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. 2024 Apr 17;19(4):e0300289.
doi: 10.1371/journal.pone.0300289. eCollection 2024.

The last giants: New evidence for giant Late Triassic (Rhaetian) ichthyosaurs from the UK

Dean R Lomax  1   2 , Paul de la Salle  3 , Marcello Perillo  4 , Justin Reynolds  5 , Ruby Reynolds  5 , James F Waldron  6
Affiliations

The last giants: New evidence for giant Late Triassic (Rhaetian) ichthyosaurs from the UK

Dean R Lomax et al. PLoS One. .

Erratum in

Abstract

Giant ichthyosaurs with body length estimates exceeding 20 m were present in the latest Triassic of the UK. Here we report on the discovery of a second surangular from the lower jaw of a giant ichthyosaur from Somerset, UK. The new find is comparable in size and morphology to a specimen from Lilstock, Somerset, described in 2018, but it is more complete and better preserved. Both finds are from the uppermost Triassic Westbury Mudstone Formation (Rhaetian), but the new specimen comes from Blue Anchor, approximately 10 km west along the coast from Lilstock. The more complete surangular would have been >2 m long, from an individual with a body length estimated at ~25 m. The identification of two specimens with the same unique morphology and from the same geologic age and geographic location warrants the erection of a new genus and species, Ichthyotitan severnensis gen. et sp. nov. Thin sections of the new specimen revealed the same histological features already observed in similar giant ichthyosaurian specimens. Our data also supports the previous suggestion of an atypical osteogenesis in the lower jaws of giant ichthyosaurs. The geological age and giant size of the specimens suggest shastasaurid affinities, but the material is too incomplete for a definitive referral. Ichthyotitan severnensis gen. et sp. nov., is the first-named giant ichthyosaur from the Rhaetian and probably represents the largest marine reptile formally described.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Distribution of the Triassic rocks in the Bristol Channel–Severn Estuary area and the three key ichthyosaur localities (where specimens discussed herein were found) referred to in the text.
Modified from Lomax et al. 2018 [7].
Fig 2
Fig 2. The holotype of Ichthyotitan severnensis gen. et sp. nov., a newly collected specimen (BRSMG Cg3178) comprising a very large, but incomplete right surangular (the ‘BAS Specimen’).
A. All associated pieces with an approximate outline of the complete surangular, in medial view. The surangular is separated into two main parts, Part #A to the right and Part #B to the left (see text). B. A close-up of the coronoid process in lateral view, showing moderate eminence. C. Bulbous coronoid process in dorsal view with lateral displacement. D. Subcircular cross section at the level of the coronoid process (posterior view, medial to the left). E-F. Comparison of the massively developed M.A.M.E. ridge observed in BAS (E) and the Lilstock surangular (F); arrows indicate top of the ridge. G. Oblique view of the medial surface highlighting part of the overhanging shelf that encloses the Meckelian canal. H. Ventromedial view of the mid-posterior portion of the surangular showing a distinct, continuous, and straight thin groove that might be a suture and could indicate two distinct bones (perhaps including a damaged angular).
Fig 3
Fig 3. Comparison of the holotype (BRSMG Cg3178, A and C right surangular, BAS specimen) and referred specimen (BRSMG Cg2488, B and D left surangular, Lilstock specimen) of Ichthyotitan severnensis gen. et sp. nov. To ease comparison, A and C have been reversed.
A-B. Lateral view of both surangulars showing same unique shape; note the upturned, almost 90-degree angle bend and the spatulate-shaped posterior end. C-D. Medial view of both surangulars displaying same morphology posteriorly; anteriorly the Lilstock specimen (D) has been heavily eroded and distorted along its length (see Discussion in Lomax et al. 2018 for more details). Note the position of an elongated foramen on the lateral surface (A-B), identified as part of the fossa surangularis that passes through the bone into the Meckelian canal. See also the damaged (?)angular that is articulated with the surangular and defined by a continuous groove (?suture) as seen in Fig 2H.
Fig 4
Fig 4. Invertebrate and trace fossils found on the bone surface of the BAS surangular, BRSMG Cg3178.
A-B. Associated bivalves, including Atreta intrusstriata (A) and Plagiostoma giganteum (B); it is worth noting that a small group of the latter are preserved adjacent to the coronoid process, see Fig 2C. C-D. Examples of the probable scavenging marks that are also observed in the Lilstock surangular, see Lomax et al. 2018, Fig 4.
Fig 5
Fig 5
A. Comparable sections for core drill(s) sampling position indicated by a white circle of (from left to right) BRSMG-Cb-3869 (an Aust bone, most probably a surangular), BRSMG Cg3178 (BAS surangular), BRSMG-Cg-2488 R-101 (Lilstock surangular). White arrows point to elongated surangular foramen. B. Binary drawings produced from stitched photos of the thin sections (respectively BRSMG-Cb-3869, BRSMG Cg3178 and BRSMG-Cg-2488 R-101) showing longitudinal vascularization and larger nutrient canals. Blue bars (upper) indicate extension of outer cortex, orange (middle) for deep cortex and pink (lower) for spongious trabecular bone.
Fig 6
Fig 6. Histological overview of BRSMG Cg3178 (BAS surangular).
A. Composite image of thin section under circular polarized light. B. Close-up of the external margin of the outer cortex, showing the presence of multiple growth marks (GMs), open vascular canals and cortical vascular canals with all degrees of maturity (simple canals, primary osteons and secondary osteons), supporting an ongoing active and continuous growth. Note the evident darker border of the lumen of a diagonal canal running from the top left toward the margin of the large nutrient canal (NC) showing further longitudinal vascularization. C. Concentric secondary osteon in the outer cortex under lambda filter. D. Close-up of the upper margin of the nutrient canal under crossed polarized (left) and circular polarized light (right). The growth marks appear as alternated tightly packed rows of brighter and darker periosteal intrinsic fibres (PIF). The same tight packing of the GMs occurs also deeper in the cortex. E. Lateral margin of the nutrient canal under circular polarized view. PIF are evident as bright yellow and blue coiled structures. The presence of simple canals alongside osteons, indicates primary deposition of bone along the margin of the large nutrient canal. F. Concentric secondary osteon in the trabecular bone under transmitted light. It is evident the high amount of osteocyte lacunae and the presence of plump irregular shaped ones in the lamellar bone. G. Trabecular bone under circular polarized view. The presence of primary matrix and concentric secondary osteons indicate that the trabeculae are secondary, produced from compact bone made cancellous. White arrows (D, E, G) point at PIF; white arrowheads point at resorption lines in concentric osteons (C, F, G); white dotted lines indicate borders of primary osteons (C, F, G); yellow arrow heads (B, D) point at rows of GMs. Abbreviations. LB, Lamellar bone; NC, Nutrient canal; OC, Open periosteal canal; PO, Primary osteon; RC, Resorption cavity; SC, Simple canal; SO, Secondary osteon.
Fig 7
Fig 7. Surangular comparisons between the holotype (BRSMG Cg3178, BAS specimen) and referred specimen (BRSMG Cg2488, Lilstock specimen) of Ichthyotitan severnensis gen. et sp. nov., with a comparable section of surangular from a specimen of Ophthalmosaurus icenicus (MJML K2577).
A. BRSMG Cg3178 and MJML K2577 illustrating the distance between the M.A.M.E. and coronoid process. B. BRSMG Cg2488 and MJML K2577 are positioned obliquely in lateral view (with MJML K2577 rotated and held closer to the camera), illustrating the general shape of the ichthyosaurian surangular.
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