SKIN IMPRESSIONS FROM THE TAIL OF A HADROSAURIAN DINOSAUR IN THE KAIPAROWITS FORMATION (UPPER CRETACEOUS), GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT

During the excavation of the tail of a hadrosaurian dinosaur in 2001, we discovered skin impressions with counterpart casts of the same impressions, preserved in exceptional detail. The skeleton was in a fluvial siltstone bed of the Upper Cretaceous Kaiparowits Formation (Campanian/Maastrichtian Age) in the Grand Staircase-Escalante National Monument (GSENM), Kane County, Utah. The skeleton occurred with limonitic and carbonaceous plant impressions and fragments. The skeleton consists of a nearly complete, articulated tail with chevrons and ossified tendons; and fragmentary sacral vertebrae, dorsal vertebrae, ribs, ilium, and elements of the rear limb including astragalus and femur. Its taxonomic identity is uncertain; we tentatively identify it as a lambeosaurine hadrosaur. The impressions originated from four anatomical positions: dorsal to the last sacral and first caudal vertebrae, dorsal to caudal 10, the lateral surface of the tail beneath caudals 24-28, and in a block of rock from an unknown position but probably from the middle region of the tail. All seem to have originated from the underside (anatomically left side) of the carcass as it lay on its side during burial. The impressions range from diamond-shaped polygons in regular diagonal rows, to irregular 5- and 6-sided polygons in poorly defined rows. Some impressions contain a weakly expressed radial pattern. Their size is remarkably uniform, generally 1-2 cm wide. All impressions appear to indicate edge-to-edge contact, with no indication of overlap or imbrication. In life, the skin of the tail consisted of convex, non-imbricating scales, with no indication of larger tubercles as described for several other hadrosaur skeletons with skin impressions. Genesis of the impressions and counterparts seems to have originated by impression of the external surface of the skin in soft sediments, and decomposition of the skin without destruction of the skin impressions. The majority of previously described dinosaur skin impressions are hadrosaurian. The only other published record of dinosaur skin impressions from the state is a hadrosaur of undetermined identity in the Neslen Formation of east-central Utah. This specimen highlights the potential for the discovery of similarly well-preserved dinosaurs in the Kaiparowits Formation within the GSENM.