Fossils Challenge Theory of Rapid Animal Evolution in Cambrian

John Roach
For National Geographic News
July 20, 2001
Most major animal groups appear for the first time in the fossil record
some 545 million years ago in a relatively short period of time known as
the Cambrian explosion. The explanation of this sudden arrival is a
scientific conundrum.

The fossil record suggests that exceptional evolutionary activity took place over 10 million years at the base of the Cambrian and generated the ancestors of nearly all the animal groups living on Earth today, as well as others that failed to see modern times.

But many scientists believe that such rapid evolution is not possible. They postulate that there was an extended period of evolutionary progression that left behind a scant fossil record.

Now, two well-preserved fossils half a millimeter (one-fiftieth of an inch) long of crustaceans—ancient relatives to crabs, shrimp, and lobsters—found in 511 million-year-old limestone deposits in Shropshire, England, lend credence to the theory that a long evolutionary fuse preceded the Cambrian explosion.

"It proves you had advanced arthropods very early on," said Mark Williams, a researcher at the British Geological Survey in the United Kingdom and co-author of a research paper on the crustacean fossils in the July 20 issue of Science. "It suggests that there was a protracted period of arthropod evolution prior to the Cambrian."

Exquisitely Preserved

The fossils are the oldest crustaceans ever found and are particularly unique in that the animals' soft body parts have been exquisitely preserved. Usually soft body parts, such as legs, mouth, and abdomen, decay and only the shells remain.

"Shells are not uncommon, but they don't really give us the lowdown on their biological affinities," said Richard Fortey, a paleontologist at the Natural History Museum in London and author of a companion piece to the research article in Science. "You need very special conditions to preserve the soft parts."

Williams and his collaborator David Siveter of the University of Leicester thought such conditions might exist in the Cambrian limestone in Shropshire. So they went and dug some up of the limestone and processed it in a laboratory at the University of Leicester. They used acetic acid to dissolve the limestone, which leaves behind a residue of fossils preserved by calcium phosphate minerals.

Their labor yielded the crustacean fossils. The soft body parts are intact and preserved in three dimensions, like mummy remains. Microscopic images show the shell, head appendages that include antennae and mouth, and the body of the specimens.

"It has all the morphology to prove it was a crustacean," said Williams. "It is a wonderful fossil."

Earlier Emergence

The fossils indicate that a crustacean was already evolved in the early Cambrian. The scientists said this suggests that the necessary steps in the evolutionary tree of arthropods that led to the design of crustaceans must have begun in the late Precambrian.

Scientists are now scouring other deposits like the Shropshire limestone for fossils of other multi-celled animals, as additional evidence to support the idea that a long evolutionary fuse preceded the Cambrian explosion.

Regardless of how long the evolutionary fuse to the Cambrian explosion may have been, the Cambrian is marked by the sudden appearance of animals of increased size and with shells.

"It is still clear that something very important happened at the Cambrian, and the choice is between the appearance of animals themselves or changes in pre-existing animals that forced the explosion," said Fortey. "It's asking a lot of evolution to manage all the business together at once, as some have claimed."

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