Ancient Symbionts in a Sulfurous World?

Farmers? These olenid trilobites may have grown bacteria on their gills for food.

Trilobites were some of the most successful arthropods ever to scuttle through the oceans. Up to a meter in length, these ubiquitous invertebrates evolved a number of feeding strategies before they died out at the end of the Permian, 250 million years ago. Ferocious trilobites cruised the open sea snatching prey with rasped jaws, while others scavenged on the bottom for decaying flesh or filtered mud for bits of muck. Now a paleontologist argues in the 6 June issue of the Proceedings of the National Academy of Sciences that some trilobites even grew bacteria for food. If so, it would be the earliest example of symbiosis yet.

For years, Richard Fortey, a trilobite expert at The Natural History Museum in London, United Kingdom, had puzzled over a family of trilobites called the Olenidae. From 505 to 445 million years ago, they lived in deep water, buried in mud that was rich in sulfur and poor in oxygen--a nasty neighborhood that made Fortey wonder how the critters managed to survive, even flourish. Then a few years ago, he heard well-known symbiosis expert Lynn Margulis of the University of Massachusetts, Amherst, lecture about bacteria that thrive in sulfur-rich environments.

Fortey began to suspect that the olenid trilobites might have relied on a diet of symbiotic bacteria, just as some modern shrimp and bivalve molluscs do. When he took another look at specimens, he noticed that many olenid species had a wide, flat thorax--indicating that the gill branches would have provided ample acreage for farming bacteria. They also had a miniscule plate, called a hypostome, on the underside of their heads. Other trilobites use a much larger version of this structure to help catch prey or shovel muck into their mouths, so it makes sense to Fortey that the olenids had a more convenient source of food. Some olenids may simply have swallowed part of their homegrown culture frequently, says Fortey, whose new book Trilobite! Eye witness to evolution will be published in the United Kingdom this month. Other trilobites may have left the microorganisms alone, but absorbed the nutrients they excreted.

So far, the most ancient inferred symbiotic relationships have been found in late Ordovician reefs, some 425 million years old. While Fortey admits that symbiosis is "a hard thing to prove," he suspects that it may have been common in other kinds of trilobites that resemble olenids and may have lived in sulfur-rich mud. "It's a fun, speculative paper, and Fortey has a knack of being right about such things," says Nigel Hughes, a trilobite paleontologist at the University of California, Riverside, who expects the find will spur others to investigate these ancient, harsh environments more closely.

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