Morphofunctional Diversity in Toarcian (Early Jurassic) Ichthyosaurs (Reptilia: Diapsida)
Moon, B.C., Benton, M.J., Williams, M. 2019 Morphofunctional Diversity in Toarcian (Early Jurassic) Ichthyosaurs (Reptilia: Diapsida). International Congress of Vertebrate Morphology (Prague)
Abstract
The Early Jurassic has the highest diversity of ichthyosaur taxa in the Mesozoic, yet not the highest disparity. Two lagerstätten – the Lias Group and Posidonia Shale – have preserved abundant specimens representing this diversity, however, three‐dimensional specimens are rare, and testing of ecological adaptation and niche occupation has thus far been infrequent. We use μCT‐scans of two complete, 3D‐ichthyosaur skulls from the Lias (Toarcian) Strawberry Bank lagerstätte of Somerset, U.K. – Hauffiopteryx typicus and Stenopterygius triscissus – to reconstruct the internal cranial hard and soft anatomy.Hauffiopteryx typicus possesses a narrow, elongate snout, and anteroposteriorly short postorbital region and supratemporal fenestra. The adductor cavity is large providing broad areas for muscle attachment. In Stenopterygius triscissus the snout is broader and more robust; the postorbital region longer and supratemporal fenestra larger than H. typicus. A feature of Neoichthyosauria is the posterior position of the paracoronoid process on the surangular, close to the jaw articulation. We find this – coupled with extensive muscle attachment in this region – gives a high moment action on the lower jaw, allowing the jaw to be closed rapidly even in the viscous medium of water. Morphofunctional differences in jaw action, particularly movement of the lower jaw through water, is considered important in the form of the skull. The teeth of S. triscissus are longer and greater in diameter than in H. typicus, and retained in both taxa through ontogeny. We suggest that these morphologies evidence niche partitioning between the taxa. No direct evidence of diet was found in the specimens, however, the lagerstätte preserves numerous fishes and insects and has been interpreted as a ‘nursery’ for ichthyosaurs. There is evidence for similar ecologies between H. typicus and the crocodyliform Pelagosaurus typus.