauroid dinosaurs in lacking a prominent crest on the upper part of the hip.

If the three traits mentioned by Tsuihiji and co-authors truly distinguish Raptorex from other tyrannosaurs, then it may be a unique species. It may turn out that Raptorex is the juvenile form of a large tyrannosaur species from which the adult is not yet known. Frustratingly, though, the PLoS One authors disagree with Tsuihiji’s group about whether the tiny crest on the hip—the most important of the differentiating traits on Raptorex—is present or absent. The paper by Tsuihiji and colleagues states that the crest is absent, but a personal observation by Peter Larson in the new paper is cited as evidence that a “subtle crest” is present. Fowler and colleagues agree that the Raptorex skeleton may represent a unique dinosaur taxon—a distinct genus or species—but, overall, the differences between it and juvenile Tarbosaurus are slight. At the very least, the idea that Raptorex was near adulthood and indicates that the famous tyrannosaur body plan evolved at small size is in doubt. Additional fossils with detailed geological data will be needed to settle this argument. For now, the tiny tyrant sits in paleontological limbo.

References:

Fowler, D.; Woodward, H.; Freedman, E.; Larson, P.; Horner, J. (2011). Reanalysis of “Raptorex kriegsteini”: A Juvenile Tyrannosaurid Dinosaur from Mongolia PLoS One, 6 (6) DOI: 10.1371/journal.pone.0021376

Tsuihiji, T., Watabe, M., Tsogtbaatar, K., Tsubamoto, T., Barsbold, R., Suzuki, S., Lee, A., Ridgely, R., Kawahara, Y., & Witmer, L. (2011). Cranial osteology of a juvenile specimen of Tarbosaurus bataar (Theropoda, Tyrannosauridae) from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia Journal of Vertebrate Paleontology, 31 (3), 497-517 DOI: 10.1080/02724634.2011.557116

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.