Scavengers, Predators and Dinosaurs of the Bearpaw Formation of Alberta

LAFRAMBOISE, Marilyn C.; Royal Tyrrell Museum of Palaeontology, Drumheller, AB


The Bearpaw Formation (74-71 ma) is an extensive marine unit deposited during the last transgressive cycle of the epicontinental Western Interior Seaway. The Bearpaw Formation extends from immediately northeast of Edmonton, Alberta southward in ever thickening intervals to northern Montana, and outcrops as far east as southwest Saskatchewan. In southern Alberta the Bearpaw Formation is wedged between the overlying mostly non-marine St. Mary and Horseshoe Canyon Formations and the underlying non-marine Belly River Group.  The Bearpaw along the St. Mary River near Lethbridge, Alberta is primarily laminated shale and siltstone with sandstone beds and kaolinitic claystone. Ammonite mining along the St. Mary River has exposed the Bearpaw Formation with particularly fossiliferous layers known as the Muddy Unit 1 or Baculites compressus Zone (4).  The top of this unit has a distinct volcanic ash layer called the 10” (25 cm) Bentonite layer, and the underlying Lethbridge Coal Zone (Belly River Group) have been used to measure the vertical position of vertebrate remains.

The selection of scavengers, predators and dinosaurs of the Muddy Unit 1 presented here indicates a complex near shore habitat.  Both exceptionally preserved mosasaurs and plesiosaurs are found to be biostratigraphically separate and equivalent plus both apex predators show scavenging by sharks.  Stomach contents include turtles and fish including the much under studies Enchodus.  Two partial skeletons of Prosaurolophus and theropod skull elements can be correlated to the same biostratigraphic level as two mosasaurs with signs of scavenging although the dinosaurs show none.

This important site can be useful in future research to correlate this particular bentonite signature in Bearpaw exposures in southeast Alberta and southwest Saskatchewan by the presence or absence of scavengers, predators and dinosaurs.