Is this the largest marine reptile ever?

UK fossils dating to the Late Triassic (also known as the Rhaetian; 237–201 million years ago) period could be from the largest marine reptile ever.

Ichthyotitan severnensis was discovered on the coast of southwest England. A new ancient jawbone was discovered about 10 km from where the same team discovered a similar jawbone. They believe the fossils represent the same, previously undescribed species. The creature is described in a paper published in the open access journal PLOS ONE.

It would have been about 25 metres long – about twice the length of a bus.

It doesn’t quite reach the wildly inaccurate size of the marine reptile Mosasaurus as depicted in the Colin Trevorrow’s 2015 film Jurassic World (the movie monster was 40 m, while real mosasaurs were probably 14–18 m long). But it’s still among the largest animals ever.

While blue whales routinely reach more than 30 metres, I. severensis would have been about as long as an average blue whale.

The animal belongs to a group called ichthyosaurs which conquered Earth’s oceans during the time of the dinosaurs. Though these animals had a similar body shape to today’s dolphins, they are reptiles.

Ichthyosaurs went extinct 94 million years ago. This is much earlier than the other massive marine and flying reptiles, as well as the non-avian dinosaurs, which disappeared at the end of the Cretaceous period 66 million years ago.

But the largest ichthyosaurs appear to have lived earlier on in their evolutionary history during the Triassic.

Fossils suggest that ichthyosaurs measuring more than 15 m were common in the Late Triassic.

In fact, the authors suggest ichthyosaurs discovered in Nevada, USA, and detailed in a 2021 paper, reached massive sizes only 3–5 million years after they first evolved about 250 million years ago.

But pinning down the largest of them has not been straightforward.

In 2018, the earlier fossil jawbone was suggested to have come from an animal measuring anywhere from 22 to more than 30 metres. The newer jawbone, however, is much better preserved and helps pin down the animal’s massive size.

Two large marine reptile ichthyosaurs swimming flock of seabirds
A giant pair of swimming Ichthyotitan severnensis. Credit: Gabriel Ugueto.

“Both specimens were discovered in Somerset, UK,” the authors write, “and were collected from strata dating to the latest Rhaetian, approximately 202 Ma [million years ago].”

At the end of the Triassic, the first of the geological periods of the “Age of Dinosaurs,” was a mass extinction event. It is considered the most poorly understood of the “Big Five” mass extinctions in Earth’s history.

The authors write: “It is probable that this lineage of giant ichthyosaurs vanished during the end-Triassic mass extinction event, and that ichthyosaurs never reached this size again before their extinction in the early Late Cretaceous.”

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