Thalattosuchian crocodylomorphs from the Sinemurian (Early Jurassic) of the UK

Thalattosuchian crocodylomorphs were a ubiquitous component of shallow marine ecosystems during the Jurassic and Early Cretaceous. Alas, their origins remain a mystery. Here we describe three specimens from the Sinemurian (and possibly Early Pliensbachian) of the UK: a partial cranial rostrum, a series of cervical vertebrae, and two dorsal vertebrae adhered with matrix. These specimens are amongst the oldest known thalattosuchian fossils, with the partial cranial rostrum being the oldest known non-neothalattosuchian thalattosuchian. This partial cranial rostrum has a unique combination of rostral characters never seen before in any crocodylomorph, and helps to elucidate early thalattosuchian internal rostrum evolution, suggesting that the reduction in thalattosuchian paranasal sinuses was not related to either the reorganization of rostral neurovasculature seen in later diverging taxa or the increased cancellous bone microstructure. Based on our CT sample, a shift in cranial bone microstructure occurred in the Eoneustes + Metriorhynchidae subclade, one that coincided with the enlargement of the salt glands and decoupling of the external antorbital fenestra from the paranasal sinuses. Without extensive histological sampling we cannot determine whether the shift to an obligate aquatic lifestyle occurred prior to the evolution of Metriorhynchidae.