Ichthyotitan

and Severn Estuary Lilstock specimen The first specimen later referred to Ichthyotitan, BRSMG Cg2488 (the "Lilstock specimen"), was found in 2016 by researcher and fossil collector Paul de la Salle in the Westbury Formation. It consists of a partial left surangular measuring long. In 2018, Dean Lomax, Paul de la Salle, Judy Massare, and Ramues Gallois identified the Lilstock specimen as belonging to a shastasaurid. This also prompted the authors to reinterpret large bones in nearby Aust Cliff, previously considered to be from large terrestrial archosaurs, as possible giant ichthyosaur fragments from the surangular, hyoid or other jaw bones. Another smaller jaw fragment from a giant ichthyosaur is known from Lilstock; it is stored in a private collection and still remains undescribed. While incomplete, the surangular, an element representing only part of the entire lower jaw length, has been estimated to have measured more than in total. It is scheduled to be displayed at the Bristol Museum and Art Gallery. The morphology of the posterior jaw was considered unique at the time of discovery, but has since been linked to Ichthyotitan specimens. == Description ==

Ichthyotitan is the only shastasaurid and giant ichthyosaur to be known from the Rhaetian, found in the fossil record 13 million years after their relatives. The 2024 study describing Ichthyotitan pointed out inaccuracies in the Besanosaurus scaling, due to a misidentification of the coronoid process with the nearby MAME (muscle adductor mandibulae externus) process. Comparing the position of the MAME process in the BAS specimen to that in Besanosaurus, they provided a revised length estimate of , likely making it the largest marine reptile ever described. Bone anatomy Apart from its size, features of the surangular bone distinguish Ichthyotitan from other shastasaurids. The surangular is spatulate at its posterior end and shows an almost 90-degree upwards turn. This is consistent in both the Lilstock and BAS specimens, ruling out taphonomic distortion. In comparison, other shastasaurids show a much less marked curvature. An extensive MAME process is present for muscle attachment. Another thin process, posterior to the latter, shows vertical ridges and furrows on its medial side, and has also been reported in the Cuers specimen. The coronoid process is also less prominent laterally than in Shonisaurus, while the shaft shows a subcircular rather than oblong cross-section at that position. While less well-preserved, the anterior part of the surangular bears a lateral groove believed to represent the continuation of the fossa surangularis, also known from the Cuers specimen. Another bone fragment is believed to correspond to the angular bone by comparison with Cymbospondylus youngorum, running ventrally across the entire length of the surangular in the BAS specimen. While a suture is present between the two bones, it disappears in a section anterior to the coronoid process. Along with the continuous bone structure, this implies that the bones were possibly fused in life, a unique condition among ichthyosaurs. Researchers speculate that this was related to Ichthyotitan large size and the individual's maturity. This morphology was also observed in one of the Aust bones, and is believed to also be present in the Lilstock specimen despite poorer preservation. Similarly, unique patterns of periosteal growth in Ichthyotitan are believed to have played a role in approaching the biological size limits in vertebrates. == Paleoecology ==

Older studies have suggested that shastasaurids were suction-feeders, but current research indicates that the jaws of shastasaurid ichthyosaurs do not fit the suction-feeding profile. This is because their short and narrow hyoid bones are unsuitable to withstand impact forces for such kind of feeding and some species like Shonisaurus had robust sectorial teeth with gut contents of mollusc shells and vertebrates. Ichthyotitan is believed to have been a predator, hunting smaller prey including other marine reptiles in a fashion similar to an orca. This has been taken as evidence for the richness of marine food webs throughout the Triassic, believed to be built upon newly evolved forms of plankton, and would show that shastasaurids were flourishing until their disappearance in the end-Triassic extinction. Their ecology made their remains vulnerable to scavengers, with one of the fossils showing signs of scavenging before burial, explaining the scarcity of known fossils. == References ==