Ichthyotitan

''Ichthyotitan'' ( ) is an extinct genus of giant ichthyosaur from the Late Triassic (Rhaetian), known from the Westbury Mudstone Formation in Somerset, England. It is believed to be a shastasaurid, extending the family's range by years up to the latest Triassic. The discovery of ''Ichthyotitan'' has been considered evidence that shastasaurids were still thriving until their disappearance in the Triassic–Jurassic extinction event.

The genus contains a single species, ''I. severnensis''. It is known from two fragmentary surangular bones of the lower jaw, discovered in separate places in 2016 and 2020. Other specimens throughout Western Europe have been linked to the species based on similar osteological features, although their affiliation is uncertain. Estimates scaling up the bones from other ichthyosaur species put ''Ichthyotitan'' body length at nearly , which would make it the largest marine reptile currently known.

Discovery and naming

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.

Holotype and description

The ''Ichthyotitan'' holotype specimen, BRSMG Cg3178 (the "BAS specimen"), was discovered in sediments of the Westbury Formation near Blue Anchor in Somerset. The first fragment was found on 28 May 2020 by 11-year-old Ruby Reynolds, while looking for fossils on the beach at Blue Anchor with her father, fossil collector Justin Reynolds. They contacted researcher Dean Lomax, who himself reached out to Paul de la Salle. Subsequent expeditions were led by the team, revealing additional pieces until 16 October 2022, and known parts of the surangular, a lower jaw-bone, were reassembled in that same year. 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.

The specimen consists of fragments of a right surangular, more complete than the Lilstock specimen, with some fragments possibly belonging to the angular bone. Histological features suggest that the specimen was still growing, so was either a subadult or a young adult. Multiple encrusting organisms, including bivalves, are present, as well as possible signs of scavenging. It is scheduled to be displayed at the Bristol Museum and Art Gallery.

Other fragmentary remains were discovered but not identified with the holotype specimen, including two large rib sections from another potential giant ichthyosaur, found at a higher stratigraphic level.

In 2024, Lomax et al. described ''Ichthyotitan severnensis'' as a new genus and species of probable shastasaurid ichthyosaur based on BRSMG Cg3178 and BRSMG Cg2488. The generic name, ''Ichthyotitan'', combines a reference to ''Ichthyosaurus''—whose name means "fish lizard", combining the Greek words (), meaning "fish", and (), meaning "lizard"—with the Greek suffix (), meaning "giant". The specific name, ''severnensis'', references the Severn Estuary near the type locality.

Other possible remains

Other fragmentary remains of giant ichthyosaurs of a similar age to ''Ichthyotitan'' have also been reported from Germany ( district of Warburg) and France (Autun and Cuers).

The Cuers specimen is known from fragments discovered in two separate excavations, but believed to represent a single specimen. A small rostrum fragment believed to be a premaxilla (MHNTV PAL-1-10/2012) and a long mandible fragment (MHNTV PAL-2/2010) are known, as well as vertebral centra and rib fragments associated to both collection numbers. It was originally described as shastasaurid-like, with flattened centra similar to ''Shonisaurus'' and ''Himalayasaurus'', and an aulacodont dentition analogous to '' Shastasaurus''. 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 lineage is believed to have gone extinct immediately after in the Triassic–Jurassic extinction event, with later ichthyosaurs never reaching similar sizes until their extinction in the early Late Cretaceous.

Size estimates

While the incompleteness of the material referred to ''Ichthyotitan'' makes its size difficult to determine, it was clearly very large. By comparing the Lilstock surangular to the same bone in ''Shastasaurus sikanniensis'' as a model, the researchers estimated the Lilstock ichthyosaur to have been around long or 25% larger by direct scaling, nearly the size of a blue whale. Scaling based on the height at the coronoid process compared to ''Besanosaurus'' found a shorter length estimate of . Researchers pointed out that, while differences in proportions between species make these estimates speculative, they were conventional given the scarcity of material.

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.

The Aust specimens, tentatively linked to ''Ichthyotitan'', have been informally estimated to be even larger at long.

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

{{Taxonbar, from=Q125509005

Fossil taxa described in 2024
Fossils of England
Ichthyosauromorph genera
Ichthyosaurs of Europe
Late Triassic ichthyosaurs
Triassic England