Ichthyotitan severnensis May Have Been the Largest Marine Reptile Ever

Fragments of bone embedded in ancient rock point to a genuinely colossal leviathan that once ruled the seas.

Researchers in the UK and US say fossilised remains from two separate jawbones unearthed in south-west England belong to an unknown late Triassic ichthyosaur genus. It appears to have been far larger than any other marine reptile known to have lived on Earth.

Ichthyotitan severnensis: a giant ichthyosaur

The team has named the new genus and species Ichthyotitan severnensis. They estimate that it may have exceeded 25 metres in length - around twice the length of a standard bus - making it an extraordinary animal even by modern standards and approaching the 30 metres reached by a blue whale.

"I was amazed by the find," says palaeontologist Dean Lomax of The University of Manchester in the UK.

"In 2018, my team (including Paul de la Salle) studied and described Paul's giant [ichthyosaur] jawbone and we had hoped that one day another would come to light. This new specimen is more complete, better preserved, and shows that we now have two of these giant bones – called a surangular – that have a unique shape and structure. I became very excited, to say the least."

Fossils found on the Somerset coast

De la Salle found the first fossilised bone section in 2016, on the coast at Lilstock in Somerset, England. Scientists soon realised it was something remarkable. Although it consisted only of the rear section of a jawbone, known as a surangular, its immense size suggested it had come from an ichthyosaur larger than any previously identified. With no further evidence available, however, there was little more they could establish about the creature, which became known as the Lilstock Monster.

A major development followed in 2020. Justin Reynolds and his daughter Ruby, who was 11 at the time, were looking for fossils on a beach at Blue Anchor in Somerset when Ruby found the first fragment of another jawbone. They contacted Lomax, who then contacted de la Salle, launching a new search.

The final section was recovered in October 2022. Palaeontologists then began assembling the pieces and determining which animal the jaw had belonged to. The newly found surangular displayed the same distinctive features as the Lilstock Monster jawbone.

The two bones are alike in both size and morphology, and were discovered at a similar time and place: each came from late Triassic mudstone deposits along the Somerset coastline. Yet the more recently found specimen was considerably more complete and better preserved, enabling scientists to gain further insight into the elusive animal.

Why giant ichthyosaurs remain elusive

Ichthyosaurs first appeared roughly 250 million years ago and could grow beyond 15 metres long. The truly enormous forms did not arise until the Late Triassic, however, and palaeontologists think they survived only briefly before being eliminated in the Triassic-Jurassic extinction event 200 million years ago.

This could account for the limited number of giant ichthyosaur bones in the fossil record, although some fragments indicate that certain huge specimens exceeded 20 metres. Appearing around 13 million years after their giant relatives, Ichthyotitan surpassed even those animals in size.

There is also evidence that the two individuals - whose bones are at last revealing these animals to us hundreds of millions of years later - had not attained their full adult size when they died.

"We could confirm the unique set of histological characters typical of giant ichthyosaur lower jaws: the anomalous periosteal growth of these bones hints at yet to be understood bone developmental strategies, now lost in the deep time, that likely allowed late Triassic ichthyosaurs to reach the known biological limits of vertebrates in terms of size," says palaeontologist Marcello Perillo of the University of Bonn in Germany.

"So much about these giants is still shrouded by mystery, but one fossil at a time we will be able to unravel their secret."

The team's findings have been published in PLOS ONE.