Ichthyotitan

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Ichthyotitan
Temporal range:
Ma
↓
Holotype (A, C) and referred (B, D) specimens of Ichthyotitan
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: †Ichthyosauria
Family: †Shastasauridae (?)
Genus: †Ichthyotitan
Lomax et al., 2024
Species:
†I. severnensis
Binomial name
†Ichthyotitan severnensis
Lomax et al., 2024

Ichthyotitan (

shastasaurid, extending the family's range by 13 million 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's body length at nearly 25 metres (82 ft), which would make it the largest marine reptile currently known.

Discovery and naming

Localities where Ichthyotitan-affiliated specimens have been found around the Bristol Channel and Severn Estuary

Lilstock specimen

The first specimen later referred to Ichthyotitan,

hyoid or other jaw bones.[1][2]

Another smaller jaw fragment from a giant ichthyosaur is known from Lilstock; it is stored in a private collection and still remains undescribed.[3]

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.[4][5] While incomplete, the surangular, an element representing only part of the entire lower jaw length, has been estimated to have measured more than 2 metres (6.6 ft) in total.[3]

The specimen consists of fragments of a right surangular, more complete than the Lilstock specimen, with some fragments possibly belonging to the angular bone.

Bristol Museum and Art Gallery.[4]

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.[3]

In 2024, Lomax et al.

type locality.[3]

Other possible remains

Other fragmentary remains of giant ichthyosaurs of a similar age to Ichthyotitan have also been reported from Germany (Bonenburg [de] district of Warburg) and France (Autun and Cuers).[6][1]

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

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.[7] The morphology of the posterior jaw was considered unique at the time of discovery, but has since been linked to Ichthyotitan specimens.[3]

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.[5] 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.[3]

Size estimates

Size of Ichthyotitan severnensis compared to a human

While its incompleteness made the size of the animal 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 26 metres (85 ft) 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 22 metres (72 ft). Researchers pointed out that, while differences in proportions between species make these estimates speculative, they were conventional given the scarcity of material.[1][8]

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 25 metres (82 ft), likely making it the largest marine reptile ever described.[3]

The Aust specimens, tentatively linked to Ichthyotitan, have been informally estimated to be even larger at 30 to 35 metres (98 to 115 ft) long.[9]

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.[3]

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.[3]

Another bone fragment is believed to correspond to the

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's 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.[3] Similarly, unique patterns of periosteal growth in Ichthyotitan are believed to have played a role in approaching the biological size limits in vertebrates.[5]

Paleoecology

life restoration

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[10] and some species like Shonisaurus had robust sectorial teeth with gut contents of mollusc shells and vertebrates.[11][12]

Ichthyotitan is believed to have been a predator, hunting smaller prey including other marine reptiles in a fashion similar to an

scavengers, with one of the fossils showing signs of scavenging before burial, explaining the scarcity of known fossils.[13]

References

  1. ^
    PMID 29630618
    .
  2. ^ Lomax, Dean (2018-04-10). "How we found a giant ichthyosaur almost as big as a blue whale". Archived from the original on 2024-04-20. Retrieved 2024-04-20.
  3. ^
    PMID 38630678
    .
  4. ^ a b Strickland, Ashley (2024-04-17). "Ancient fossils lead to discovery of largest known marine reptile". CNN. Archived from the original on 2024-04-20. Retrieved 2024-04-20.
  5. ^ a b c University of Manchester (2024-04-17). "Paleontologists unearth what may be the largest known marine reptile". SciNews. Archived from the original on 2024-04-17. Retrieved 2024-04-20.
  6. PMID 38618574
    .
  7. PMID 25256640
    .
  8. ^ News Staff (2018-04-11). "Giant Triassic Ichthyosaur is One of Biggest Animals Ever | Paleontology | Sci-News.com". SciNews. Archived from the original on 2024-04-20. Retrieved 2024-04-20.
  9. ^ Marshall, Michael (2022-12-29). "Largest ever animal may have been Triassic ichthyosaur super-predator". NewsScientist. Retrieved 2024-04-22.
  10. PMID 24348983
    .
  11. S2CID 254874088
    .
  12. S2CID 129908740. Archived
    from the original on 2024-04-22. Retrieved 2024-04-21.
  13. ^ a b Black, Riley (2024-04-20). "This 80-foot-long sea monster was the killer whale of its time". National Geographic. Archived from the original on 2024-04-19. Retrieved 2024-04-20.
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