Dinosaur skin impressions are pretty rare, and, even among the known collection of these soft-tissue traces, not all dinosaurs are equally well-represented. There are plenty of skin impressions from hadrosaurs, but stegosaurs are among the dinosaurs in which the skin texture is still largely unknown. Now, as reported by paleontologists Nicolai Christiansen and Emanuel Tschopp, an exceptional specimen from northern Wyoming gives scientists a first look at the skin and other body coverings from a North America stegosaur.
The individual described by Christiansen and Tschopp, nicknamed "Victoria," is an approximately 150-million-year-old, nearly complete skeleton of the stegosaur Hesperosaurus mjosi. Discovered in 1995, it came from the well-known Howe-Stephens quarry site, where soft tissue impressions of other Jurassic dinosaurs have been found before. Based on the state of the skeleton, it appears that the dinosaur died, was partially buried, and then completely buried by a second flow of sediment, with the best preserved elements being found on the dinosaur's right side.
The soft-tissue impressions found in association with the skeleton were scattered around the section of the ribs just before the hips and on one of the large armor plates on the dinosaur's back. The preservation was not complete, but rather shows bits and pieces within these areas. Even so, enough of the skin impressions were preserved to show what the skin of Hesperosaurus was like. Overall it consisted of the same kind of honeycomb scale pattern seen in hadrosaurs, horned dinosaurs and another stegosaur from Asia called Gigantspinosaurus. Rather than being uniform, however, the scale pattern differed over the dinosaur's body, with larger, domed scales surrounded by the smaller tubercles found on skin impressions from its back.
Among the most remarkable aspects of Victoria's remains were the soft tissue impressions from the plate. For decades paleontologists have debated what the plates would have looked like, how they were arranged, and what function they might have had, and while this new specimen probably won't resolve the ongoing discussions about the purpose of stegosaur plates, it appears to show a relatively smooth plate covering marked by shallow grooves. That this preserved material is really from a kind of plate sheath cannot be confirmed without any doubt, but Christiansen and Tschopp make the case that this interpretation is the most consistent with the structure of the material and the existing hypothesis that stegosaur plates were probably covered in this kind of material. If further remnants of these plate sheaths can be found, they can help paleontologists better understand the anatomy of these armored dinosaurs and better test ideas about the function of their plates.
The report was published in the Swiss Journal of Geoscience as part of the proceedings from the Symposium on Stegosauria held last year. The papers cover a range of topics, from new species to the bite mechanics of Stegosaurus, and several of this week's Dinosaur Tracking posts will feature new findings presented at the meeting. Stay tuned for more on this bizarre group of dinosaurs.
Christiansen, N., & Tschopp, E. (2010). Exceptional stegosaur integument impressions from the Upper Jurassic Morrison Formation of Wyoming Swiss Journal of Geosciences DOI: 10.1007/s00015-010-0026-0