A Remarkable Vertebrate Assemblage from the Lance Formation, Niobrara County, Wyoming.

L. Spencer, L. Turner and A.V. Chadwick

During the process of an on-going taphonomic study of the Lance vertebrates, a small quarry was opened up on a sandstone ridge abutting a calcareous cemented cobbly sandstone bone bed one meter in thickness.  The deposit concerned here occurs in a poorly cemented well-sorted immature white sandstone and appears as a lag deposit on the trailing edge of the larger mass of the bone bed itself.  Transport of the entire unit appears to have been to the south, based upon imbrication of the flat pebble clasts contained in the unit.  The bone bed contains a transported assemblage of disarticulated bones in all size categories, ranging from small bone chips and ossified tendons to whole femora. The lag deposit contains a variety of bones, but they tend to be smaller than those of the main bone bed, and the entire thickness of the lag deposit is less than 20 cm.  The profile of the residual tails and disappears in the white sandstone over a distance of two meters upstream of the bone bed.  The materials described below were recovered from 20 cm depth in an area of two square meters in this lag.

The lag deposit contained a variety of bones of Edmontosaurus, fragments of turtle and fish bones, fish scales, and numerous teeth.  While many of the teeth were those of Edmontosaurus, and a dentary of a juvenile Edmontosaurus was recovered, teeth of several other species of dinosaur were recovered, including Triceratops horridus, Tyrannosaurus rex, Nannotyrannus sp., and Troodon formosus. Associated with the dinosaur teeth were teeth from crocodilians, fish, multituberculates and other forms still under study.  The accumulation of such a variety of species in such a small area is consistent with a residual deposited upstream of a high energy debris accumulation.