From colorful, enigmatic octopuses, to oysters with their iridescent pearls, molluscs today are as beautiful as they are diverse. But it turns out that their ancient relatives may have looked like the love child of a snail and a hedgehog.
Soft-bodied creatures are a rarity in the fossil record as their tissues decay rapidly after death. However, researchers say they have found a rare exception in eastern Yunnan province, in the southwest of Chinain fossils dating back to about 514m years ago.
The specimens, which are only a few centimeters long, are believed to be the remains of a proto-mollusk.
Called Shishania aculeata, to the eminent paleontologist Zhang Shishan, and the Latin word for “spiky,” the fossils reveal a flat, snail-like animal covered in hollow, cone-shaped spikes.
While many of the specimens were poorly preserved, with only the animal’s spiky exterior visible, others showed traces of its funny parts – including a muscular foot on its underside.
“Within those rare cases of us having this fossil to begin with, there are some that are just pristine preserved that haven’t decayed much while they’ve been preserved,” said Dr Luke Parry, co-author of the study from the University of Oxford.
Write in the journal Science, Parry and colleagues report that they also found traces of tiny tubules inside the spines, suggesting that the spines once contained microscopic projections called microvilli. This, the researchers suggest, would have secreted a sticky substance called chitin, which itself formed the spines.
While the specimens are somewhat younger than what are thought to be the oldest mollusk fossils, the team suggests that the spiny snail-like creature branched off the tree of life before molluscs evolved.
As a result, they say the creature could help shed light on what early mollusk ancestors might have looked like — a long-standing mystery given the enormous diversity of molluscs and the small number of early mollusk fossils.
Indeed, while it is thought that the last common ancestor of all molluscs would had a single shell, Parry said the new fossils indicate a spiky exterior that appeared even earlier on the family tree. “The new fossil is an extra piece of the puzzle as it shows us what molluscs looked like before they developed a shell,” Parry said.
The spines, he suggested, were subsequently lost in the branch of the mollusk family tree that gave rise to creatures such as clams and snails and octopuses, but were retained in the branch that gave rise to other molluscs including chitons – marine molluscs which filled spines with biominerals.
Dr Martin Smith, of Durham University, who was not involved in the work, welcomed the discovery.
“At first blush, I may have overlooked the importance of this spiky ‘pancake,'” he said, adding that he remembered seeing traces similar to the cone-shaped spines in other deposits. If this [features] can be recognized in extant microfossil assemblages, we may be able to deal with the timing of early mollusc evolution with a much higher resolution than we currently have,” he said.
Dr Julia Sigwart, from Queen’s University Belfast, also said the discovery of a proto-mollusc was exciting, but noted that it was possible that spines appeared more than once during mollusk evolution. “One shape we find in the fossil record doesn’t necessarily mean that all the other early fossils looked like that too,” she said.
