Let’s get this out of the way first: The penis worms are a group of marine invertebrates named for their penislike shape. All species of the tubular animals, which can reach lengths of 39 cm, have an extensible mouth called a proboscis that is lined with sharp hooks, teeth, and spines. Some species still exist today, but during the Cambrian period, about 500 million years ago, they were among the most common organisms on the planet, and are preserved en masse in the Burgess Shale—a famous deposit of fossils in the Canadian Rockies. New research, published online today in Palaeontology, suggests that the Priapulida phylum, the group that contains all the penis worms, may have been more diverse than previously realized, and that the differences between species are revealed by their tiny teeth. Using a suite of microscopy techniques, researchers analyzed fossils of penis worm mouth parts and discovered a previously unrecognized penis worm. Dental imprints left behind in the rock show that the most common group of penis worm, Ottoia prolifica, should actually be two separate species. The team has proposed the name O. tricuspida for the newly discovered species to reflect a unique three-pronged tooth that distinguishes it from other penis worms. Because the difference is so subtle, the scientists speculate that many fossils previously characterized as O. prolifica may actually belong to O. tricuspida, suggesting that one of the most widespread and ecologically important animals of the Cambrian era may be more diverse than previously thought.

(Credit for linked PDF: M. R. Smith et al., Palaeontology [2015])