Stegosaurian skin impressions from the Upper Jurassic Shangshaximiao Formation, Zigong, Sichuan, China: A new observation

Xing Li-da¹*, Peng Guang-zhao², Shu Chun-kang²

1, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037

2, Zigong Dinosaur Museum, Zigong, 643013

Abstract: Describes a skin impression fossil of Gigantspinosaurus sichuanensis from Shangshaximiao Formation, Upper Jurassic discovered in Zigong Sichuan. The skill fossil is preserved on the dorsal face of the left shoulder. The lithology is sage green sandstone mixed with extremely thin carmine mudstone. The total area of the fossil is 414cm². The scales present are generally arranged net like structure, most of them are pentagonal with a few being quadrilateral and hexagonal. The maximum inner radius of most scales range from 5.7 to 9.2mm. The scales connect to each other in an intercrossing arrangement, jointed by grooves. Scattering around are large or tubercular, pentagon and hexagon in shape and are surrounded by 13-14 scales. The phenomenon odd large scales scattered among regular scales are found in the skin impression fossil of Titanosaur embryos of Sauropods, Hadrosauridae, Edmontosaurus annectens, Corythosaurus casuarius of Ornithopods and Chasmosaurus belli, Monoclonius nasicornis, Psittacosaurids of Ceratopsians. It suggests that this was a common skin format for herbivorous dinosaurs, most likely for defense. The scale format of Gigantspinosaurus sichuanensis is very similar to the throat armor of the Stegosaurus stenops DMNH 2818 specimen from the Morrison formation (Kimmeridgian-Tithonian) Garden Park of Upper Jurassic Colorado, America. The maximum inner radius of the scales of Gigantspinosaurus sichuanensis is less than that of the DMNH 2818 specimen while the groove on the scales of the DMNH 2818 specimen is wider than that of Gigantspinosaurus sichuanensis. The surface of the scales of Gigantspinosaurus sichuanensis is rough and with a string like ridge. The similar non-overlaped polygonal shape is found in the mandible skin of the Hadrosaurs specimen (UW 39449) from the Lance Formation (Maastrichtian), Upper Cretaceous Wyoming, America. Such a structure was reduces glare. A body with a smooth surface would produce glare under the sun and interfere with concealment. The stringy ridge on the scales of Gigantspinosaurus sichuanensis made the surface of the scales uneven. Such an uneven surface scatters reflected light into different directions which help camouflage. Based on the exaction location and the scattering of the scales, we believe the skin impression fossil of Gigantspinosaurus sichuanensis is from the elbow of the forelimb, the relevant upper arm and lateral area of the body.

Key words: Stegosaurian skin impressions, Gigantspinosaurus, Shangshaximiao Formation, Upper Jurassic, Diffuse reflection

Gigantspinosaurus (Draw by Chuang Zhao)

The scales on the left of the upper right part of Gigantspinosaurus sichuanensis skin impressions