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| Stellasaurus ancellae
Wilson, Ryan & Evans, 2020
DOI: 10.1098/rsos.200284
Illustration: Andrey Atuchin
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Abstract
Ceratopsids are among the most ubiquitous dinosaur taxa from the Late Cretaceous terrestrial formations of the Western Interior of North America, comprising two subfamilies, Chasmosaurinae and Centrosaurinae. The Two Medicine Formation of northwestern Montana has produced numerous remains of centrosaurine dinosaurs, which represent three taxa previously considered valid: Rubeosaurus ovatus, Einiosaurus procurvicornis and Achelousaurus horneri. Here, we reassess the previous referral of specimens to Rubeousaurus ovatus and demonstrate that this taxon is represented solely by its holotype specimen, which was first diagnosed as Styracosaurus ovatus. One of the specimens previously referred to ‘Rubeosaurus’ ovatus instead represents a new eucentrosauran centrosaurine taxon diagnosed here, Stellasaurus ancellae gen. et sp. nov. Stellasaurus expresses a unique combination of eucentrosauran centrosaurine characters, including an elongate nasal horncore, diminutive supraorbital horncores, and a parietal bearing straight, elongate P3 processes, semi-elongate P4 processes and non-elongate P5, P6 and P7 processes. Within the stratigraphic succession of Eucentrosaura, Stellasaurus occurs intermediate to Styracosaurus albertensis and Einiosaurus, and likewise reflects intermediate morphology. Assessed within the stratigraphic, geographical, taphonomic, ontogenetic and phylogenetic framework of Unified Frames of Reference, we fail to reject the hypothesis that Stellasaurus ancellae represents a transitional taxon within an anagenetic lineage of eucentrosauran centrosaurines.
Keywords: centrosaurine, evolution, ceratopsid, dinosaur, anagenesis, Cretaceous
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| Figure 4. Left lateral parietal bar of Stellasaurus ancellae holotype MOR 492 in dorsal and ventral views.
EPS, epiparietosquamosal; PSC, parietosquamosal contact. Scale bar 10 cm.
Parietal line drawing modified from Evans & Ryan [2015], Public Library of Science (PLoS), used under Creative Commons Attribution 4.0.
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Systematic palaeontology
Dinosauria Owen, 1842
Ornithischia Seeley 1887
Ceratopsia Marsh, 1890
Neoceratopsia Sereno, 1986
Ceratopsidae Marsh, 1888
Centrosaurinae Lambe, 1915
Stellasaurus gen. nov.
Stellasaurus ancellae gen et. sp. nov.
Etymology: The genus name Stellasaurus, ‘star lizard', is derived from Stella, Latin for star, and –saurus, Greek for lizard, in reference to the overall star-like appearance of the cranial ornamentation, and in homage to the song ‘Starman' by David Bowie. The species name ancellae honours Museum of the Rockies field palaeontologist and fossil preparator Carrie Ancell, who discovered and prepared MOR 492, the holotype specimen of Stellasaurus ancellae, as well as the holotype of Achelousaurus horneri, and co-discovered the holotype of Einiosaurus procurvicornis, and whose decades of extraordinary fossil preparation have furthered vertebrate palaeontology beyond measure.
Holotype: MOR 492, an isolated partial skull of a centrosaurine ceratopsid preserving the left lateral parietal bar, proximal portion of the midline parietal bar, near-complete paired and fused nasals, partial left premaxilla, and partial left postorbital and associated supraorbital ornamentation.
Locality, horizon and age: MOR 492 was recovered from Lithofacies 5 of the uppermost Two Medicine Formation, 65 m below the upper contact between the Two Medicine Formation and the overlying Bearpaw Formation. ....
Diagnosis: Centrosaurine ceratopsid exhibiting a unique combination of characters intermediate in distribution between the stratigraphically preceding Styracosaurus albertensis and stratigraphically successive Einiosaurus procurvicornis: elongate, erect and recurved nasal horncore and diminutive supraorbital ornamentation with pointed apex, as in Styracosaurus albertensis; parietal with elongate, straight P3 processes (spikes), partially elongate P4 processes (spikes) less than half as long as P3, and non-elongate P5, P6 and P7 processes, as in Einiosaurus procurvicornis; unique P4 elongation, intermediate between that of Styracosaurus albertensis and the stratigraphically successive lowest bonebed of Einiosaurus. As stated above, elongate, erect nasal horncores are variably present in Centrosaurus apertus, though rarely with the same recurvature and degree of hypertrophy as in both Styracosaurus albertensis and Stellasaurus. Einiosaurus procurvicornis specimens of equivalent maturity (e.g. MOR 456 8-9-6-1; young adult, based on degree of P7 and P6 parietal process imbrication and bone surface texture) express strongly procurved nasal horns and rounded masses of bone as supraorbital ornamentation, differing significantly from the facial ornamentation seen in Stellasaurus.
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| Figure 11. Stratigraphic and temporal relationship of the centrosaurine taxa hypothesized as representing an anagenetic lineage, with the evolution of their parietal ornamentation, within their respective geological formations. Letters are colour coded by taxon to match transformation of parietal ornamentation listed. (a) Styracosaurus albertensis, (b) Stellasaurus ancellae, (c) and (c') Einiosaurus procurvicornis MOR 373 and MOR 456, respectively, indicating slight stratigraphic separation with associated morphological change of the P4 process, (d) Achelousaurus horneri, (e) Pachyrhinosaurus lakustai. Generalized stratigraphic columns are based on [Fowler, 2017].
Relative taxon placement is based on stratigraphic occurrence of specimens within their respective geological formations and the temporal relationships of host strata based on [Fowler, 2017] (see Material and methods and Discussion). Styracosaurus ovatus is omitted due to its uncertain stratigraphic placement within the Two Medicine Formation. Parietal line drawings used and modified from Evans and Ryan ([2015] fig. 15), Public Library of Science (PLoS), used under Creative Commons Attribution 4.0.
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| Figure 12. Comparison of the two major evolutionary mode hypotheses for the Two Medicine Formation centrosaurines and the associated lines of evidence supporting or absent. Parietal line drawings modified from Evans & Ryan ([2015] fig. 15), Public Library of Science (PLoS), used under Creative Commons Attribution 4.0. |
John P. Wilson, Michael J. Ryan and David C. Evans. 2020. A New, Transitional Centrosaurine Ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the Evolution of the ‘Styracosaurus-line' Dinosaurs. Royal Society Open Science. DOI: 10.1098/rsos.200284
