Look at any part of the seabed and you are likely to find star-shaped creatures with long, slender arms wriggling across the bottom.
These are not fish, but invertebrates called brittle stars, and they are relatives of starfish and sea urchins.
They are also known as snake stars because of the way they move their arms. Palaeocoma was an early kind of brittle star. Wary of predators, Palaeocoma, like modern brittle stars, may have hidden in cracks in rocks and corals, coming out only at night to feed.
■ When: Nearly 200 million years ago (Early Jurassic)
■ Fossil location: Europe
■ Habitat: Seafloor
■ Size: 2–4 in (5–10 cm) across
■ Diet: Remains of plants and animals
Palaeocoma had a flat, central disklike body. Extending from it were five long, spiny arms, with which it moved swiftly along the seabed. When disturbed, it could escape quickly, pulling and pushing its body using the muscles in its arms. On the underside of its body was a star-shaped mouth containing five toothed jaws.
When feeding, it used the tiny, muscular tube feet underneath its arms to sweep food into its mouth. It had no eyes, but may have been able to sense light through its feet.
Drifting with the tide
Like many sea creatures, brittle stars live on the seafloor as adults but spend the early part of their lives as plankton—tiny organisms that float freely in the sunlit upper waters of the ocean.
They drift with the currents for weeks, traveling hundreds of miles before finally sinking to the seafloor, where they change into adult brittle stars.
Around 2,000 species of brittle star exist today, in icy seas and warm waters all over the world. These, often brilliantly colored, creatures—some with patterned bodies—have five snakelike arms. When attacked, they shed an arm, which wriggles for a while, confusing the predator. Brittle stars quickly regrow the lost arm.