Arctic fossils record evolutionary burst

Science News, Nov 7, 1998 by R. Monastersky

Working near the northern limit of land, on Canada's remote Victoria Island, scientists have unearthed a mother lode of microscopic fossils, chronicling an evolutionary explosion among complex cells more than 800 million years ago.

"This is a case of exceptional fossil preservation. We're seeing an awful lot more [species] than people have seen in similarly aged deposits," says Nicholas J. Butterfield of the University of Cambridge in England, who studied the Arctic rocks along with Robert H. Rainbird of the Geological Survey of Canada in Ontario.

Some of the fossils from Victoria Island are bacteria, but many represent larger, eukaryotic cells--the branch on the tree of life that harbors all animals, plants, fungi, and a vast microcosm of single-celled protists including amoebas and paramecia. Like most fossils of early eukaryotes, the Victoria Island forms are so simple that they cannot be assigned to any modern group. Paleontologists call these acritarchs, a catch-all category of unicellular eukaryotic fossils.

Though the researchers cannot unequivocally identify what the original organisms were, the diversity of the acritarchs is startling, says Butterfield, who discussed the discovery in the November Geology and in Toronto last week at a meeting of the Geological Society of America. The researchers found 30 acritarch species, 13 of which had never been seen before.

Butterfield and Rainbird propose that some of the new acritarchs closely resemble modern eukaryotes called dinoflagellates, which account for much of the photosynthesis in the oceans and thus provide food for larger creatures.

Some modern dinoflagellates survive winter by producing tough-walled cysts that drop to the ocean floor in the cold months and then crack open when the cells are ready to resume growing. Geologists have identified fossils of these cysts only as far back as the Silurian period, roughly 420 million years ago.

Butterfield and Rainbird suggest that this record stretches at least twice that long. Some of the Victoria Island acritarchs, they say, have jagged, polygonal edges, a pattern characteristic of dinoflagellate cysts that have broken open. Despite the similarity, Butterfield and Rainbird say they cannot firmly identify the acritarchs because the fossils lack key features seen on many, but not all, dinoflagellates.

The Victoria Island discovery supports chemical studies of rocks that have found molecular indications of dinoflagellates at least as far back as 850 million years ago, says Roger Summons of the Australian Geological Survey Organisation in Canberra. He and other paleontologists have suspected for some time that a fraction of the known acritarch species represent predecessors of modern dinoflagellates.

Regardless of the acritarchs' affinities, however, the number and complexity of species found on Victoria Island indicate that eukaryotes were passing through a critical evolutionary period at that time, says Butterfield.

"All of this means something. There's a huge amount of diversity in the environment at this time," says Butterfield. It suggests that eukaryotes were rapidly evolving in the face of some new ecological pressure, and one candidate is the appearance of the earliest animals, which could have been grazing on the plentiful organisms, he suggests. Traces of animals don't appear in the fossil record until less than 600 million years ago, but scientists are currently debating whether animals split off from other eukaryotes far earlier in time.