The oldest sponges: a scratchy way of life.

A team of paleontologists working in Mongolia has discovered the earliest conclusive evidence that sponges thrived on the seafloor just before the start of the Cambrian period 544 million years ago. The find represents an evolutionary benchmark for these scientists, who regard sponges as the most primitive multicellular animal, says Martin Brasier of the University of Oxford in England.

The Mongolian sponges lived at a pivotal point in Earth's history, when the ancestors of most modern animal groups appeared suddenly in the seas. Prior to this so-called Cambrian explosion, the oceans were home to a perplexing group of creatures known as the Ediacaran fauna-simple organisms that paleontologists find it difficult to categorize (SN: 7/8/95, p. 28).

Unlike their Ediacaran contemporaries, however, the Mongolian fossils present no puzzles: They consist of tiny glassy spikes identical to the spicules of some sponges today. "These are absolutely unequivocal," says Brasier. "An awful lot of what we get out of the Ediacaran fauna and the Cambrian explosion doesn't have any clear relative today and is very paradoxical."

Measuring one-tenth of a millimeter across and made of silica, the Mongolian spicules formed a rudimentary glassy scaffolding-one of the earliest animal skeletons. Still common today, such glass-barbed sponges are quite different from their smoother relatives, which can be harvested for use in the bathtub.

Several researchers have reported other finds of Precambrian sponge spicules, but many turned out to be volcanic shards or mineral crystals and all remain controversial, says Brasier. In Australian rocks of Ediacaran age, paleontologists have found round fossils that they identify as the compressed bodies of sponges. Some researchers, however, have questioned whether these spiculeless impressions were indeed sponges.

Because the Mongolian spicules are easy to interpret, paleontologists are confident that the creatures lived as modern sponges do, by filtering nutrients from the water. The Precambrian remains therefore provide the earliest example of filter feeding, an extremely common mode of life in the modern seas.

Paleobiologist Andrew H. Knoll of Harvard University says that Brasier and his colleagues, who report their discovery in the April Geology, have clearly documented the existence of sponges in the latest stage of the Precambrian period. This find confirms what many researchers have long believed-that sponges and more complex animals must have evolved by the time the Ediacaran fossils appeared around the world.