A Juvenile Elasmosaur Skull Preserved in a Concretion from the Upper Cretaceous Bearpaw Formation of Saskatchewan, Canada

STREET, Hallie P; Royal Saskatchewan Museum, T. rex Discovery Centre, Eastend, Saskatchewan, Canada. MEKARSKI, Michelle M C; Canada Science and Technology Museum, Ottawa, Ontario, Canada. BAMFORTH, Emily L; Royal Saskatchewan Museum, T. rex Discovery Centre, Eastend, Saskatchewan, Canada. SMITH, Anthony J; Redpath Museum, McGill University, Montreal, Quebec, Canada.  TAHARA, Rui; Redpath Museum, McGill University, Montreal, Quebec, Canada. LARSSON, Hans CE; Redpath Museum, McGill University, Montreal, Quebec, Canada.

Abstract 

During the Cretaceous the North American continent was inundated by multiple incursions of the Western Interior Seaway.  The Bearpaw Sea represents one of the final transgressions of this sequence and extended across parts of Alberta, Montana, and Saskatchewan during the late Campanian.  Typically, the Bearpaw Formation consists of dark gray shales, but across Saskatchewan various near-shore localities with greater terrestrial influences exhibit differing lithologies.  An unusual outcropping of the Bearpaw Formation from southwestern Saskatchewan preserves fossil material in concretions.  Breaking the concretions open has revealed bivalve, ammonite, crustacean, echinoderm, and vertebrate material.  Additionally, the layer that produces the concretions is underlain by a stratum that is highly bioturbated and preserves abundant infilled burrows.

One concretion from this site contains the skull and a vertebra of a very small elasmosaur.  This specimen is interpreted as representing a young juvenile based on both its small size (skull ~13 cm long) and the poorly ossified state of the cranial bones.  Due to the mode of preservation within the concretion, much of the preserved bone is quite brittle.  Mechanical preparation was therefore not recommended, and the entire concretion was scanned using micro-computed tomography.  Models rendered from the micro-CT scans reveal that the skull underwent little to no deformation during preservation.  The scans also show that there is greater contrast between the matrix of the concretion and cranial elements with endochondral developmental origins than elements with dermatocranial origins.  The poor contrast between the dermatocranial bones and the matrix could indicate that these elements were less well ossified than the endochondral elements.  Considering that the dermatocranium typically ossifies first, this finding was unexpected.  It is more difficult to study developmental patterns for fossil groups with no living descendants, but this specimen provides a rare example of early ossification in the plesiosaur skull.