The age and identity of an ichthyosaur reputedly from the Purbeck Limestone Group, Lower Cretaceous, Dorset, southern England

Abstract

Ichthyosaurs are widespread in Mesozoic marine sequences. The marginal marine to terrestrial strata of the Cretaceous Purbeck Limestone Group of Dorset are an unlikely source for the remains of such animals. A specimen in the collections of the Oxford University Museum of Natural History, acquired in the nineteenth century, is recorded as collected from these strata. Despite the suggestion that this specimen might represent a relict taxon of a much earlier lineage in the evolution of ichthyosaurs [Delair, J.B., 1969. The first record of the occurrence of ichthyosaurs in the Purbeck. Proceedings of the Dorset Natural History & Archaeological Society 90, 128–132], its age and source have not been questioned. A comprehensive investigation of the matrix, including a palynological study, confirms a Purbeck Limestone Group origin for the specimen. Reassessment of the preserved postcranial skeleton provides evidence that the specimen, though strictly indeterminate, is consistent with attribution to a juvenile of an ophthalmosaurid such as Brachypterygius. The ‘notched’ phalanx previously considered to be ‘primitive’ is an artefact of damage to the specimen, either as the slab broke away from the cliff or shore, or during collection and subsequent preparation.