Early Cretaceous plesiosaurs (Sauropterygia: Plesiosauria) from northern Alberta: Palaeoenvironmental and systematic implications

Oil sands mining in northern Alberta by Syncrude Canada Ltd. has resulted in the serendipitous discovery of seven plesiosaurs and two ichthyosaurs from the Lower Cretaceous Clearwater Formation. Facies analysis has revealed that deposition of the Wabiskaw Member, from which eight of the nine specimens were recovered, occurred in the lower shoreface to offshore transition zone. The systematic and consistent manner in which overburden is removed, coupled with the non-random distribution of the specimens, both vertically and laterally within the succession, suggest a habitat preference for this setting by marine vertebrates compared to deeper water environments. Analysis of matrix blocks associated with the most complete and articulated plesiosaur, TMP 94.122.01, indicates its rapid burial by storm sands.; The Syncrude fossils significantly increase our knowledge of Early Cretaceous plesiosaur diversity. TMP 94.122.01 is a new taxon of short-necked plesiosaur, and is one of the most complete and well-preserved specimens from North America A second, partially articulated skeleton is referred to Polycotylidae, as traditionally recognized. Three of the specimens are referable to Elasmosauridae, of which two represent early and late ontogenetic stages of a new taxon. Two of the plesiosaurs are fragmentary and taxonomically indeterminate. Both ichthyosaurs are tentatively referred to Platypterygius.; 185 characters applicable to plesiosaur systematics are described and critically reappraised. A phylogenetic analysis of 30 terminal units scored for 152 characters indicates a basal division of plesiosaurs into two major clades of short-necked (traditional pliosauroids) and long-necked (plesiosauroids) taxa. Among short-necked plesiosaurs, a sister group relationship between TMP 94.122.01 and traditionally recognized Polycotylidae (Dolichorhynchops + Edgarosaurus) is recovered. This clade, in turn, is the sister taxon to Leptocleididae, consisting of Leptocleidus and an unnamed Australian taxon, AM F99374. Together, these two clades constitute a newly recognized taxon of Cretaceous short-necked plesiosaurs, Leptocleidoidea. Contrary to predictions, TMP 94.122.01 and Leptocleidus are not sister taxa.