Starfish ancestor
The 430m-year-old starfish ancestor had five limbs and a peculiar arrangement of tubular limbs.Proceedings of the Royal Society B: Biological Sciences 2017. DOI: 10.1098/rspb.2017.1189

An ancient invertebrate related to starfish, sea urchins and sea lilies has been discovered in a rich fossil bed in the west of England.

The fossil, part of the echinoderm group, is an astonishing 430 million years old and incredibly well preserved. The creature was found at a fossil bed in Herefordshire, and is described in a study in the Proceedings of the Royal Society B.

The 430-million-year-old starfish ancestor, named Heropyrgus disterminus, is about 3cm in length. Five plates cover its body, from which protruded a set of unusual tubular limbs.

"Echinoderms are unique in having a water vascular system with tube feet, which perform a variety of functions in living forms," the authors, led by Derek Briggs of Yale University, write in the study.

"Here, we report the first example of preserved tube feet in an extinct group of echinoderms."

Echinoderms use their tube feet for everything from walking around to oxygen exchange and feeding. In this ancient species, the tube feet have an arrangement never seen in another echinoderm. There are two sets of feet, an upper and a lower, that attach to the inner surface of the plates, which make up the creature's main body and surrounding its oral cavity.

"The tube feet may have captured food particles that entered the oral area and/or enhanced respiration," the authors propose.

The preservation of this soft and delicate tissue is very rare. Usually the fossilisation doesn't retain delicate tissue that can easily rot. Fossil beds where large amounts of soft tissue are preserved – known as Lagerstättes – are very rare.

At the site in Herefordshire, the animals died soon after a volcanic eruption, when hot ash rained down on them. The ash covered the creatures, sealing their bodies in an air-tight bed while still alive. Once they died, no living microbes were present to break down the bodies, hence its excellent structural preservation.