The bones, tucked away in fossil drawers and storage, belong to a pliosaur and suggest that this ancient sea creature could grow even larger than scientists previously thought.

Megan Jacobs/University of PortsmouthA depiction of a pliosaur.

Paleobiology professor David Martill was looking through fossil drawers at Abingdon County Hall Museum in the United Kingdom when he came upon a large vertebrae. To his delight, Martill then found out that the museum had three more in storage — and realized that he’d stumbled upon fossils of a pliosaur, an ancient Jurassic “sea monster.”

“Dave opened the drawer and there was a huge backbone in it – it was dinner-plate sized,” PhD student Megan Jacobs, who was photographing an ichthyosaur skeleton while Martill looked through the fossil drawers, explained to the BBC. “We got it out had a look and concluded it wasn’t a dinosaur but a huge marine reptile.”

Martill and Jacobs quickly realized that the vertebrae belonged to a pliosaur — but the museum staff had no idea what had been lurking in their archives. Not only were they pliosaur bones, but they were extraordinarily large pliosaur bones. In fact, the newly discovered bones suggest that pliosaurs could grow much larger than scientists had previously thought.

“They had no idea that these were ginormous [bones] for this type of animal,” Jacobs said.

As Live Science reports, Martill scanned the vertebrae and determined that the unknown pliosaur species once stretched between 32 feet to 47 feet long. That makes it the largest pliosaur ever found, as the second largest, Kronosaurus queenslandicus, was between 33 to 36 feet long.

Megan JacobsThe four pliosaur vertebrae discovered by chance at Abingdon County Hall Museum.

Some 150 million years ago, pliosaurs terrorized the ancient seas. They could grow to be twice the size of orcas and had, according to Jacobs, “a mouth full of enormous banana-sized teeth… you wouldn’t have wanted to go swimming in the late Jurassic seas.”

Indeed, pliosaurs were one of the most powerful predators of their age. They had large heads like a crocodile, a short neck, a short tail, and four flippers, which they used to propel themselves through the water.

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

Ian Sutton/FlickrA model of a pliosaur from the Kronosaurus Korner in Richmond, Queensland, Australia.

Martill added: “They were at the top of the marine food chain and probably preyed on ichthyosaurs, long-necked plesiosaurs and maybe even smaller marine crocodiles, simply by biting them in half and taking chunks off them. We know they were massacring smaller marine reptiles because you can see bite marks in ichthyosaur bones in examples on display in The Etches Collection in Dorset.”

The vertebrae that Martill came across in the fossil drawers at the at Abingdon County Hall Museum were originally discovered at Warren Farm in the River Thames Valley in Oxfordshire. They come from the Kimmeridge Clay Formation, a Late Jurassic deposit that’s approximately 152 million years old.

At some point, they were forgotten in the museum. But their discovery has shed new light on pliosaurs. Now, Martill is hopeful that similar groundbreaking discoveries will be made in the future.

“It’s wonderful to prove there was indeed a truly gigantic pliosaur species in the Late Jurassic seas,” he said. “It wouldn’t surprise me if one day we find some clear evidence that this monstrous species was even bigger.”