Ancient “penis worms” reveal a fascinating insight into early animal evolution

It’s a small, fleshy creature, no more than an inch in length, with elongated spines and menacing pharyngeal teeth used to crush food. It lived in muddy shallow waters some hundreds of millions of years ago.

It also looks strikingly like a human penis.

The Eximipriapulus was what’s called a priapulan worm and it lived around 500 million years ago. Its unusual, phallic-like resemblance is how it (and its descendants) became known as a “penis worm.” Up to now, scientists have known little about this worm’s evolution.

A study published Monday in the journal Current Biology adds to our sparse understanding, revealing priapulan worms displayed a specific behavior similar to modern-day hermit crabs.

Co-author Martin R. Smith, an associate professor in paleontology at Durham University, tells Inverse the team didn’t expect to find this sort of behavior — the use of a shell as shelter — in the Cambrian era.

“Getting into a shell that's the right fit, working out when it's time to move on, and sizing up a replacement — this all takes a degree of behavioral and neurological sophistication that I for one wasn't necessarily expecting to find quite so early in animal evolution,” Smith says.

What are penis worms?

Priapulids, or penis worms, are predatory animals that still live in muddy marine ecosystems. The biggest penis worms can reach up to 4 to 6 inches in length.

“Priapulids today are perhaps slightly unfamiliar but are well-known to scientists,” Smith says. “They tend to live in places where predators struggle to get a foothold, such as fetid oxygen-poor waters.”

Unlike many other worms, penis worms aren’t segmented. Their body can be broken down into three parts: a tail, a trunk, and a spiny appendage known as an introvert (that’s the part that looks like a penis).

But the specific penis worm examined in the new study, Eximipriapulus, emerged not long after the infamous “Cambrian explosion.” During this time there was a burst of new life, resulting in the emergence of several modern animal groups that persist to this day. Priapulan-like worms were “one of the more abundant organisms in the oceans” in this period, explains Smith.

How the discovery was made — Smith and colleagues studied certain fossil deposits in southern China. The fossils in this area, known as the Guanshan biota, are known for revealing diverse animal life from the Cambrian era.

The scientists discovered new fossils in this area, which they identified as an ancient penis worm Eximipriapulus.

But that wasn’t the most interesting part of their discovery.

What’s new — The fossils revealed the ancient penis worm lived in a shell. But not just any shell — it sported the conical shell of another, long-extinct marine animal, called hyoliths.

Through this revelation, the scientists discovered the Eximipriapulus exhibited an odd behavior known as “hermiting.” This is when an animal adopts another organism’s exoskeleton.

A modern-day example of this behavior occurs in the hermit crab, which wears the shell of other snails and slugs for protection. Similarly, the researchers speculated the penis worm used the shell of other animals to defend itself against predators.

The discovery is the “first direct evidence of a ‘hermiting’ life strategy... in the priapulans within the Palaeozoic era,” the study team writes.

The findings are especially striking because they were discovered in a creature “that's, shall we say, not renowned for its ingenuity today,” Smith says.

Why it matters — This discovery explains helps explain why there were so many penis worms living after the Cambrian explosion, while also giving insight into the evolution of early modern animals.

Scientists used to think there were so many of these worms during this time because “predators were fewer and less fearsome in the Cambrian, so these fleshy and no doubt tasty worms didn't need to worry too much about getting eaten,” Smith says.

But this study disproves this older theory, showing penis worms actually “did go to some lengths to defend themselves” Smith says.

Penis worms “did go to some lengths to defend themselves.”

While they may have looked like simple creatures, the behavior of these ancient worms resembled that of modern animals. Previously, hermit behavior was thought to have originated around 240 million years ago, during the Mesozoic marine revolution.

“This is building up a picture of a Cambrian period that was much more 'modern' in its character than we used to think,” Smith says.

What’s next — These findings were an unexpected surprise — and it’s unclear whether or not any living penis worms still hermit.

“Were hermits a Cambrian oddity, or did they characterize the rest of the fossil record?” Smith asks.

Ultimately, this study joins others in challenging what we know about the evolution of modern animal life.

“It's easy to think of evolution as a gradual but unstoppable trend towards greater complexity and ‘better’ solutions to life's problems,” Smith says. “This study is a useful rejoinder to that idea, showing that sophisticated behavior can evolve early before apparently ‘dropping out’ of the fossil record — only to be reinvented much later.”

Abstract: The Cambrian ‘explosion’, about 530 million years ago, marks a rapid diversification of the major animal lineages ¹ . A concomitant increase in the complexity of ecosystems is believed to have accelerated this evolutionary radiation ² , but direct evidence of the ecological modes of Cambrian taxa is nevertheless scarce — even in exceptional Burgess Shale-type deposits ³ . Here, we present new fossil material from the Cambrian (Stage 4) Guanshan biota in southern China that reveals the consistent occurrence of the priapulan worm ?Eximipriapulus⁴ within the conical shells of hyoliths. This represents the first direct evidence of a ‘hermiting’ life strategy — the adoption of a different organism’s exoskeleton — in the priapulans and within the Palaeozoic era. It highlights the intense degree of convergent evolution during the Cambrian radiation. Hermiting behaviour has previously been linked with the escalation of predation pressure during the Mesozoic marine revolution ⁵ ; such intensity of predation may also have characterised early Cambrian oceans.