Sedimentological and taphonomic observations on the “Dragon's Tomb” Saurolophus (Hadrosauridae) bonebed, Nemegt Formation (Upper Cretaceous), Mongolia

The famous “Dragon's Tomb” Saurolophus bonebed at Altan Uul II preserves multiple complete skeletons with skin impressions, and is exceptional among Nemegt Formation dinosaur localities as the only mass accumulation of articulated individuals. Here, we present results from an on-going paleontological project focused on the Baruungoyot-Nemegt succession in order to document this unique locality. Sedimentological data allow us to confirm that the fossils and host sediments were deposited in the thalweg of a meandering paleochannel that experienced a variety of high and lower energy flow conditions. A variety of taphonomic data were collected from exposed fossils, including those displaced by poaching. Three-dimensional maps of the host sediments and fossils were generated, and 13 m2 of exposed, in-situ bones were mapped. Previously undocumented portions of at least 13 additional Saurolophus individuals and a partial subadult Tarbosaurus skeleton were identified. The Dragon's Tomb is a monodominant bonebed with at least three size classes (juvenile, subadult, and large adult) of Saurolophus preserved. The bonebed has a minimum size of ~ 2000 m2, suggesting that over 100 Saurolophus carcasses may have contributed to the thanatocoenose. Although the data provide evidence for a catastrophic mass death of a social group of Saurolophus and provide the first evidence of gregariousness in this taxon, the cause of death cannot be determined.