Abstract
A well-preserved large chasmosaurine ceratopsid premaxilla (MOR 1122 7-22-00-1) collected from the basal sandstone of the Cretaceous Hell Creek Formation (HCF) represents one of the stratigraphically lowest ceratopsid occurrences in the formation. The specimen was discovered in 2000, during the excavation of a large Torosaurus latus skull (MOR 1122) which was later hypothesized to represent an advanced growth stage of the more commonly recovered HCF ceratopsid Triceratops. MOR 1122 7-22-00-1 compares favorably with the incomplete premaxillae of the MOR 1122 skull and reveals details of premaxilla morphology from this stratigraphic zone. It preserves large, closely spaced ventromedial foramina, a narrow triangular process, and a thin septal flange at the base of the narial strut. The nasal process is narrow, caudally inclined and has a forked dorsal surface which appears to represent an intermediate between the morphology expressed in the slightly stratigraphically lower ceratopsid Eotriceratops xerinsularis from the Horseshoe Canyon Formation of Alberta and specimens recovered higher in the HCF. MOR 1122 7-22-00-1 expresses a deep recess extending medial to the strut of the triangular process, a feature shared with other HCF ceratopsid specimens but not Eotriceratops or other earlier occurring triceratopsin taxa. The morphology of MOR 1122 7-22-00-1 is consistent with noted stratigraphic trends in HCF ceratopsids and highlights the increased complexity of the narial region in uppermost Cretaceous triceratopsins.
Highlights
The uppermost Cretaceous Hell Creek Formation (HCF) of Montana and surrounding regions contains the remains of some of the last non-avian dinosaurs to roam western North America (Horner et al 2011; Clemens and Hartman 2014)
Museum of the Rockies (MOR) 1122 7-22-00-1 was found associated with MOR 1122, in which the premaxillae are not well preserved (Farke 2007); that skull expresses a narrow, caudally inclined strut extending from the triangular process and closely spaced ventromedial foramina, consistent with the morphology of MOR 1122 7-22-00-1 (Fig. 10)
The forked surface of the nasal process of the premaxilla (NPP) of MOR 1122 7-22-00-1 appears to be intermediate between the morphology of Eotriceratops and specimens found stratigraphically higher in the HCF (Fig. 7); direct comparisons to other specimens from the lower half of the HCF are complicated by the fact that the morphology of this process in large individuals is unknown due to non-preservation (e.g., MOR 1186), taphonomic damage (e.g., MOR 6653; Scannella et al 2014), or obfuscation due to fusion with adjacent elements (e.g., MOR 981, MOR 1122)
Summary
The uppermost Cretaceous Hell Creek Formation (HCF) of Montana and surrounding regions contains the remains of some of the last non-avian dinosaurs (hereafter referred to as ‘dinosaurs’) to roam western North America (Horner et al 2011; Clemens and Hartman 2014). Scannella — Chasmosaurine premaxilla from the Hell Creek Formation (Scannella and Horner 2010, 2011; Horner and Lamm 2011) This synonymy hypothesis has been the subject of ongoing study and Torosaurus is considered a distinct taxon by other authors (see Farke 2011; Longrich and Field 2012; Maiorino et al 2013). Initial excavation was undertaken by Ken Olson and MOR; later excavation was completed by Bob Harmon, Nels Peterson, and the 2000 MOR paleontology field crew This specimen (MOR 1122; Fig. 2) represents one of the stratigraphically lowest ceratopsid specimens recovered from the HCF of Montana (Scannella et al 2014).
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