Tryblidium, with its flattish, ornamented shell, looked like a snail but had one unique anatomical feature: some organs (gills, kidneys and muscles) were repeated over and over, in multiple series.

It was long thought that this type of mollusc, common in the Silurian, had died out more than 350 million years ago — until living specimens were found in the ocean deep.

Like its modern descendants, Tryblidium fed by scraping up algae and bacteria from stones, shells and other hard surfaces.