A cretaceous ichthyosaur from North Canterbury.

Abstract

Ichthyosaurs were a group of secondarily aquatic marine reptiles that colonised the marine realm shortly after the Permian-Triassic mass extinction and were major components of Mesozoic marine ecosystems for more than 150 million years. The partial skeleton of an ichthyosaur has been discovered in a concretion from the Split Rock Formation in North Canterbury. This concretion dates from the Cenomanian approximately 98 million years ago, only four million years before the final extinction of the ichthyosaurs. The concretion, having been broken into several blocks, was CT scanned to image the interiors of the concretionary blocks, and identify bones within the rock matrix. The blocks were also subject to physical and chemical preparation to produce better CT imaging results, as well as revealing the bones themselves. The CT data allowed for digital reconstruction of the skeletal elements of GS15687 and their 3D distribution within the concretion. The skeletal elements of GS15687 were described and measured using these reconstructions, and this morphological data was used to conduct a phylogenetic analysis. A large number of bones were discovered to be within the concretionary blocks, including a complete basioccipital, left scapula, left pelvic girdle, and much of the proximal left hindfin. In addition, a partial nasal, left coracoid, left humerus, and right ischiopubis have been recovered, as well as multiple vertebrae, neural arches, ribs and gastralia from the posterior trunk of the animal. The heavy disarticulation of the skeleton suggests that GS15687 was subjected to significant disturbance, including scavenging, before fossilisation.

The phylogenetic analyses place GS15687 within the derived ichthyosaur subfamily Platypterygiinae, with results suggesting that GS15687 is a new taxon of ichthyosaur. This finding is supported by a number of unique apomorphies, including a protrusion on the ventral surface of the scapula and a depression on the anterior ischiopubis face. The complete left pelvic girdle of GS15687 is one of the few well preserved Cretaceous ichthyosaur pelvises and adds to the data set of known derived ichthyosaur ischiopubes. The phylogenetic results generated in this thesis produced a topology similar to many other recent analyses and finds that the derived platypterygiine taxa are divided into two clades and that GS15687 is a basal member of one of these clades. The two clades additionally appear to show different biogeographic patterns, with potential origins in South America and Australasia. GS15687 represents the most complete ichthyosaur specimen known in New Zealand and is the first one that can be described to a valid genus and species.