Namibian fossils reveal ancient oddities.

Paleontologists working in Namibia have unearthed the remains of an evolutionary enigma--a fossil so bizarre that it doesn't fit easily into the modern categories of animals, plants, or any other kingdom. The organism, which lived more than half a billion years ago, flourished alongside a broad array of unclassifiable species that filled the oceans immediately before the explosion of animal life in the Cambrian period.

The newly identified fossils, called Swartpuntia, look somewhat like a revolving door the size of a hand. In life, the organism had at least three vertical sheets attached to a central stalk that grew up from the seafloor, says Guy M. Narbonne of Queen's University in Kingston, Ontario. Narbonne and his colleagues report their discovery in the November Journal of Paleontology.

Swartpuntia is the most recent addition to a group of organisms known as the Ediacaran biota, which first appeared about 600 million years ago and went extinct at the start of the Cambrian, 543 million years ago. Found around the world, Ediacaran fossils represent the first large, complex creatures to appear on Earth. The Namibian site captures a snapshot of these species immediately before they disappeared.

"These are probably the youngest Ediacaran fossils in the world. They provide us with a glimpse of the climax of Ediacaran evolution just prior to the Cambrian explosion," says Narbonne.

When researchers first uncovered a large number of these fossils in the 1940s in South Australia, they interpreted the Ediacaran biota as the earliest animals. In the 1980s, however, Adolf Seilacher of Tubingen University in Germany argued that they were neither animal nor plant but belonged instead to an extinct kingdom of life constructed like fluid-filled versions of the Michelin Man, a well-known tire company mascot. Seilacher named them vendobionts (SN: 7/8/95, p. 28).

The Namibian discovery provides new support for part of Seilacher's idea. The flat sheets of Swartpuntia consist of a series of inflated chambers, matching the vendobiont model, says Narbonne. They grew in shallow waters and may have harbored photosynthetic algae or bacteria. He categorizes Swartpuntia and similarly constructed species as members of an extinct group that may fit into the animal kingdom or may represent an extinct kingdom of life.

First unearthed in Namibia in 1995, Swartpuntia has more recently turned up in North America, according to Benjamin M. Waggoner of the University of Central Arkansas in Conway. Waggoner found a specimen this February while conducting fieldwork in southern Nevada, he reported last month at a meeting of the Geological Society of America.

Waggoner agrees that Swartpuntia and several other Ediacaran species belonged to a group of sheetlike, inflated creatures. "I'm not so sure where they should be placed on the tree of life. I'm remaining agnostic on that larger issue."

Not all Ediacaran species remain mysterious, however. Earlier this year, Waggoner identified a fossil called Kimberella as a close relative of a mollusk (SN: 8/30/97, p. 132). Other Ediacaran fossils record the tracks that worms made in the seafloor.

Even as they struggle to decipher what the various Ediacaran organisms were, paleontologists are attempting to understand what happened to these diverse life forms. Narbonne lists three possible reasons for their demise.

One is that these large, soft, defenseless creatures could not survive after the first predators evolved. Another possibility is that some Ediacaran species actually persisted into the Cambrian but failed to fossilize because worms started to churn up the seafloor.

A third explanation arises from studies of the ratio of two carbon isotopes, which reveal an abrupt change in seawater chemistry at the end of Precambrian time. "This is the most profound, sharpest excursion in carbon isotopes in Earth's history," says Narbonne. The geochemical upheaval could have hastened the Ediacarans' extinction, he says.