Worming Its Way into History

The mysterious little fossil plates appeared in marine sediments all over the world, along with dozens of other extinct invertebrates.

The tiny pieces resembled horizontally ridged potato chips or, to a paleontologist perhaps, parts of mollusks, barnacles, sea urchins or some other strange armored animal.

This is the kind of puzzle invertebrate paleontologists grab onto and don’t let go, because the plentiful fossils might reveal an important facet of the history of life.

Experts were puzzled because the small plates were not associated with any whole organism. Scientists categorized them as machaeridians, and the mystery persisted for about 150 years.

Finally, fossil experts from Yale University and Belgium’s Ghent University identified the bearer of the elegant, diminutive plates. Peter Van Roy of Belgium happened upon an extremely unusual fossil – - a few plates plus traces of the soft body that bore them.

The body — which otherwise would have decayed rapidly, leaving no trace — was preserved through rapid burial and chemical reactions on a sea floor 480 million years ago.

Since then, the sea floor has been thrust upward and now its sediments are rocky foothills to the Atlas Mountains in Morocco. Van Roy happened upon the extraordinary fossil amid the mineralized remains of trilobites, sponges, echinoderms and other invertebrates of the Ordovician period.

The newly discovered 1-inch-long fossil bore the unmistakable shape of a worm-like creature, preserved in yellowish iron oxides.

Van Roy contacted a Yale graduate student in geology and geophysics, Jakob Vinther, knowing that Vinther was studying mysterious extinct wormy animals.

Vinther and Derek E.G. Briggs, professor of geology and geophysics and curator of invertebrate paleontology at the Peabody Museum of Natural History, recognized the animal as an armored relative of earthworms, leeches and the sea mouse, a kind of marine worm found in the North Atlantic Ocean.

The puzzling fossils suddenly made sense. Machaeridians were annelid worms, sheathed from head to tail in overlapping armor plates. When a worm died, the plates were left behind.

The freshly deceased worm that became the fossil was probably quickly buried in silt without oxygen and loaded with anerobic bacteria that produce hydrogen sulfide, the compound that gives low tide its characteristic mephitic smell.

The hydrogen sulfide reacted to form iron sulfide, otherwise known as iron pyrite, or "fool’s gold," which weathered to the iron oxide image that Van Roy found.

The annelid ancestor was flat and equipped with side bristles, similar to some species of modern marine bristle worms that are frequently used as bait.

The plates were probably defensive, and the worm would have added segments as it grew, rather than molting like a trilobite or other arthropod, Briggs said.

Despite the defense, the prescence of a strong jaw in its living relatives suggests that the enigmatic creature may have been an active predator, he said.

Yale’s Peabody Museum collection includes rare trilobite fossils whose legs and antennae were preserved, again, in iron pyrite. Another graduate student in Briggs’ lab is working on the chemistry of environments that yield pyritized fossils.

Studying early invertebrates without shells is difficult because they leave little behind. Briggs said that more machaeridian fossils with pyritized soft parts will likely be found as paleontologists continue to explore and collect specimens.

Just where the machaeridians are evolutionarily related to other annelid worms, mollusks and brachiopods is not yet clear. Other annelids first appeared in the fossil record about 50 million years before the machaeridians, Briggs said.

The machaeridians thrived for about 180 million years and then slowly diminished and disappeared, rather than suddenly becoming extinct. This longevity makes them remarkably successful animals.

Why they disappeared while modern annelid worms, such as aquatic bristle worms, garden earthworms and leeches, continue to thrive, is another mystery of biology.

The machaeridian environment may have changed, or newer organisms may have displaced them, Briggs said. "They were at the wrong place at the wrong time," he said.

Why pursue the lineage of worms when vertebrate paleontologists get all the attention with dinosaur fossils?

"Our research helps us work out the relationships of living groups, and the nature of life on Earth," said Briggs, who will assume directorship of the Peabody Museum in July.

Worms may seem unimportant, but they may exert a significant environmental influence on the ocean bottom, other bodies or water and on land.

Vinther, Van Roy and Briggs published their research in the prominent science journal Nature.