480-million-year-old fossil sheds light on 150-year-old paleontological mystery

Fossil (left) and depiction (right) of extinct armored machaeridian worm. Colors indicate the trunk (yellow), limb (red), bristles (gray), attachment of shell plates (green), gut (purple) and dorsal linear... Click here for more information.

New Haven, Conn. — Discovery of an exceptional fossil specimen in southeastern Morocco that preserves evidence of the animal’s soft tissues has solved a paleontological puzzle about the origins of an extinct group of bizarre slug-like animals with rows of mineralized armor plates on their backs, according to a paper in Nature.

While evolution has produced great diversity in the body designs of animals, over the course of history several highly distinct groups, such as trilobites and ammonites, have become extinct. The new fossil is of an unusual creature known as a machaeridian, an invertebrate, or animal without a backbone, that existed for about 180 million years from 485 to 305 million years ago.

“The new specimen unequivocally identifies machaeridians as annelid worms, an extremely successful and diverse group of animals that includes familiar living animals like the sea mouse, the earthworm and the leech,” said Jakob Vinther, graduate student in the Department of Geology & Geophysics at Yale. The specimen was found in an area that had earlier been identified as a rich source of exceptionally preserved fossils including sponges, trilobites, echinoderms and other less-familiar invertebrates.

Jakob Vinther. Click here for more information.

First described over 150 years ago, armor plates of these strange animals have been found in marine fossil deposits worldwide covering the time span of their existence, and indicating that they were an important component of ancient seafloor ecosystems. Until now there was little information about their body design or how they might be related to other ancient — or currently living — animals.