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Crustaceans Against Change
4 December 2003 (All day)
Over the past half-billion years, evolution has dished up an almost endless variety of novelties. So when paleontologists find a creature that doesn't change, they take note. In this week's issue of Science, a team describes a fossil crustacean and suggests that its descendents have had essentially unchanged anatomy for a record-breaking 425 million years.
Ostracodes are tiny crustaceans with a clamlike carapace. Fossil ostracode shells are so common and so varied that geologists use them to date and analyze rocks. The new fossil, however, preserves a jaw-dropping amount of detail in its soft tissue, because it was buried in volcanic ash, then rapidly mineralized. A few years ago, a team of paleontologists led by David Siveter of the University of Leicester, U.K., figured out a way to reveal the intricate detail. They grind the rock away, photographing polished cross-sections as they go. Then they assemble the digital images into a three-dimensional reconstruction, a technique that has already uncovered a strange, soft-bodied mollusk (Science, 23 March 2001, p. 2292), a king crab arthropod, and a bristle worm.
Colymbosathon ecplecticos is the latest wonder from this trove, located in Herefordshire, U.K. "The whole animal is amazing," David Siveter says. Features include limbs used for sensing, feeding, and swimming; a "furca," probably used to grasp prey and carrion; and a stout copulatory organ--the oldest penis in the fossil record. (The creature's name is Greek for "amazing swimmer with a large penis.") Six pairs of gills help peg it as a member of a living family called the Cylindroleberididae, Siveter says.
Ostracode specialists are stunned. "This is a demonstration of unbelievable stability," says Tom Cronin of the U.S. Geological Survey in Reston, Virginia. While ostracodes diversified into some 33,000 living and extinct species, "these guys have just been plodding along totally unfazed." The new fossil indicates that a basic ostracode body plan was already present in the Silurian period. It could also help sort out evolutionary relationships of fossil ostracodes.