Ichthyotitan severnensis, Lomax & de la Salle & Perillo & Reynolds & Reynolds & Waldron, 2024

Lomax, Dean R., de la Salle, Paul, Perillo, Marcello, Reynolds, Justin, Reynolds, Ruby & Waldron, James F., 2024, The last giants: New evidence for giant Late Triassic (Rhaetian) ichthyosaurs from the UK, PLOS ONE (e 0300289) 19 (4), pp. 1-21 : 8

Ichthyotitan severnensis gen. et sp. nov.

LSID for the genus: urn:lsid:zoobank.org:act:57B85E76-0A91-4EBF-9634-5B5A5FB10B60

LSID for the species: urn:lsid:zoobank.org:act:53F19051-D7E5-4ADB-8129-48D66B79C7A2

Etymology. Giant fish lizard of the Severn. Ichthys derived from Greek meaning fish, taken from ichthyosaur meaning “fish lizard”, and - titan (Greek for giant), after the large size. Severn after the River Severn Estuary, Somerset, UK, where the remains were discovered and Latin - ensis pertaining to the location.

Holotype. BRSMG Cg3178 , a large right surangular comprising the posterior end and parts of the middle and anterior sections.

Referred material. BRSMG Cg 2488 , a large left surangular comprising the posterior end and a portion of the shaft.

Type locality and horizon. The type specimen was collected from the Upper Triassic Westbury Mudstone Formation (latest Rhaetian) at Blue Anchor, Somerset, UK. The referred specimen was collected from the Upper Triassic Westbury Mudstone Formation (latest Rhaetian) 0.8 m below the junction with the Cotham Formation, at Lilstock, Somerset, UK.

Diagnosis. Giant, probable shastasaurid ichthyosaur distinguished by the presence of the following unique characters of the surangular: upturned, almost 90 degree angle bend at posterior end; subcircular cross section morphology of the shaft at the position of the coronoid, oblong in Shonisaurus ; minor eminence of coronoid process in lateral view, compared with prominent projection in Shonisaurus ; bulbous coronoid process displaced laterally and only occupying half of the width of the dorsal surface; massively developed dorsoventral M.A.M.E. ridge; spatulate shaped posterior end; and possibly dorsoventral height of posterior end in adults being more than 20% larger than in either Shonisaurus popularis or Shonisaurus sikanniensis .

Remarks. Further to the last character in the diagnosis, this feature can be used to distinguish the new taxon from other giant ichthyosaurs, as mentioned, but this could be problematic for assigning immature or juvenile individuals of I. severnensis that would be smaller than adult specimens of Shonisaurus spp. Therefore, we feel that this character can be used to compare large, presumably adult specimens but might not be useful (unless scaling is taken into consideration) in assigning smaller individuals that may be discovered in the future. Another notable feature that might be of interest is a continuous, straight thin groove in the ventral surface of the BAS specimen. For further details, see the description below. Probable shastasaurid affinities are based on the large size of the ichthyosaur combined with the Upper Triassic age, a time when the largest-known ichthyosaurs (the Shastasauridae ) existed.