Original paper

Skeletal anatomy of an exceptionally complete specimen of a new genus of plesiosaur from the Early Cretaceous (Early Albian) of northeastern Alberta, Canada

Druckenmiller, Patrick S.; Russell, Anthony P.

Abstract

Skeletal anatomy of an exceptionally complete specimen of a new genus of plesiosaur from the Early Cretaceous (Early Albian) of northeastern Alberta, Canada by Patrick S. Druckenmiller and Anthony P. Russell With 17 figures and 1 table in the text Zusammenfassung Ein neuer Plesiosaurier Taxon, Nichollsia borealis, gen. et sp. nov, aus der Wabiskaw Member der Clearwater Formation (Unterkreide, Alb) in Nordost-Alberta, Kanada, wird beschrieben. Das besonders gut erhaltene, fast vollkommene, voÈllig artikulierte Exemplar stellt den aÈltesten bekannten und vollstaÈndigsten Plesiosaurier aus der Kreidezeit in Nordamerika dar. Computertomographische Untersuchungen liefern Information uÈber SchaÈdelstruktur. Nichollsia wird mit anderen Gattungen aus der Kreidezeit verglichen, einschlieûlich Leptocleidus, mit dem es aÈhnliche Proportionen des Skeletts, GroÈûe, und das Vorhandenseins eines dorsomedianen Grats auf der Premaxillare gemeinsam hat. Nichollsia besitzt jedoch zahlreiche Autapomorphien, einschlieûlich des Besitzes eines grazilen, schmalen SchaÈdels ohne rostrale Verengung, ein vertikal ausgerichtetes Suspensoriums, einer schuppenartigen Squamosumspitze ohne Kamm, laÈngslaufende Skulpturleisten auf der lateralen FlaÈche des Dentale, eines verhaÈltnismaÈûig homodonten Gebisses, dem caniniforme ZaÈhne fehlen, und weiterer Eigenschaften des axialen Skeletts und der ExtremitaÈten. Nichollsia bewohnte das Boreal-Meer, die erste groÈûere Transgression im noÈrdlichen Teil des Western Interior Basin in der Unterkreide, vor der Bildung des Western Interior Seaway. S c h l uÈ ss e l w oÈ r te r: Sauropterygia ± Plesiosauria ± Unterkreide ± Kanada ± Computertomographie Summary 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. K ey w or d s: Sauropterygia ± Plesiosauria ± Early Cretaceous ± Canada ± computed tomography 0375-0442/08/0283/0001 $ 14. 85 ° 2008 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart Palaeontographica Abt. A 283 Lfg. 1±31±33 Stuttgart, Februar 2008 Addresses of the authors: Patrick S. Druckenmiller, Vertebrate Morphology Research Group, Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada. Current address: University of Alaska Museum of the North and Department of Geology and Geophysics, 907 Yukon Drive, Fairbanks, Alaska, 99775, USA. E-mail: ffpsd@uaf. edu Anthony P. Russell, Vertebrate Morphology Research Group, Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada. E-mail: arussell@ucalgary. ca Introduction Plesiosaurs are a clade of secondarily aquatic, carnivorous marine tetrapods that existed throughout much of the Mesozoic. Our understanding of their diversity and distribution, beginning in the Late Triassic and extending through to the end of the Cretaceous, is both temporally and geographically restricted. The majority of plesiosaurs are known from Jurassic successions of Europe (specifically Early and late Middle to early Late Jurassic) (Martill 1991, Benton & Spencer 1995) and the Late Cretaceous of North America (Welles 1962, Carpenter 1996, Storrs 1999). Specimens recovered from deposits of Early Cretaceous age are comparatively Contents Introduction. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 2 Institutional abbreviations. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 3 Materials and methods. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 3 Preservation. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 3 Provenance. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 4 Age. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 4 Systematic Palaeontology. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 6 Description. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 6 Ontogenetic status. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 6 Cranium. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 12 Skull roof. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 12 Braincase. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 14 Palate. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 16 Mandible. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 17 Dentition. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 19 Postcranium. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 19 Axial skeleton. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 19 Appendicular skeleton. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 23 Discussion. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 26 Relationships. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 26 Comparison with other plesiosaur taxa. .. .. .. .. .. .. .. .. . 26 The question of a nasal. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 30 Gastroliths. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 30 Conclusions. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 31 Acknowledgements. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 31 References.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . 31 Text-fig. 1. Map showing the location of the Syncrude Canada Ltd. Base Mine and the type locality of TMP 94. 122. 01 in northeastern Alberta, Canada. ± 2 ±