Ellen Druffel of the University of California, Irvine, and her colleagues
have used radiocarbon dating to assess the age of three specimens of Gerardia
collected in the Bahamas in 1982 by the US Navy’s Alvin submersible. The
animals lived 620 metres below the surface, and had apparently been around for
between 1500 and 2100 years (Geochimica et Cosmochimica Acta, vol 59, p 5031).
“I was astounded,” says Druffel. “I figured that a lifetime of one to two
millennia was just too long for a single animal.”

Whether Gerardia can really be considered a single animal, however, is a
moot point. Like corals, Gerardia are colonies of tiny animals, known as
polyps, living together on a branching skeleton which they deposit in
successive layers. “It more resembles a modern plastic than a natural tissue,”
says Druffel. Eventually the organism resembles a tree, with a trunk and
branches.

To calculate the anemones’ age, Druffel and her colleagues measured the
relative amount of carbon-14, a radioactive isotope, in several sections of
the anemones’ skeletons, including the oldest, innermost layers of their
trunks. Since carbon-14 decays at a known rate, the researchers were able to
determine how much time had elapsed since those layers were deposited.

Fred Grassle, director of the Institute of Marine and Coastal Sciences at
Rutgers University in New Jersey, says that scientists have previously found
evidence of corals living for hundreds of years – but not for millennia. “This
is a surprise. I think it’s very exciting,” he says.

Nevertheless, the anemones’ longevity may not be unique. Most colonial
animals can reproduce asexually, and therefore have no finite lifespan. This
means that they can, in theory, survive as long as their environment stays the
same. “I wouldn’t be surprised if some reef corals lived into the thousands of
years,” says Ted Bayer, a zoologist at the Smithsonian Institution in
Washington DC. “It’s just that no one has looked into it yet.”

Whether or not the deep-sea Gerardia are record-breakers, their longevity
is a bonus for scientists. The carbon in their skeletons came from the food
they ate, presumably plankton or organic debris that rained down from the
surface. So the anemones may serve as a kind of time capsule, preserving a
record of the ocean’s productivity over the past two thousand years.

These extinctions, which occurred in at least eight distinct bursts, are known collectively as the late Devonian crisis. At least 70 per cent of all marine animal species were wiped out in what was a very selective crisis, killing off many tropical bottom-dwelling animals, such as corals and other reef organisms, while leaving many high-latitude species unharmed. Although scientists have proposed several explanations for the crisis, including a meteorite impact, none of them convincingly matches these particular patterns of extinction.

Now Thomas Algeo of the University of Cincinnati and his colleagues think that they have discovered the root of the problem – literally. Algeo notes that land plants made massive evolutionary strides during the late Devonian and argues that these advances would have had profound evolutionary consequences. For example, about 385 million years ago land plants began to develop supporting tissues that transformed them from small, creeping species into the first large trees. By 363 million years ago, plants had evolved seeds, which allowed them for the first time to spread widely over dry, upland areas.

Each of these evolutionary advances would have caused an increase in the total volume of land plants. Today, plant roots have a stabilising influence on most soils. But as root-bearing plants first began to colonise previously barren areas, their initial effect would have been to break up the surface both chemically and physically, making it more susceptible to weathering and erosion. The emergence of trees, in particular, would have had a dramatic effect as their large root systems descended into the ground and started to break up rocks.

By rendering soils more susceptible to weathering and erosion, Algeo argues, the burgeoning populations of Devonian plants caused large amounts of soil particles and dissolved nutrients to be washed into rivers and oceans. The nutrients fertilised the waters, he argues, causing an explosive growth of algae. As these algae died, their decomposition would have used up the oxygen from the oceans’ deeper layers, suffocating many marine animals.

This fits neatly with the fact that rocks called black shales, which seem to contain large quantities of algal material, were laid down during the periods of rapid extinction. “Finally someone has linked up what’s going on on land with what’s going on in the ocean,” says Fred Mackenzie, a geochemist at the University of Hawaii. “That’s an important contribution.”

The new theory also explains why the late Devonian crisis consisted of distinct “pulses” of extinctions: each pulse may correspond to the population explosion of a particular land plant, following a single evolutionary advance. Algeo also points out that his theory explains why the tropics were hardest hit: they were simply the most crowded with plants.

Algeo believes that the success of seedforming plants triggered the last Devonian extinction pulse, about 362 million years ago, since seeds evolved just 1 million years earlier. And he suspects that the most devastating extinction some 367 million years ago was tied to the spread of Archaeopteris, a relative of modern conifers.

Algeo and his colleagues describe their theory in the March issue of GSA Today (vol 5, p 45), published by the Geological Society of America.