Thescelosaurus assiniboiensis, Brown & Boyd & Russell, 2011

Brown, Caleb Marshall, Boyd, Clint A. & Russell, Anthony P., 2011, A new basal ornithopod dinosaur (Frenchman Formation, Saskatchewan, Canada), and implications for late Maastrichtian ornithischian diversity in North America, Zoological Journal of the Linnean Society 163 (4), pp. 1157-1198 : 1187

THESCELOSAURUS IN THE FRENCHMAN FORMATION

The occurrence of a new species of Thescelosaurus from the Frenchman Formation necessitates a review of other material referred to Thescelosaurus from the formation. Three other specimens of Thescelosaurus , exhibiting a large size range, have been retrieved from this unit. Unfortunately, none of these preserves cranial material and therefore cannot be identified at the specific level. The large articulated specimen RSM P 2415.1 does, however, preserve both ilia. They exhibit the distinctive dorsal kink at the posterior margin common to T. neglectus (USNM 7757) and other specimens of Thescelosaurus (MOR 979, NCSM 15728). This character may prove to be diagnostic for T. neglectus . If this is the case, it indicates the presence of both T. neglectus and T. assiniboiensis sp. nov. in the Frenchman Formation, and suggests spatial and temporal overlap of these two taxa. The discovery of additional diagnostic material referable to Thescelosaurus , as well as an increased understanding of the stratigraphy of the Frenchman Formation, are required to explore these questions further.

The discovery of a multitaxa bone bed within the Frenchman Formation (the ‘convenience store’ locality) has yielded isolated limb elements from several individuals of Thescelosaurus , all of very small size. All limb elements preserved are smaller than their respective elements in RSM P 1225.1, which is the smallest known associated, reasonably complete skeleton of Thescelosaurus . The simplest interpretation is that this site preserves a rich sample of juvenile specimens of Thescelosaurus . All material of Thescelosaurus recovered from the Frenchman Formation, with the exception of RSM P 2415.1, is smaller than the smallest specimens recovered from the Hell Creek, Lance, and Scollard formations (RSM P 1225.1 and CMN 22039). This suggests a general trend towards the preservation of ontogenetically young individuals of Thescelosaurus in the Frenchman Formation , a pattern repeated for Tyrannosaurus , for which large volume of juvenile and subadult material has been collected and reported (RSM P 2347.1, partial fragmentary juvenile skeleton; 2693.1, juvenile metatarsal; 2990.1, subadult lacrimal; 2416.82, juvenile metatarsal), and for Triceratops (RSM P 2299.1, subadult supraorbital horncore; 2613.1, subadult supraorbital horncore; 2982.1, juvenile skull) ( Tokaryk, 1997b; Snively & Longrich, 2009). Given the sample size of the collected material from the Frenchman Formation, the volume of juvenile material is proportionally much larger than that retrieved from the Hell Creek, Lance, and Scollard formations. At present, however, it remains uncertain whether this represents a biological signal, or is the result of preservational biases based on associations of lithology and ontogeny ( Goodwin & Horner, 2010).