The fossilised remains of a foot-long slimy sea creature dating from 100m years ago suggest that the last common ancestor of all vertebrates looked less like a squishy eel and more typically “fish-like”, researchers claim.

They say the fossil, unearthed around eight years ago in Lebanon, is an early hagfish, a peculiar creature that has no jaws, eyes or true vertebrae but that boasts the ability, when threatened, to squirt out a mixture that turns into an expanse of slime.

Hagfish are typically found at the bottom of oceans, where they burrow into and feed on dead marine creatures. Given that they lack bones, the scientists say the discovery of an ancient hagfish was a surprise.

A modern hagfish. Photograph: Tetsuto Miyashita/University of Chicago

“It’s as rare as finding a fossilised sneeze,” said Prof Phillip Manning, a co-author of the study and chair of natural history at the University of Manchester.

Writing in the Proceeding of the National Academy of Sciences, the team say analysis of the fossil, including a mapping of chemical elements, reveals a number of features seen in hagfish today – slime glands included.

As well as being astonishing in its own right, the fossilised creature – given the name Tethymyxine tapirostrum, or “slimy fish from the Tethys Sea with a long tapering snout” – could lay to rest a row among experts about where hagfish sit on the evolutionary tree of vertebrates.

Some believe the form of hagfish suggests similar creatures belong near the base of that tree, with jawless creatures called lampreys, armoured fish, and other vertebrates evolving as time went on. But there is a problem: analysis of DNA from hagfish and lampreys is at odds with this picture, suggesting instead that these species both evolved from an ancient creature that had branched off early on.

“For many years the data you got from the DNA didn’t match with what we saw in the fossil record,” Manning told the Guardian. Without fresh fossils, the debate raged on. But now, says Manning, the conundrum appears to have been solved.

“There are some pretty funky [characteristics] which are present in this animal relatively early,” he said, adding that such features – including slime glands and an unusual feeding mechanism– suggested a hagfish-type creature was not the last common ancestor of all vertebrates.

Manning said the branching that gave rise to hagfish and lamprey probably took place place about 500m years ago. But quite what the common ancestor of these creatures and all other vertebrates looked like is still something of a mystery, although the team say it was probably more typically “fish-like”.

“This ancestor, X, which exists somewhere, gives rise directly to the hagfish and lampreys but also gives rise to a line of other jawless fish and the true fishes and vertebrates such as us,” said Manning.

The hunt for that creature continues. “There is going to be this ancestor lurking either in a museum drawer or still in the ground,” Manning said. “Which is fun. That means there is still work for palaeontologists to do.”