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 : 1186

THESCELOSAURUS ASSINIBOIENSIS SP. NOV.

Thescelosaurus assiniboiensis sp. nov. represents the smallest ornithischian dinosaur recovered from the Frenchman Formation of Saskatchewan, and may represent the smallest ornithischian from the late Maastrichtian of North America. This time interval is generally characterized by large dinosaurs, in many cases, the largest to evolve within their respective lineages, such as Tyrannosaurus rex and Triceratops horridus . Although T. neglectus represents a noted size increase from the putative sister taxon of Thescelosaurus , Parksosaurus warreni , the holotype of T. assiniboiensis sp. nov. is of equal size to the holotype and only known specimen of Parksosaurus (ROM 804). This suggests that although many of the latest Maastrichtian dinosaur taxa show a potential size increase, the small herbivore niche probably remained occupied.

In RSM P 1225.1 the neurocentral sutures of the anterior dorsals are not fused, leaving their neural arches dissociated from their respective centra. The posterior dorsals, although articulated, retain visible sutural lines throughout the series, whereas the neurocentral sutures are firmly fused in the sacral and caudal vertebrae, with suture lines being obliterated. The lack of fusion of the neurocentral sutures is a common occurrence in Thescelosaurus , regardless of the absolute size of the animal (MOR 1106, 1164, 1165, NCSM 15728, and SDSM 7210). The closure of neurocentral sutures serves as a conserved sizeindependent indicator of ontogeny in extant crocodilians, and has been suggested to have developed similarly in extinct crocodilian relatives ( Brochu, 1996). Irmis (2007), however, found that this trend was not uniformly expressed across phytosaur ontogeny, and suggested that its consistency should not be assumed for extinct archosaurs. Although as yet inadequately documented, the retention of open neurocentral sutures occurs more broadly than solely in Thescelosaurus , and is common in many basal ornithopod specimens, presumed to be at, or near, adult size [ Camptosaurus (USNM 4697), Dryosaurus (CM 21786), Hypsilophodon (NHMUK R 196), Orodromeus (MOR 623), Zephyrosaurus (MCZ 4392)], as well as in several undescribed North American basal ornithopod taxa (C.M. Brown, pers. observ.). Two hypotheses can be erected to explain this occurrence: firstly, sutural closure is ontogenetically conserved, as it is in extant crocodilians, and the sample of basal ornithopod specimens is disproportionately represented by juvenile material; secondly, and alternatively, neurocentral sutural fusion in many basal ornithopod taxa is either not tightly correlated with ontogeny or is ontogenetically postdisplaced, and sutures may remain open for the majority of the life of the animal. A more extensive investigation of this phenomenon and testing of these hypotheses, in conjunction with the exploration of other aspects of the ontogeny of Thescelosaurus , is currently underway.

All autapomorphies of T. assiniboiensis sp. nov., as well as two synapomorphies (posterior half of ventral edge of jugal offset ventrally and covered laterally with obliquely inclined ridges, and frontals wider midorbitally than across posterior margin) and two potential synapomorphies (presence of a Y-shaped indentation on the dorsal edge of opisthotics and palpebral dorsoventrally flattened and rugose along the medial and distal edges) of Thescelosaurus , are preserved in the cranial skeleton of the holotype (RSM P 1225.1). The postcranial skeleton of this specimen, however, preserves many of the features that are more similar to those of Parksosaurus and Orodromeus than to the other species of Thescelosaurus , specifically in the ilium and femur. These are interpreted here as plesiomorphies. The regional segregation of character occurrence (the cranial skeleton preserving derived characters, and the postcranial skeleton preserving putatively ancestral characters) suggests that cranial synapomorphies of Thescelosaurus initially differentiated this genus from Parksosaurus , and that many characters associated with the postcranial skeleton were acquired within Thescelosaurus , and may have arisen in association with increased body size.

As was found by Boyd et al. (2009), our cladistic analysis recovered a clade composed exclusively of basal ornithopods from the Cretaceous of North America ( Fig. 23 ). It is subdivided into an earlier Cretaceous small-bodied clade ( Orodromeus , Oryctodromeus , and Zephyrosaurus ), and a larger-bodied later Cretaceous clade ( Thescelosaurus and Parksosaurus ). This topology greatly reduces the ghost lineages for many of these taxa (most notably Thescelosaurus ) that were implied in previous phylogenetic hypotheses ( Sereno, 1986; Weishampel & Heinrich, 1992). Additionally, the monophyly of the Cretaceous North American taxa suggests that they may represent an exclusively North American radiation of basal ornithopods during the Cretaceous, a pattern not previously recognized. Further work incorporating newly discovered taxa and examining taxa that fall outside of this clade can be used to test this hypothesis, and may reveal this radiation to encompass more taxa, and to be more widespread than is currently understood.

Figures

Figure 23. The single most-parsimonious tree (tree length 361 steps) recovered from the cladistic analysis using the implicit enumeration search option in TNT. Bootstap values (1000 replicates) above 50% are reported below the branches, and Bremer support values greater than 1 are reported above.