ECOLOGY AND EVOLUTION

Symbiotic Trilobites

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Science  30 Jun 2000:
Vol. 288, Issue 5475, pp. 2285
DOI: 10.1126/science.288.5475.2285c

Oxygen-poor habitats associated with sulfide-rich sediments are widespread in the oceans. To counteract the toxic effects of sulfide, many marine metazoa have evolved physical associations with autotrophic sulfur bacteria, which use reduced sulfur as an energy source and, in doing so, render the sulfur nontoxic. Metazoa with chemoautotrophic bacterial symbioses tend to exhibit morphological specializations, such as reduction of the mouthparts.

When did such symbioses first evolve? Fortey has investigated trilobites of the family Olenidae, one of the early arthropod groups that populated the late Cambrian and early Ordovician seas (505-445 million years ago), and interprets their morphology as being consistent with symbiotic associations with sulfur bacteria. The olenids, which appear to have been sluggish organisms meandering about the seabed at the margins of the sulfur-rich zones, have extended pleural areas (gill filaments)—a possible site for bacterial cultivation—and atrophied mouthparts. Moreover, their fossils are found in sulfur-rich nodules in Cambrian shales. Thus, these trilobites would represent the oldest known chemoautotrophic symbionts. – AMS

Proc. Natl. Acad. Sci. U.S.A.97, 6574 (2000).

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