Description of a new Upper Jurassic ophthalmosaurid ichthyosaur from the Slottsmøya Member, Agardhfjellet Formation of central Spitsbergen

• Main Author: Roberts, Aubrey Jane
• Other Authors: Glenn-Peter Sætre og Jørn Harald Hurum
• Format: Master Thesis
• Language: English
• Published: 2013
• Subjects: VDP::470; Agardhfjellet; Norway; Slottsmøya; Svalbard; Spitsbergen
• Online Access: http://hdl.handle.net/10852/35773; http://urn.nb.no/URN:NBN:no-34861

Ichthyosaurs were a group of marine reptiles that thrived in the Mesozoic seas. They evolved to be highly specialized marine predators, and were the first reptilian group to be entirely aquatic. This project’s goal was to describe and determine the systematic status of a new ophthalmosaurid ichthyosaur specimen PMO 222.654, from the Upper Jurassic (Middle Volgian) Slottsmøya Member of the Agardhfjellet Formation, Svalbard, Norway. The Slottsmøya Member is the uppermost of four members in the Agardhfjellet Formation. PMO 222.654 was compared to other globally distributed Upper Jurassic and Cretaceous ophthalmosaurid ichthyosaurs as well as newly discovered specimens from the same unit in Svalbard. The comparative analysis resulted in the recognition of a new taxon and adds to our rapidly growing knowledge of ophthalmosaurid diversity. The specimen includes a partial laterally compressed skull, partial left forefin, right humerus, near complete shoulder girdle, near complete pelvic girdle, cervical vertebrae and accompanying ribs, caudal vertebrae and gastralia. PMO 222.654 was placed into the family Ophthalmosauridae on the basis of morphological characters found in the members of this group. These included a reduction of the extracondylar area of the basioccipital, an anterior preaxial accessory element on the humerus, an anterior twisting dorsal process on the humerus and long lateral exposure of the angular. PMO 222.654 was compared to the other members of the family described from Europe, North America and South America. In general cranial morphology the specimen resembled that of Ophthalmosaurus in the rostrum, mandible, orbital and nasal regions, although with several similarities to that of Cryopterygius, Caypullisaurus and Palvennia. The supratemporal region resembles more to that found in Palvennia and Platypterygius. There are more differences in the braincase elements: The stapes morphology can be compared to that in Palvennia and Acamptonectes, but the basioccipital resembles more to that found in Sveltonectes. In post-cranial general morphology the specimen is similar to that of Arthropterygius and Undorosaurus. A phylogenetic analysis was for the first time run on the family that includes the specimens described from the Upper Jurassic of Svalbard. The results were mainly inconclusive, and the Svalbard ophthalmosaurid ichthyosaurs do not appear to form a monophyletic group. Specimen PMO 222.654 and the undescribed specimen from Svalbard PMO 222.667 group with Arthropterygius and Undorosaurus, a hypothesis that is also confirmed by the general post-cranial morphology. If substantial cranial material becomes available for these two taxa, further comparisons should be made to better understand the phylogenetic relationship between these taxa and PMO 222.654.