Bizarre Ancient Sea Creature's Soft Innards Reveal Its True Identity

Around half a billion years ago, the world's oceans were home to a bizarrely shaped creature measuring up to 4cm in length. Called stylophoran, the species had an unusual appearance that included a single long appendage and a flat body, making it difficult for scientists to determine where it falls on the animal family tree. After more than 150 years of mystery, a new study has the answer.

At the heart of the mystery was the stylophoran's unusually long paddle-like appendage. Scientists have speculated whether the appendage was something like a tail or if it was used for feeding; the creature's skeletal anatomy wasn't enough to settle the debate, but its preserved innards are.

It turns out that stylophorans were echinoderms, the ancient relative of modern-day starfish, sea urchins, and sea cucumbers. The determination was made using high-quality fossils discovered in Morocco, ones preserved in the ferrous mineral pyrite. The long appendage wasn't a tail, but rather something similar to the arm of a starfish.

This discovery was made possible by mapping the fossil's iron and using it to analyze the appendage's structure, particularly the soft tissues that often do not fossilize. The arm-like appendage featured a water vascular system not unlike a modern starfish.

The appendage would have been used to help the flat creature move around and eat. A notable difference between the stylophorans and starfish, of course, is the lack of five-rayed symmetry, which they may have evolved away from. The full study can be read here.