Thursday, May 19, 2016

[Paleontology • 2016] Spiclypeus shipporum • A Boldly Audacious New Chasmosaurine Ceratopsid (Dinosauria: Ornithischia) from the Judith River Formation (Upper Cretaceous: Campanian) of Montana, USA

Spiclypeus shipporum 
Mallon, Ott, Larson, Iuliano & Evans, 2016


This study reports on a new ceratopsid, Spiclypeus shipporum gen et sp. nov., from the lower Coal Ridge Member of the Judith River Formation in Montana, USA, which dates to ~76 Ma (upper Campanian). The species is distinguished by rugose dorsal contacts on the premaxillae for the nasals, laterally projecting postorbital horncores, fully fused and anteriorly curled P1 and P2 epiparietals, and a posterodorsally projecting P3 epiparietal. The holotype specimen is also notable for its pathological left squamosal and humerus, which show varied signs of osteomyelitis and osteoarthritis. Although the postorbital horncores of Spiclypeus closely resemble those of the contemporaneous ‘Ceratops’, the horncores of both genera are nevertheless indistinguishable from those of some other horned dinosaurs, including Albertaceratops and Kosmoceratops; ‘Ceratops’ is therefore maintained as a nomen dubium. Cladistic analysis recovers Spiclypeus as the sister taxon to the clade Vagaceratops + Kosmoceratops, and appears transitional in the morphology of its epiparietals. The discovery of Spiclypeus adds to the poorly known dinosaur fauna of the Judith River Formation, and suggests faunal turnover within the formation.

Systematic palaeontology

Dinosauria Owen, 1842

Ornithischia Seeley, 1887

Ceratopsia Marsh, 1888
Neoceratopsia Sereno, 1986

Ceratopsidae Marsh, 1888
Chasmosaurinae Lambe, 1915

Spiclypeus gen. nov.

Spiclypeus shipporum, gen. et sp. nov.

Etymology: The genus name (pronounced ‘spick-LIP-ee-us’) derives from the Latin for spike (spica) and shield (clypeus), referring to the many large, spike-like epiossifications about the margin of the parietosquamosal frill. The specific epithet (pronounced ‘ship-OR-um’) honours Dr. Bill and Linda Shipp, the original owners of the holotype, and their family.

Holotype: CMN 57081, a partial skull and postcranium.

Locality, horizon, and age: The holotype and only known specimen is from Fergus County, Montana, near the town of Winifred. The type locality occurs within the lower Coal Ridge Member of the JRF, several meters above the mid-Judith discontinuity, which dates to 76.24 Ma ± 0.18 Ma [10]. The top of the member has been dated to 75.21 ± 0.12 Ma [10], which marks the upper age bracket of the species.

Diagnosis: Chasmosaurine ceratopsid with autapomorphic rugose nasal contact on the lateral surface of the dorsal process of the premaxilla. Spiclypeus is also diagnosed by the following unique combination of characters: (1) postorbital horncores project dorsolaterally; (2) all six epiparietals fused at bases; (3) epiparietals P1 and P2 curl anteriorly from posterior margin of frill; (4) epiparietal P3 projects posterodorsally.

With respect to other chasmosaurines from the Judith River Formation (i.e., Judiceratops, Medusaceratops, Mercuriceratops), Spiclypeus can further be distinguished by the large, triangular epiossifications laterally on the parietal and squamosal. It also differs from Judiceratops [6] in the medial embayment of the posterior parietal bar. Spiclypeus lacks the laterally directed epiparietals that characterize Medusaceratops [7], and the hatchet-shaped lateral margin of the squamosal that characterizes Mercuriceratops [8].

Jordan C. Mallon, Christopher J. Ott, Peter L. Larson, Edward M. Iuliano and David C. Evans. 2016. Spiclypeus shipporum gen. et sp. nov., A Boldly Audacious New Chasmosaurine Ceratopsid (Dinosauria: Ornithischia) from the Judith River Formation (Upper Cretaceous: Campanian) of Montana, USA. PLoS ONE. 11 (5): e0154218.  DOI: 10.1371/journal.pone.0154218


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