Unusual Ceratopsian Frill Morphologies from the uppermost Maastrichtian Frenchman Formation (66 Ma) of Saskatchewan, Canada

BAMFORTH, Emily L.; Royal Saskatchewan Museum, Eastend, SK, Canada. STREET, Hallie P.; Royal Saskatchewan Museum, Eastend, SK, Canada

Abstract

Large-bodied ceratopsians from the latest Maastrichtian (66 Ma) of North America are traditionally classified into two genera, Triceratops and Torosaurus. Debate exists as to whether these belong to a single ontogenetic series, represent one taxon with taphonomic/pathologic variances, or truly represent distinct taxonomic groups.  The Frenchman Formation of southern Saskatchewan represents a northern extension of the upper Hell Creek Formation into Canada. In the Frenchman Formation, ceratopsian material accounts for approximately 70% of all dinosaur fossils recovered, the majority of which is referred to Triceratops prorsus. Although not necessarily reflective of taxonomic diversity, two unusual ceratopsian specimens under Royal Saskatchewan Museum curation suggest an underappreciated morphological diversity in ceratopsians from this time period. 1) Specimen EMP 16.1, collected in the 1920s and originally identified as ‘cf. Torosaurus’, is represented by a parietal–squamosal frill and associated post-crania. The frill is large, broad and quadrangular in shape, lacking epoccipitals. The parietal bar is missing taphonomically, but the parietal fenestrae are suggested to be symmetrical, and larger and more oblong than what is typical for Torosaurus. The specimen also displays smaller symmetrical, oblong squamosal fenestrae. EMP 16.1 may represent an atypical morph of Torosaurus, an undescribed ontogenic stage of the same, or possibly a new species. 2) Specimen RSM P3218.1, collected in 2017-2018, consists of a partial frill, nasal horncore, nasals and partial rostrum. Immediately adjacent to the parietal-squamosal suture, the parietal frill displays a distinct thinning of the bone, ending in defined, crenulated margins less than 1 cm thick. These margins form the edge of two partially preserved oblong fenestrae. The thinning out of the bone surrounding fenestrae has been suggested to represent ontogenic parietal thinning in older ceratopsian animals. Interestingly however, RSM P3218.1 also appears to display sub-adult characters. The recovery of more of this specimen may help to elucidate these unusual features. The anomalous characters displayed in EMP 16.1 and RSM P3218.1 may be reflective of ontogenic/morphological variations in Triceratops and/or Torosaurus, could represent northern regional variants, or may be new species. Future research will help to establish ceratopsian taxonomic diversity in the Frenchman Formation, as well as the range of phenotypic plasticity that may be expected in ceratopsian skulls from a single taxon.