Wu, Brinkman, Eberth & Braman, 2007
Cretaceous Late Maastrichtian
Ornithischia Genasauria Cerapoda Marginocephalia Ceratopia Neoceratopia Ceratopidae Chasmosaurinae
Horseshoe Canyon Formation, Dry Island Buffalo Jump Provincial Park, Alberta, Canada
Genus - Typespecies - Skull
Eotriceratops xerinsularis is a large chasmosaurine that differs from other chasmosaurines in a unique set of features in the premaxilla, nasal horn core, squamosal frill, and epijugal, it is the first associated vertebrate skeleton found within the upper 20 m of the Horseshoe Canyon Formation.
The most striking of those features includes an extremely tall, non-recessed narial process of the premaxilla; the presence of greatly elongate, spindle-shaped epoccipitals on the squamosal frill; a deep, well-demarcated fossa on the anteroventral surface of the squamosal frill; a sharply conical epijugal with a pronounced proximoposterior process and separate fossa-like facets for the jugal and quadratojugal; and the presence of an obliquely extending vascular trace meeting a transverse vascular trace ventrally on the anterior surface of the nasal horn core. Our phylogenetic analysis suggests that E. xerinsularis is nested within a clade including Triceratops, Diceratops, and Torosaurus, which are all from late Maastrichtian deposits.
The upper 20 m of the Horseshoe Canyon Formation comprises a coal-rich interval (Carbon-Thompson coal zone, unit 5), which previously has been assigned to upper Maastrichtian magnetochrons 31n and 30r, and the Mancicorpus gibbus miospore subzone. The ceratopsid specimen was collected from between the Carbon and Thompson coal seams, and thus, is inferred to (1) occur near the top of magnetochron 31n and (2) have an age of 67.6-68.0 Ma. Large chasmosaurine ceratopsids, such as Triceratops and Torosaurus, have not previously been described from the Horseshoe Canyon Formation or from magnetochron 31n or the M. gibbus miospore subzone. Thus, Eotriceratops is distinctly older than any other ceratopsid in the "Triceratops" group, and the discovery of E. xerinsularis helps fill a biostratigraphic gap between early and late Maastrichtian chasmosaurines.
Holotype: RTMP 2002.57.7 an incomplete and disarticulated skeleton, inlcuding right ristral, both premaxillae, both maxillae, left superorbital horn core with lacrimal, prefrontal, frontal, postorbital and jugal, left epijugal, right quadratojugal, left quadrate, partial parietal, left squamosal frill, braincase, syncervical, cervicals 4 and 5, a string of vertebrae including posterior cervicals and anterior dorsals, some ribs and fragments of ossified ligaments.
Locality and horizon: Whitin Dry Island Buffalo Jump Provincial Park (northeastern quarter), southern Alberta; between coal seams 11 and 12, in the upper 20 meter of the Horseshoe Canyon Formation.
Etymology: Generic name Eotriceratops implies that it is an early member of the Triceratops group: specific name xerinsularis refers to Dry Island Buffalo Jump Provincial Park, where the specimen was collected.
Diagnosis: A large chasmosaurine ceratopsid with a skull length of about 3 meter, measured from the tip of the snout to the posterior edge of frill. It differs from other chasmosaurines in having the following combination of derived features: (1) tall lamina-like narial process of premaxilla lacking any fossa or recess and having its dorsal margin well above ventral border of interpremaxillary fenestra; (2) epoccipitals of squamosal frill extremely elongate, spindle-shaped, and contacting each other; (3) a well-demarcated elliptical depression or fossa on anteroventral surface of squamosal frill; (4) a deep, slightly oblique trace for blood vessels meeting a transverse trace ventrally on anterior surface of nasal horn; and (5) sharply conical epijugal with a pronounced proximposterior process and separate fossa-like articular facets for jugal and quadratojugal. (Wu et al, 2007)