Known as hyoliths, these marine creatures evolved over 530 million years ago during the Cambrian period and are among the first animals known to have produced mineralized external skeletons.
Long believed to belong to the same family as snails, squid and other molluscs, a study published today in the scientific journal Nature shows that hyoliths are instead more closely related to brachiopods — a group of animals which has a rich fossil record, although few living species remain today.
Brachiopods have a soft body enclosed between upper and lower shells (valves), unlike the left and right arrangement of valves in bivalve molluscs. Brachiopods open their valves at the front when feeding, but otherwise keep them closed to protect their feeding apparatus and other body parts.
Although the skeletal remains of hyoliths are abundant in the fossil record, key diagnostic aspects of their soft-anatomy remained critically absent until now.
“Our most important and surprising discovery is the hyolith feeding structure, which is a row of flexible tentacles extending away from the mouth, contained within the cavity between the lower conical shell and upper cap-like shell,” said Moysiuk. “Only one group of living animals — the brachiopods — has a comparable feeding structure enclosed by a pair of valves. This finding demonstrates that brachiopods, and not molluscs, are the closest surviving relatives of hyoliths.
“It suggests that these hyoliths fed on organic material suspended in water as living brachiopods do today, sweeping food into their mouths with their tentacles,” Moysiuk said.