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

The Cambrian period, which lasted from 543 million to 490 millions years ago, saw the first major explosion in biodiversity on Earth. It also saw the first appearance of the ancestors to nearly all modern animals. The penis worm is one of the most fearsome.
Technically known as priapulids after Priapus, the well-endowed Greek god for male genitals penis worms, they are a group of marine worms that has survived in the oceans for over 500 million years. Their descendants are now largely unknown and live in deep, muddy burrows below the surface, sometimes frightening fishermen with their flat, phallus-shaped bodies. However, fossils from the early Cambrian reveal that penisworms were once a problem in ancient seas. They were widely distributed and had fang-lined mouths which could be used to make snacks of any marine creature that passed them.

Penis worms were no less fearful than they appeared to be. Researchers discovered four priapulid flora in a study published in Current Biology on Nov. 7.

Related: Image gallery: Bizarre Cambrian creatures

Researchers believe that all the worms were in the same shell type and in approximately the same location, which suggests that they had taken the shells as homes.

If this is true, it would mean that penisworms invent the "hermit" lifestyle many millions of years before the crustaceans who made it popular.

One fossil showing a penisworm chilling inside the shell of a dead Hyolith. (Image credit: Zhang Xiguang)

Martin Smith, a Durham University associate professor of paleontology, stated via email that "the only explanation that made sense" was that the shells were their homes.

The four penis fossils of a hermit were discovered in the Guanshan fossil deposit, south China. These fossil deposits date to the early Cambrian (525 million years ago), and are well-known for their preservation of not only hard structures like teeth or shells but also soft tissue such as the bodies of priapulids. This is a rare find in the fossil record.

Each shell contains a worm's bottom that sits in the bottom of the cone. The worm's head, mouth, and head hang out on the side, almost like soft-serve icecream. Researchers found that the fossil region had dozens of empty shells but no free-living priapulids. This suggests that the connection between them was not accidental. Each worm was snugly contained in its sheath. This suggests that the creatures used their shells to protect themselves from Cambrian predators and not as temporary refuge.

Researchers wrote that this type of "hermiting behavior" has never been seen before in priapulids, nor in any species prior to the Mesozoic Era (250 million to 65 millions years ago). Smith said that it is "mind-boggling," that such complex behavior could have arisen so quickly after the Cambrian explosion, which occurred more than 500 millions years ago. Even the most fearsome penisworms had to find creative ways in the harsh environment of the early ocean.

Original publication on Live Science