Exceptional stegosaur integument impressions from the Upper Jurassic Morrison Formation of Wyoming

Abstract

Dinosaur skin impressions are rare in the Upper Jurassic Morrison Formation, but different sites on the Howe Ranch in Wyoming (USA), comprising specimens from diplodocid, camarasaurid, allosaurid and stegosaurian dinosaurs, have proven to be a treasure-trove for these softtissue remains. Here we describe stegosaurian skin impressions from North America for the first time, as well as the first case of preservation of an impression of the integument that covered the dorsal plates of stegosaurian dinosaurs in life. Both have been found closely associated with bones of a specimen of the stegosaurian Hesperosaurus mjosi CARPENTER, MILES and CLOWARD 2001. The scales of the skin impression of H. mjosi are very similar in shape and arrangement to those of Gigantspinosaurus sichuanensis OUYANG 1992, the only other stegosaurian dinosaur from which skin impressions have been described. Both taxa show a ground pattern of small polygonal scales, which in some places is interrupted by larger oval tubercles surrounded by the small scales, resulting in rosette-like structures. The respective phylogenetic positions of G. sichuanensis as a basal stegosaurian and H. mjosi as a derived form suggest that most stegosaurians had very similar skin structures, which also match the most common textures known in dinosaurs. The integumentary impression from the dorsal plate brings new data to the long-lasting debate concerning the function of dorsal plates in stegosaurian dinosaurs. Unlike usual dinosaur skin impressions, the integument covering the dorsal plates does not show any scale-like texture. It is smooth with long and parallel, shallow grooves, a structure that is interpreted as representing a keratinous covering of the plates. The presence of such a keratinous covering has affects on all the existing theories concerning the function of stegosaurian plates, including defense, thermoregulation, and display, but does not permit to rule out any of them.

Ontogenetic histology of Stegosaurus plates and spikes

“…(2005) and Christansen and Tschopp (2010) have also suggested that plates were covered in keratin. Particularly, Christansen and Tschopp (2010) noted that they found what was possibly keratin on a plate. The external and internal vascular canals in plates and spikes probably contribute to the production of the keratin sheath.…”

“…Main et al. (2005) and Christansen and Tschopp (2010) have also suggested that plates were covered in keratin. Particularly, Christansen and Tschopp (2010) noted that they found what was possibly keratin on a plate.…”

The Postcranial Skeleton of an Exceptionally Complete Individual of the Plated Dinosaur Stegosaurus stenops (Dinosauria: Thyreophora) from the Upper Jurassic Morrison Formation of Wyoming, U.S.A.

“…It seems possible, therefore, that the section exposed at the Red Canyon Ranch may correlate with the middle part of the Morrison Formation at Red Gulch ( Fig 4 ). The Howe and Howe-Stephens quarries, which are close by and have both produced a diverse range of dinosaur taxa, also appear to be from the middle part of the Morrison Formation in this area [ 5 , 6 ]. A radiometric date of 151.5±3 million years was obtained from a bentonite near the base of the Morrison Formation at the nearby Red Gulch dinosaur tracksite [ 7 ], indicating that the entirety of the Morrison Formation in this area could have been deposited during the Tithonian.…”

The skull and neck of a new flagellicaudatan sauropod from the Morrison Formation and its implication for the evolution and ontogeny of diplodocid dinosaurs

“…2). Kvale et al (2001) interpreted a second site approximately 10 m higher in stratigraphy (Howe-Stephens Quarry, see Schwarz et al 2007b;Christiansen & Tschopp 2010) as being of 147 Ma in age. This implies a latest Kimmeridgian to earliest Tithonian age for the Howe Quarry.…”