Ancient armor. Early mollusks may have had shell plates like these.

Spiny Beast Escapes Volcanic Tomb

Paleontologists have created a virtual reconstruction of a strange wormlike mollusk that died some 425 million years ago. The fossil has a mix of traits that supports, although not conclusively, a controversial idea about the identity of the earliest mollusks.

Mollusks include snails and clams and also several groups of more puzzling organisms. Chitons, for example, have segmented shells and superficially resemble large pillbugs. Aplacophorans, meanwhile, look like odd worms. Some aplacophorans lack key traits shared by many modern mollusks, such as a shell and a muscular foot, leading many malacologists to think the aplacophorans resemble the first mollusks. Yet none had been found in the fossil record of mollusks, which stretches back more than 500 million years to the early Cambrian--until now.

The creature, described in the 22 March issue of Nature, was entombed in an ash bed in Herefordshire, United Kingdom. Its body rotted away, and calcite filled the cavity. To reveal this and other fossils, paleontologist Mark Sutton of the University of Oxford ground down the rock 30 micrometers at a time. At each step, he polished the end of the rock and took a digital photograph. "We didn't have a clue what it was," Sutton says. But after a computer had stacked up several hundred slices into a 3D replica, "everything fell into place."

The fossil, named Acaenoplax hayae, has several aplacophoran traits, such as features that may have been gills and a lack of the typical molluscan foot. It also has shell plates, like chitons do. Sutton and his colleagues say that Acaenoplax suggests that shell plates were originally a common feature of both aplacophorans and chitons, and that the two form a natural group--perhaps even the first branch of the molluscan family tree.

Mollusk fans are happier than a clam. "This is a most amazing beast," says aplacophoran expert Amélie Scheltema of the Woods Hole Oceanographic Institution (WHOI) in Massachusetts. But she's not sure whether Acaenoplax is an aplacophoran, because it's so different from modern ones. To Bruce Runnegar of the University of California, Los Angeles, that's part of the attraction. "It gives us a great deal of information about early molluscan evolution," he says.

Related sites

Movies of Acaenoplax hayae

Information on aplacophorans at a WHOI Web site

All about mollusks, from Virginia Tech