Dinosaurs are famous for their size, but the largest known animal to have ever existed is still with us today: the blue whale. Now ancient marine reptiles may be closing the gap, as a new study reports the discovery of a huge fossilized jawbone belonging to an unknown species of ichthyosaur that may have measured up to 26 m (85 ft) long.

Looking like a cross between a fish and a dolphin, ichthyosaurs thrived throughout the reign of the dinosaurs, and since their remains are regularly dug up across Europe and the UK, they're among the most well-understood ancient creatures. Most measured about 2 to 4 m (6.5 to 13 ft) long, but the largest known species could have been over 20 m (66 ft).

That title could be about to change hands though. Fossil collector Paul de la Salle found pieces of a jawbone on a beach in Lilstock, UK, which together measured about 1 m (3.3 ft) long.

"Initially, the bone just looked like a piece of rock but, after recognising a groove and bone structure, I thought it might be part of a jaw from an ichthyosaur and immediately contacted ichthyosaur experts Dean Lomax (University of Manchester) and Prof. Judy Massare (SUNY College at Brockport, NY, USA) who expressed interest in studying the specimen," says de la Salle.

The researchers were able to identify it as part of a surangular, a bone on the lower jaw. The team compared it to several other ichthyosaurs, including Shonisaurus sikanniensis, the largest known specimen at 21 m (69 ft) long. The new fossil appeared to be from a similar, albeit larger, species that's still unknown.

"As the specimen is represented only by a large piece of jaw, it is difficult to provide a size estimate, but by using a simple scaling factor and comparing the same bone in S. sikanniensis, the Lilstock specimen is about 25 percent larger," says Lomax. "Other comparisons suggest the Lilstock ichthyosaur was at least 20 to 25 m (65.6 to 82 ft). Of course, such estimates are not entirely realistic because of differences between species. Nonetheless, simple scaling is commonly used to estimate size, especially when comparative material is scarce."

If the specimen turns out to be towards the higher end of that scale, it's starting to bump up against the length of the blue whale, which commonly grow to be 26 to 28 m long. That said, it's got nothing on the Patagotitan, the largest land animal to ever walk the Earth at 37 m (122 ft) long and weighing around 70 tons. But length isn't everything, and when it comes to weight both Patagotitan are dwarfed by the blue whale, which typically tips the scales at about 200 tons.

The new study might also help clarify previously misidentified fossils found in Aust Cliff, UK, in 1850. The researchers believe these bones, which were thought to be the limbs of land-dwelling dinosaurs, may be other giant ichthyosaur jawbone fragments, but it's hard to know for sure until a more complete skeleton turns up.

"One of the Aust bones might also be an ichthyosaur surangular," says Lomax. "If it is, by comparison with the Lilstock specimen, it might represent a much larger animal. To verify these findings, we need a complete giant Triassic ichthyosaur from the UK – a lot easier said than done!"

The research was published in the journal PLOS One.

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