A new partial skeleton of a cryptocleidoid plesiosaur from the Upper Jurassic Sundance Formation of Wyoming

Abstract

ABSTRACT-Cryptocleidoid plesiosaurs from the Upper Jurassic are well known from the Oxford Clay (Callovian) of the United Kingdom. The plesiosaurs of the nearly coeval Sundance Formation (Oxfordian) of North America are poorly known, but are thought to include two cryptocleidoid taxa: Pantosaurus striatus and Tatenectes laramiensis. Here we present two specimens recently recovered from the Bighorn Basin of Wyoming. The first specimen comprises three articulated adult cervical vertebrae and fragments of a fourth. This specimen preserves a posteriorly directed cervical neural spine, a character diagnostic of Pantosaurus striatus. It also resembles Pantosaurus in the morphology of its cervical rib articulations. The second specimen is a partial articulated skeleton comprising a complete pelvic girdle, dorsal, sacral, and caudal vertebrae, and numerous ribs and gastralia. This specimen displays a number of unique characters, including posteriorly directed dorsal, sacral, and caudal neural spines, highly autapomorphic illia, and a pathologically asymmetric pelvic girdle. Despite the lack of overlapping material, it is tentatively referred to Pantosaurus on the basis of posteriorly directed neural spines. These specimens represent the first significant adult material provisionally referable to Pantosaurus striatus, as well as the first posterior axial column and pelvic girdle.

Osteology and relationships ofColymbosaurusSeeley, 1874, based on new material ofC. svalbardensisfrom the Slottsmøya Member, Agardhfjellet Formation of central Spitsbergen

“…Each of the sacral ribs preserved in PMO 222.663 is proximodistally extended and exhibits nearly equally expanded proximal and distal ends. This differs from the spatulate sacral ribs found in Tatenectes laramiensis and Pantosaurus striatus, which have anteroposteriorly expanded distal ends in dorsal view (Wilhelm and O'Keefe, 2010;O'Keefe et al, 2011). Additionally, PMO 222.663 differs from Cryptoclidus eurymerus, in which the proximal end is dorsoventrally thicker than the distal end (Brown, 1981).…”

“…In lateral view, the dorsal margin has subequal anterior and posterior expansions. This differs somewhat from the asymmetrically shaped dorsal portion of the ilium preserved in the holotype of C. svalbardensis (PMO A27745), and the more rounded dorsal end of Tatenectes laramiensis and Pantosaurus striatus (Wilhelm and O'Keefe, 2010;O'Keefe et al, 2011;Knutsen et al, 2012a). The anterior margin is concave and displays more of the acetabular facet in medial view, as in the majority of cryptoclidids (Andrews, 1910; A.J.R., pers.…”

“…As in most cryptoclidids, the sacral neural spines are positioned dorsally from the centrum (O'Keefe et al, 2011). This morphology differs from Pantosaurus striatus, where the neural spines are significantly posteriorly inclined (Wilhelm and O'Keefe, 2010). A partial sacral rib was found in articulation with this vertebra (Fig.…”

“…The middle caudal centra of Colymbosaurus are distinctive among cryptoclidids in that their chevron facets project slightly from the ventral surface and are not positioned in a deep groove (PMU 24781) or are not significantly venterolaterally projecting (e.g., Cryptoclidus eurymerus; Andrews, 1910;Kear and Maxwell, 2013). The mid-caudal centra of Colymbosaurus also have a flat ventral margin in anterior view, compared with the more rounded morphology in Cryptoclidus eurymerus, Muraenosaurus leedsii, and Pantosaurus striatus (Andrews, 1910;Wilhelm and O'Keefe, 2010).…”

“…Caudal series are well known from three genera of cryptoclidids: Cryptoclidus (e.g., NHMUK R8575), Muraenosaurus (e.g., NHMUK R2864), and Pantosaurus (USNM 536965; Wilhelm, 2010;Wilhelm and O'Keefe, 2010). The latter has a slight downward bend of the tail starting at the fourth caudal, indicating a tail bend (Wilhelm and O'Keefe, 2010).…”