Ancient oddball invertebrate finds its place on the tree of life

illustration of a hyolith

Tentacles protruding from this hyolith’s shell (illustrated) are part of a feeding organ called a lophophore, which is also found in modern horseshoe worms.

Danielle Dufault © Royal Ontario Museum

Hyoliths are evolutionary misfits no more.

This class of ancient marine invertebrates has now been firmly pegged as lophophorates, a group whose living members include horseshoe worms and lamp shells, concludes an analysis of more than 1,500 fossils, including preserved soft tissue.  

The soft-bodied creatures, encased in conical shells, concealed U-shaped guts and rings of tentacles called lophophores that surrounded their mouths. Fossil analysis suggests that hyoliths used those tentacles and spines, called helens, to trawl the seafloor more than 500 million years ago, researchers report online January 11 in Nature.

For years, paleontologists have argued over where on the tree of life these bottom-feeders belonged. Some scientists thought hyoliths were closely related to mollusks, while others thought the odd-looking creatures deserved a branch all their own. This new insight into hyolith anatomy “settles a long-standing paleontological debate,” the researchers write.

fossil of hyolith
In this hyolith fossil specimen, six tentacles of the lophophore feeding organ (right) and two spines called helens (top and bottom) are visible. Royal Ontario Museum
Cassie Martin

Cassie Martin is an associate editor. She has a bachelor's degree in molecular genetics from Michigan State University and a master's degree in science journalism from Boston University.

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