A reconstruction of Calvapilosa, showing what this primitive mollusc most likely looked like in real life. Photograph: Jakob Vinther/Model made by Esben Horn (10tons.dk)

Lumpy, hairy and with a nail-like horny patch it sounds like a hobbits toe. In fact, its a portrait of what researchers say the common ancestor of slugs, snails and squid might have looked like.

The surmise is based on the discovery of the fossilised remains of a mollusc, thought to have lived about 480 million years ago, which has short spines all over its body and fingernail-like shell over its head which housed a radula a tongue-like structure found only in molluscs with more than 125 rows of teeth.

Believed to be a very early ancestor of a group of marine molluscs known as chitons, the discovery, scientists say, suggests that the common ancestor of all molluscs likely had a similar appearance.

I would say that our animal probably is very close to the spitting image of how the ancestor of all molluscs must have looked like 530 million years ago, said Jakob Vinther, a molecular palaeobiologist from the University of Bristol and co-author of the research published in the journal Nature.

The newly discovered animal is believed to have reached up to 12cm in length although the juvenile found in the Yale collection is less than 2cm long. Its name, Calvapilosa kroegeri, is a reference to the hairy shell that covered its head together with a nod to Bjrn Krger. The palaeontologist spotted a complete version of the fossilised creature among a drawer of recently collected Moroccan rocks at Yale University almost a decade after the first incomplete fossil was found.

We had been looking through those drawers to try and see if there were any specimens and we missed it, said Vinther. [Then Krger said] Why dont you guys use this specimen it is entirely complete, and then he pulled this thing out and it was like dude, that is totally what we needed!

The discovery also sheds light on a previously discovered fossils, revealing that a number of older creatures whose nature had been debated due to a lack of preserved details were also molluscs, due to their similarity in structure to the newly discovered creature. We could bring all these other fossils into the fold of thinking [about] molluscan evolution, said Vinther.

It also reveals that an type of early animal with two shell-like plates, known as Halkieria, was also a mollusc. Despite Halkeria being older, the authors suggest that the number of plates grew during evolution, leading to modern day chitons, which bear a row of eight plates on their back. Basically our animal sits right at the base of the branch that leads to chitons, said Vinther.

With a very early non-molluscan creature called Wiwaxia known to have had scales and spikes, the researchers go further, proposing an evolutionary path in which the common ancestor of all molluscs bore spines, a single plate, and a radula before a variety of branches emerged, eventually giving rise to molluscs as diverse as snails, clams and slugs.

Martin Smith, an invertebrate palaeontologist at the University of Durham who was not involved in the research, described the new find as exciting. This is a really important fossil, he said. Theres been a lot of discussion about the common ancestor of molluscs and of course there is such a wide diversity of body plans of molluscs today ranging from squids to snails to slugs and various other things that it is very hard to work out what their common ancestor looked like.

While it has previously been suggested that the common ancestor was shell-less, the new study, says Smith, points towards a single shell and a radula forming part of the body plan of molluscs, which have since been lost, modified or multiplied in various branches over the course of evolution. It completely transforms how we see the earliest history of molluscs and how we read the fossil record, said Smith of the new find.

But Julia Sigwart of Queens University Belfast, cautioned against such an interpretation, saying that even at 480 million years old, the newly-discovered fossil is too young to draw conclusions about what the common ancestor of all molluscs would have looked like.

This is not a particularly old fossil in the context of molluscan evolution, she said. But, she added, the fossil does show how many different forms existed through the history of molluscs over the last half billion years. Any time we find these exceptionally preserved fossils, they are very important for us to understand what the body plans looked like, because the fossils are so rare.

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Lumpy, hairy, toe-like fossil could reveal the evolution of molluscs - The Guardian