A new ophthalmosaurid ichthyosaur from the Upper Jurassic (Early Tithonian) Kimmeridge Clay of Dorset, UK, with implications for Late Jurassic ichthyosaur diversity

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.

An enigmatic large mawsoniid coelacanth (Sarcopterygii, Actinistia) from the Upper Jurassic Kimmeridge Clay Formation of England

“…The material is currently exhibited in the Museum of Jurassic Marine Life. This museum opened to the public in 2016, and houses the lifetime collection made by Steve Etches, which includes several unstudied specimens new to science (see Etches and Clarke, 2010;Jacobs and Martill, 2020;Martill and Etches, 2020).…”

“…The collecting area forms part of Dorset's Jurassic Coast World Heritage Site (Martill and Etches, 2020). Several exceptionally preserved marine vertebrates have been found in this area, including chondrichthyans, osteichthyans, and marine reptiles such as pliosaurs and ichthyosaurs; but also continental vertebrates such as crocodiles, turtles, pterosaurs, and dinosaurs (e.g., Martill et al, 2006;Jacobs and Martill, 2020;Martill and Brito, 2020;Stumpf et al, 2021). Trace fossils and an abundant shelly fauna have also been reported, including numerous ammonites, which are used to divide the Formation into 13 well-defined biozones (Cox, 2020;Gallois, 2020).…”

“…Trace fossils and an abundant shelly fauna have also been reported, including numerous ammonites, which are used to divide the Formation into 13 well-defined biozones (Cox, 2020;Gallois, 2020). The rhythmic sequence of mudrocks that comprise the Formation were deposited in a shallow epicontinental sea with scattered low-relief islands, with a stratified water column including euxinic conditions reaching into the photic zone, and ocean influence from the cooler Boreal Sea to the North and the warmer Tethyan Ocean to the South (Pearson et al, 2004;Martill et al, 2006;Gallois, 2020;Jacobs and Martill, 2020;Martill, 2020) (Fig. 1B).…”

The soft tissue and skeletal anatomy of two Late Jurassic ichthyosaur specimens from the Solnhofen archipelago

“…Recent studies have revealed the structure and color of ichthyosaur skin in a few key specimens from the Early Jurassic and shown that some ichthyosaurs had blubber ( Lindgren et al, 2014 ; Lindgren et al, 2018 ). However, for later ichthyosaurs (Middle Jurassic-Cretaceous), records of soft tissue are few, including an Ophthalmosaurus specimen from the Middle Jurassic of England ( Martill, 1987 ), a Thalassodraco and an undescribed ophthalmosaurid specimen from the Upper Jurassic of England ( Jacobs & Martill, 2020 ; K1747, D. Lomax, 2022, personalcommunication) a Maiaspondylus specimen from the Lower Cretaceous of Canada ( Maxwell & Caldwell, 2006 ) and ichthyosaurs from the Solnhofen Plattenkalk.…”