Jurassic marine reptile size debate reignited by fossils

Liopleurodon, a carnivorous marine reptile that lived alongside dinosaurs between 166 million and 140 million years ago during the Jurassic period, would have been a fearsome predator. But its exact size has been subject to heated debate.

The dispute really kicked off after the 1999 BBC documentary Walking With Dinosaurs showed a 25-metre-long Liopleurodon patrolling the Jurassic seas. Commentators and experts have spent much of the last 24 years lambasting the episode, titled ‘Cruel Sea’, for gravely exaggerating the marine reptile’s size.

For what it’s worth, the Liopleurodon scenes will always rank among my favourite TV moments in history regardless of scientific inaccuracy. Four-year-old me agrees.

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“I was a consultant for the BBC’s pilot program ‘Cruel Sea’ and I hold my hands up – I got the size of Liopleurodon horrendously wrong,” admits Professor David Martill from the UK’s University of Portsmouth. “I based my calculations on some fragmentary material which suggested a Liopleurodon could grow to a length of 25 meters, but the evidence was scant and it caused a lot of controversy at the time.”

By all accounts Liopleurodon would have reached lengths of no more than 6 metres. Or so we thought.

But the recent chance discovery of four enormous vertebrae at Abingdon County Hall Museum in Oxfordshire has brought attention back to the potential size of Liopleurodon’s close relatives. And they were huge – up to 14.4 metres in length, or twice the size of a killer whale.

University of Portsmouth PhD student Megan Jacobs was photographing ichthyosaur (dolphin-like marine reptiles from the age of dinosaurs) fossils when she stumbled upon the vertebrae of a pliosaur – the group to which Liopleurodon belonged.

Pliosaurs are like plesiosaurs (think Loch Ness monster), but have larger, crocodile-like heads and shorter necks. Their four massive flippers would have made pliosaurs powerful swimmers.

These marine reptiles were the apex predators in the Jurassic seas and were common between 215 and 80 million years ago. Their extinction in the Late Cretaceous is not fully understood, but contributing factors may have been changing sea levels, falling numbers of prey animals and competition from other marine predators like mosasaurs.

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“We know these pliosaurs were very fearsome animals swimming in the seas that covered Oxfordshire 145–152 million years ago,” Martill explains. “They had a massive skull with huge protruding teeth like daggers – as big, if not bigger than a T. rex, and certainly more powerful.”

Martill and his colleagues used topographic scans to calculate that the Late Jurassic marine reptile found in the museum could have grown to between 9.8 and 14.4 metres long – as big as a bus.

“It’s wonderful to prove there was indeed a truly gigantic pliosaur species in the Late Jurassic seas,” adds Martill. “Although not yet on a par with the claims made for Liopleurodon in the iconic BBC TV series Walking With Dinosaurs, it wouldn’t surprise me if one day we find some clear evidence that this monstrous species was even bigger.”

The research is published in the Proceedings of the Geologists’ Association.