When I think of prehistory’s greatest survivors – deep lineages marked by body plans that have remained consistent for millions of years – creatures like horseshoe crabs, velvet worms, and ginkgos immediately come to mind. Penis worms belong on that list, too.
Technically, and less hilariously, known as priapulids, these marine invertebrates have been burrowing into ocean sediments since the time the famous Burgess Shale was laid down 505 million years ago. The Cambrian form Ottoia prolifica roughly resembles modern priapulids (see the video below), and, as preserved gut contents show, this archaic penis worm was an opportunist that snarfed up just about anything it could extend its pharynx around.
Paleontologist Jean Vannier cataloged the gut contents of Ottoia in a PLoS One paper published last month. While about half the specimens Vannier investigated had empty guts, hundreds of Ottoia were preserved with their last meals intact. The fossils-within-fossils revealed a varied diet for the penis worm.
In general, Vannier found that Ottoia ate polychaete worms, bits of trilobites, shelled brachiopods, cone-shaped creatures called hyolithids, the armor of pincushion-like wiwaxiids, and parts of miscellaneous arthropods. And an exceptional fossil of several Ottoia arranged in “a wreath” around a large arthropod – Sidneyia inexpectans – hint that the penis worms weren’t above scavenging from larger prey when the opportunity arose.
Unfortunately, because of our geologic distance from Ottoia, we don’t really know how the tubular critter acquired such a wide range of prey. But Vannier suspects that the priapulid obtained nourishment in different ways.
Ottoia probably caught or accidentally ingested worms and small invertebrates as it sifted through seabed sediment, and occasionally scavenged large animal carcasses. The penis worm didn’t have a rigid, specialized dietary regimen. More likely, Vannier suggests, the omnivorous priapulid was a generalist and “facultative feeder”, able to take on whatever it could send down into its gut. In the Middle Cambrian seas, brimming with life, hungry priapulids didn’t have to be picky.
Vannier, J. 2012. Gut Contents as Direct Indicators for Trophic Relationships in the Cambrian Marine Ecosystem. PLoS One. e52200. doi:10.1371/journal.pone.0052200