Pat S. Druckenmiller

Dale A. Russell

Academic paper

alaeontographica Abteilung a -Stuttgart

283

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A new plesiosaur taxon, Nichollsia borealis, gen. et sp. nov., from the Wabiskaw Member of the Clearwater Formation (Lower Cretaceous, Albian) of northeastern Alberta, Canada, is described. The exceptionally well-preserved, almost complete, fully articulated specimen represents the oldest known, and one of the most complete, Cretaceous plesiosaurs from North America. High resolution computed tomographic data of the skull provide detailed information regarding cranial structure. Nichollsia is compared to other Cretaceous genera, including Leptocleidus, with which it shares similar skeletal proportions, size, and the presence of a prominent dorsomedian ridge on the premaxillae. Nichollsia, however, possesses numerous autapomorphies, including the possession of a gracile, narrowly triangular skull lacking a rostral constriction, a vertically oriented suspensorium, a squamosal vertex that lacks a prominent crest, the presence of longitudinal grooves on the lateral surface of the dentary, a relatively homodont dentition lacking caniniforms, and other unique features of the axial and appendicular skeleton. Nichollsia inhabited the Boreal Sea, the first major marine incursion into the northern part of the Western Interior Basin in the Early Cretaceous prior to the establishment of the Western Interior Seaway.

English