Shishania aculeata, a stem-mollusk with no shell but lots of spikes

[Halichoeres]

A new stem-mollusk (that is, equally closely related to all mollusks, but closer to them than any other living animal group) called Shishania aculeata sheds a bit of light on the evolution of their shells. They show spiky, hollow keratin bristles, similar to the chetae that can be found on some annelid worms, only thicker.

A close-up of the bristly hide:


This might suggest that this was the ancestral integument for mollusks that eventually gave rise to the diverse shells of cephalopods, snails, chitons, and bivalves.

Paper (paywall): https://www.science.org/doi/10.1126/science.ado0059

popular write-up: https://www.earth.com/news/mollusks-evolution-shishania-aculeata-spiny-armor-no-shells-500-million-years-ago/