Judging a Dinosaur By its Cover

From Smithsonian: Judging a Dinosaur By its Cover

We love to bring dinosaurs back to life. From museum displays and academic papers to big-budget movies, we have an obsession with putting flesh on old bones. How much anatomical conjecture and artistic license is required to do so varies from dinosaur to dinosaur.

Some dinosaurs are known from a paltry collection of fragments and require a considerable among of reconstruction and restoration on the basis of better-known specimens of related species. Other dinosaurs are known from complete skeletons and require less osteological wrangling, but they still present the challenge of filling in the soft tissue anatomy that the skeleton supported in life. Every now and then, though, paleontologists discover skin impressions associated with the bones of dinosaurs. These rare fossils can give us a better idea of what the outside of some dinosaurs looked like.

Skin impressions are found most often with hadrosaurs. These herbivores, such as Edmontosaurus and the crested Corythosaurus, were plentiful and seemed to dwell in habitats where deceased dinosaurs could be buried rapidly by sediment, a key to the preservation of soft-tissue anatomy. In the roughly 68-million-year-old strata of Canada and Mongolia, for example, skeletons of two different species of the hadrosaur Saurolophus have been found associated with skin impressions. But these fossils can do more than help use restore the outer appearance. According to a new paper by University of Alberta paleontologist Phil Bell, subtle differences in Saurolophus skin traces can help paleontologists distinguish one species of dinosaur from on another on the basis of soft tissue anatomy alone.

In 1912, professional dinosaur hunter Barnum Brown named the hadrosaur Saurolophus osborni from skeletons found in Alberta’s Horseshoe Canyon Formation. Although not mentioned at the time, three skeletons of this species were associated with skin impressions from various parts of the body, including the jaw, hips, foot and tail. Forty years later, from skeletons found in a huge bonebed called the “Dragon’s Tomb” in Mongolia’s Nemegt Formation, paleontologist Anatoly Konstantinovich Rozhdestvensky named a second species, Saurolophus angustirostris. Numerous skin impressions were found with skeletons of this species, too. The fact that two Saurolophus species had been found with intact skin impressions provided Bell with a unique opportunity to compare the outer anatomy of two closely related dinosaurs.

Both Saurolophus species had pebbly skin. Like other hadrosaurs, the skin of these dinosaurs was primarily composed of non-overlapping scales or tubercles of varying shape. In detail, though, Bell ascertained that the skin of the two species differed enough that one species can be readily distinguished from the other.

Along the base of the tail, the North American species (S. osborni) had mosaic-like clusters of scales, while the species from Mongolia (S. angustirostris) seemed to have vertical bands of specialized scales interspersed with larger, rounded scales Bell terms “feature scales.” This pattern in S. angustirostris remained consistent in young and old individuals—evidence that this was a real pattern peculiar to this species and not just a matter of variation among individuals.

Frustratingly, the skin impressions from the North American species cover less of the body and come from fewer specimens than those from the Dragon’s Tomb. That limits the possible comparisons between the species. Still, based on the consistent differences between the Saurolophus species in the skin at the base of the tail, it appears that paleontologists might be able to use soft-tissue anatomy to identify and diagnose particular dinosaur species. This could be especially useful for the study of hadrosaurs. These dinosaurs are notoriously difficult to tell apart on the basis of their post-cranial skeleton, but Bell’s study hints that skin impressions might show prominent differences. Judging a dinosaur by its cover might not be such a bad idea.

References:

Bell, P. (2012). Standardized Terminology and Potential Taxonomic Utility for Hadrosaurid Skin Impressions: A Case Study for Saurolophus from Canada and Mongolia PLoS ONE, 7 (2) DOI: 10.1371/journal.pone.0031295