Abstract
BackgroundThe chasmosaurine ceratopsid Chasmosaurus is known from the Upper Cretaceous (Campanian) Dinosaur Park Formation of southern Alberta and Saskatchewan. Two valid species, Chasmosaurus belli and C. russelli, have been diagnosed by differences in cranial ornamentation. Their validity has been supported, in part, by the reported stratigraphic segregation of chasmosaurines in the Dinosaur Park Formation, with C. belli and C. russelli occurring in discrete, successive zones within the formation.Results/ConclusionsAn analysis of every potentially taxonomically informative chasmosaurine specimen from the Dinosaur Park Formation indicates that C. belli and C. russelli have indistinguishable ontogenetic histories and overlapping stratigraphic intervals. Neither taxon exhibits autapomorphies, nor a unique set of apomorphies, but they can be separated and diagnosed by a single phylogenetically informative character—the embayment angle formed by the posterior parietal bars relative to the parietal midline. Although relatively deeply embayed specimens (C. russelli) generally have relatively longer postorbital horncores than specimens with more shallow embayments (C. belli), neither this horncore character nor epiparietal morphology can be used to consistently distinguish every specimen of C. belli from C. russelli.Status of Kosmoceratops in the Dinosaur Park FormationKosmoceratops is purportedly represented in the Dinosaur Park Formation by a specimen previously referred to Chasmosaurus. The reassignment of this specimen to Kosmoceratops is unsupported here, as it is based on features that are either influenced by taphonomy or within the realm of individual variation for Chasmosaurus. Therefore, we conclude that Kosmoceratops is not present in the Dinosaur Park Formation, but is instead restricted to southern Laramidia, as originally posited.
Highlights
Chasmosaurus is a chasmosaurine ceratopsid known from the Upper Cretaceous (Campanian) Dinosaur Park Formation (DPF) of southern Alberta and Saskatchewan (Fig 1)
Admitting that considerable gradational variation existed among Chasmosaurus specimens, they retained C. belli and C. russelli as distinct taxa, and used the purported stratigraphic segregation of specimens assigned to these taxa in the DPF (i.e., C. belli temporally replacing C. russelli) to support their conclusions
The relatively large stratigraphic range of C. russelli is the result of recently conducted fieldwork, which revealed that the holotype (CMN 8800) of this species was recovered from the upper DPF, and not the lower DPF as previously supposed
Summary
Chasmosaurus is a chasmosaurine ceratopsid known from the Upper Cretaceous (Campanian) Dinosaur Park Formation (DPF) of southern Alberta and Saskatchewan (Fig 1). Referred to Monoclonius [1], these specimens were subsequently reassigned to the new genus Chasmosaurus [2]. Longrich [6] subsequently erected the new taxon, Mojoceratops perifania, from specimens previously referred to Chasmosaurus. The chasmosaurine ceratopsid Chasmosaurus is known from the Upper Cretaceous (Campanian) Dinosaur Park Formation of southern Alberta and Saskatchewan. Chasmosaurus belli and C. russelli, have been diagnosed by differences in cranial ornamentation. Their validity has been supported, in part, by the reported stratigraphic segregation of chasmosaurines in the Dinosaur Park Formation, with C. belli and C. russelli occurring in discrete, successive zones within the formation
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