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Soft tissue traces of the ankylosaur Tarchia. Black asterisks denote large osteoderms, scale impressions are pointed out by an arrowhead and small ossicles are identified by the arrow. From Arbour et al., 2012.

Ankylosaurs can be frustrating dinosaurs. In life, armor covered the bodies of these dinosaurs from snout to tail, but those bony adornments often fell out of place between the death and ultimate burial of the ankylosaurs. Reconstructing an ankylosaur, therefore, requires that paleontologists not only figure out the articulations of the bones but also the arrangement of the armor. Every now and then, though, researchers discover one of these dinosaurs with some armor still in place. According to an in-press Acta Palaeontologica Polonica paper, ankylosaur expert Victoria Arbour and colleagues have just identified one such specimen from the Late Cretaceous of Mongolia.

The dinosaur in question is most likely a specimen of Tarchia–an ankylosaur that could grow to about 26 feet long and, like many of its close relatives, carried a tail club. Rather than being a brand new find, though, this Tarchia was originally discovered in 1971 during the Polish-Mongolian Palaeontological Expedition and was sent to the Geological Museum in Oslo, Norway in 1998. Now, after over three decades, the dinosaur gets its time in the scientific spotlight.

What makes this Tarchia so significant isn’t the completeness of the skeleton. Only the left side of the back half of the body, including most of the tail, is preserved. What’s special is that parts of the dinosaur’s armor are still in place, including triangle-shaped bits of armor along the dinosaur’s slender tail and impressions of the tough sheaths that covered some of the armor in life. Indeed, the bony armor of dinosaurs was not exposed to the outside but was covered in a hard keratinous coating–horns, claws, plates and spikes were all covered in this, often making weapons sharper and ornaments more expansive.

While such soft tissue fossils are relatively rare, Arbour and her co-authors follow what paleontologist Phil Bell recently suggested on the basis of hadrosaur skin impressions–that preserved soft tissue impressions such as these might eventually be useful in distinguishing between different genera or species of dinosaur. In fact, this may be particularly important in cases like this exceptional ankylosaur. While the specimen is most similar to other specimens of Tarchia, it also differs in some minute tail characteristics. Are the differences the result of growth or individual variation, or could they be signs of a previously-unrecognized species? Detailed comparisons of skin impressions, in addition to skeletal differences, may help paleontologists winnow down the possibilities. We just need a better collection of ankylosaur soft tissue traces first.

Reference:

Arbour, V.M., Lech-Hernes, N.L., Guldberg, T.E., Hurum, J.H., and Currie P.J. (2012). An ankylosaurid dinosaur from Mongolia with in situ armour and keratinous scale impressions Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0081

A natural cast of a hadrosaur's skin found in southern Utah. From Herrero and Farke, 2010.

When Charles H. Sternberg and his sons excavated one of the first hadrosaur mummies ever found, in the summer of 1908, it was a major discovery. For nearly a century naturalists and paleontologists could only imagine what a dinosaur’s skin was like, but the Edmontosaurus the Sternbergs collected gave scientists an unprecedented look at the hadrosaur soft tissue anatomy. In the century since that discovery, though, so many hadrosaur skin impressions have been found that they don’t make the news anymore—only the most spectacular finds, such as the Brachylophosaurus “Leonardo,” get much attention.

Despite the number of hadrosaur skin impressions that have been found, there is still much to learn about the skin of different hadrosaurs and how the impressions came to be preserved. In the latest edition of PalArch’s Journal of Vertebrate Paleontology, for example, high school student Lucia Herrero and paleontologist Andy Farke have described the partial skin impressions associated with a disarticulated hadrosaur skeleton from southern Utah’s 76- to 74-million-year-old Kaiparowits Formation. The specimen was too broken up to determine what genus and species of hadrosaur it had been, but among the scattered bones were patches of skin imprints left in the rock.

On its surface, the association of skin impressions with a busted-up skeleton might appear to be a contradiction. The depositional environment was delicate enough for traces of soft tissue anatomy to be preserved, yet the dinosaur’s bones were moved out of place or destroyed. As hypothesized by Herrero and Farke, what this may indicate is that—in the right circumstances—dinosaur skin was durable enough to survive becoming detached from the rest of the carcass and enter the fossil record. Rather than being just an oddball case, the specimens described by Herrero and Farke represent a kind of preservation that may simply have been overlooked at other disarticulated hadrosaur sites, and the Kaiparowits Formation appears to be rich enough in both hadrosaurs and skin impressions to further investigate the way in which traces of dinosaur skin entered the fossil record.

References:

Lucia Herrero & Andrew A. Farke (2010). HADROSAURID DINOSAUR SKIN IMPRESSIONS FROM THE UPPER CRETACEOUS KAIPAROWITS FORMATION OF SOUTHERN UTAH, USA PalArch’s Journal of Vertebrate Palaeontology, 7 (2), 1-7