New Fossil Solves Old Mystery

The new fossil in comparison with a drawing of what a machaeridian might look like.

A discovery by Yale graduate student Jakob Vinther, Belgian scientist Peter Van Roy, and Derek Briggs, Director of the Yale Peabody Museum, has helped put a group of armored worms in their proper place.

Machaeridians, the creatures in question, first appeared four hundred and eighty million years ago, around the same time as nautiluses (the spiralshelled relatives of squids), and the first jawed fishes and coral reefs.

They covered themselves in rows of hard shell plates that both protected the worms and helped them burrow into the mud of the ancient sea floor.

One hundred and eighty million years after they appeared, the machaeridians mysteriously vanished from the fossil record, leaving no clues as to what happened to them or their possible descendants.

Since their discovery one hundred and fifty year ago, machaeridians have been giving paleontologists problems. Individual, triangular shell plates from these creatures are commonly found in sediments from the Ordovician through the Carboniferous periods (488-299 million years ago), but it is rare to find the plates articulated together to form a recognizable organism, much less to find soft tissue of the organism preserved in rock.

While many other groups of organisms have been re-examined and placed on the evolutionary tree in recent years, the scarce patchwork of machaeridian fossils made them nearly impossible to classify.

Over time they have bounced from group to group, including the barnacles, echinoderms (starfish and sea urchins), mollusks (snails and squids), and annelids (earthworms and some marine worms).

The plates, similar to the shell of clams and especially to the layered shell of marine mollusks called chitons, previously provided reason to believe that the two organisms were somehow related. “There was a lot of evidence for these things being mollusks,” says Vinther.

Vinther’s discovery, published in the January 10 issue of Nature, reveals a specimen of machaeridian whose shell plates are attached to fossilized body tissue—a first for this group of animals. The specimen was discovered in Morocco, in a southern locality of the country quickly becoming known for its rich collection of Ordovician fossils.

To find any fossilized soft tissue of any organism is extremely rare. Normally, the soft parts of an organism such as skin and muscle rot away before an animal has a chance to fossilize, which is why the vast majority of fossils remains are the hard parts of an animal, such as bones, teeth, and shells.

Scientists aren’t exactly sure what causes soft tissue preservation in rare cases, but they suspect that rapid burial and low oxygen levels play a role in the process.

Named Plumulites bengtsoni after the Swedish scientist Stefan Bengston, who first proposed that machaeridians were related to annelids, the specimen shows preserved skin and the body outline of the organism.

While interesting features such as the gut are visible, the most significant part of this find is the series of paired lobes along the side of the body, with bundles of thin bristles sticking out from the lobes.

These mechanisms were probably used by the tiny worms to “lever themselves from the sediments” as Jakob explains, and to move around the sea floor.

They are similar to the bristles found in modern class of marine worms known as polychaetes and are the most definitive evidence that machaeridians are an early ancestor of modern earthworms and other segmented worms.