Abstract
A new ophthalmosaurid ichthyosaur, Thalassodraco etchesi gen. et sp. nov., from the Upper Jurassic Kimmeridge Clay Formation of Dorset, UK is described. The specimen, a partial, articulated skull and anterior thorax in the Etches Collection of Kimmeridge, Dorset, is exceptionally well preserved on a slab of laminated coccolith limestone and has been expertly prepared. It comprises a near complete skull in articulation with associated anterior vertebral column and dorsal ribs, complete pectoral girdle, fully exposed left forelimb, and some elements of the right forelimb. Other elements present, including an ischiopubis are preserved on separate slabs. Presumed rapid burial of the anterior portion of the specimen in the coccolith substrate has preserved a number of ossified ligaments lying across the vertebral column and associated ribs as well as stomach contents and decayed internal organs. Aspects of the dentition, skull roof bones and the forelimb configuration distinguishes the new specimen from previously described Late Jurassic ichthyosaurs. Autopmorphies for T. etchesi include a large rounded protuberance on the supratemporal bone; a thin L-shaped lachrymal, with a steeply curved posterior border; ~ 70 teeth on the upper tooth row, and deep anterior dorsal ribs. A well resolved phylogenetic analysis shows T. etchesi as a member of a basal clade within Ophthalmosauridae comprising Nannopterygius, Gengasaurus, Paraophthalmosaurus and Thalassodraco. The new specimen adds to the diversity of the Ichthyopterygia of the Kimmeridge Clay Formation and emphasises the important contribution of amateur collectors in palaeontology.
Highlights
Ichthyosaurs were a successful group of large marine reptiles for most of the Mesozoic
The main topology of the analysis shows Ophthalmosauridae diverges into three clades; Ophthalmosaurinae and Platypterygiinae, and a third, Nannopterygius clade including Nannopterygius + Thalassodraco and Gengasaurus
A phylogenetic analysis finds Thalassodraco nested within a clade within Ophthalmosauridae, with Nannopterygius, Paraophthalmosaurus and Gengasaurus
Summary
Ichthyosaurs were a successful group of large marine reptiles for most of the Mesozoic. Well preserved ichthyosaurs are known from four Jurassic deposits in the British Isles, the Early Jurassic Blue Lias and Whitby Mudstone formations, the middle Jurassic Peterborough Member of the Oxford Clay Formation and the Upper Jurassic Kimmeridge Clay Formation of Dorset [12]. These formations have long been known to yield marine reptiles, especially ichthyosaurs, and have become of immense historical significance [13,14,15,16,17]. A new museum in the village of Kimmeridge houses the famous, but largely unstudied Etches collection that includes several new and important ichthyosaur specimens [20]
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