The broken jawbone is thought to have belonged to a giant ichthyosaur that may have been as long as a blue whale, measuring up to 26m (85ft) from its jagged front teeth to the tip of its powerful tail.
A quick ichthyosaur refresher: these marine reptiles show up in the fossil record and explode in size and number during the Triassic, get smaller but are still plentiful in the Jurassic (201-145 million years ago) and then die out during the Cretaceous, millions of years before the mass extinction that offed the dinosaurs.
After comparing the two, the researchers concluded that the new fossil is a giant shastasaurid-like ichthyosaur and larger than the Shonisaurus sikanniensis. That wasn't the case for the ichthyosaur, a giant aquatic dinosaur that swam the oceans of the Triassic period: these giant, finned lizards were predators and fed on everything from fish to squid.
"It was a giant piece of mandible from an ichthyosaur", the palaeontologist told BBC News.
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The one-metre-long bone came from the mouth of a huge predatory ichthyosaur. "Other comparisons suggest the Lilstock ichthyosaur was at least 20 to 25 m (65.6 to 82 ft)".
By comparing the Royal Tyrrell Museum's 21-meter long Shastasaurid Shonisaurus Sikanniensis' jaw with the recently found one, the scientists were able to estimate that the newly unearthed Ichthyosaur could be by 25% longer.
Paul de la Salle, a fossil collector and co-author of the study, discovered the jaw bone in May 2016.
We eventually realised the Lilstock ichthyosaur was so big it could be the largest of its kind ever discovered, comparable to a blue whale in length.
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"Since the specimen is represented only by a large piece of jaw, it is hard to provide an estimate of the size of the animal", said Dean Lomax, an expert on ichthyosaurs at the University of Manchester.
The discovery of this jaw in 2016 also led scientists to re-examine 208 million-year-old bones discovered in 1850 at Aust Cliff in Gloucestershire (western United Kingdom). They are, in fact, jaw fragments from sizeable, previously unrecognized ichthyosaurs.
"Of course, such estimates are not entirely realistic because of differences between species", Lomax said in a statement. Over the decades, the partial Aust Cliff bones have been described as limb bones of a number of different dinosaurs and reptiles.
Massare and I agreed the fossil was an incomplete "surangular" bone from the lower jaw of a giant ichthyosaur because it had several notable features such as a prominent groove in the surface facing towards the middle of the body. Four other similarly incomplete bones were also found and described.
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Lomax and Massare's findings coincided with that of Darren Naish from the University of Southampton. "Nonetheless, simple scaling is commonly used to estimate size, especially when [a] comparative material is scarce", Lomax explained.