Ferocious "Penis Worms" Were the Hermit Crabs of the Ancient Seas

DannyB writes:

Ferocious 'penis worms' were the hermit crabs of the ancient seas:

[...] Technically known as priapulids - named for Priapus, the well-endowed Greek god of male genitals - penis worms, as they're commonly known, are a division of marine worms that have survived in the world's oceans for 500 million years. Their modern descendants live largely unseen in muddy burrows deep underwater, occasionally freaking out fishermen with their floppy, phallus-shaped bodies. But fossils dating back to the early Cambrian show that penis worms were once a scourge of the ancient seas, widely distributed around the world and in possession of extendible, fang-lined mouths that could make a snack out of the poor marine creature that crossed them.

But, fearsome as they were, penis worms themselves were not without fear. In a new study published Nov. 7 in the journal Current Biology, researchers discovered four priapulid fossils that were nestled into the cone-shaped shells of hyoliths, a long-extinct group of marine animals.

[...] In each shell, the worm's bottom sits squished into the bottom of the cone, while the worm's head and mouth dangle out over the side - sort of like a melting swirl of soft-serve ice cream. According to the researchers, the fossil region contained dozens of other empty shells, but no other free-living priapulids, suggesting the connection between the two was no mere accident. Furthermore, each worm fit snugly in its sheath, suggesting the creatures chose their shells for permanent protection from Cambrian predators, rather than as temporary refuge.

Penis worms were the hermit crabs of their time:

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