An ancient type of marine community typical of 450 million years ago has resurfaced in Antarctic fossils of near-modern age. A National Science Foundation-sponsored expedition to Seymour Island off the Antarctic Peninsula unearthed an ecological anomaly: fossil communities only 40-million-years-old dominated by brittle stars and sea lilies (marine invertebrates like starfish).
The authors of the paper detailing the findings, published in the October, 1997 issue of the journal Geology, believe that as Antarctica entered its current deep freeze, cooling ocean temperatures suppressed predation and spurred a dramatic increase in nutrients upwelling in the Southern Ocean surrounding the continent. "This discovery is a good example of how global climate change can have severe impacts on marine life," said Richard Aronson, senior marine scientist at Dauphin Island Sea Lab and lead author of the paper.
The community structure reflected in the Seymour Island fossils -- that is, the comparative numbers of different organisms occupying particular ecological niches -- was much more typical of the shallow seas of 150-to-450 million years ago. After that, predation by newly evolved fish and other creatures confined brittle stars and sea lilies to habitats in the depths of the sea.
The paper's authors believe that when Antarctic temperatures began to plummet, however, predation was disrupted -- some predator populations shrank and others went extinct -- and the archaic community structure reappeared. In fact, the brittle stars and sea lilies clustered in dense beds of fossils show few arm injuries, an indication that predation was light.
Bottom dwellers such as brittle stars and sea lilies also require abundant nutrients. "Global cooling accelerated about 40 million years ago in the late Eocene, and this longterm trend was accompanied by increased upwelling in the Southern Ocean, including around the Antarctic Peninsula," the authors said, which made more nutrients available.
The authors also point out that today, living bottom-dwelling communities in Antarctic waters also show archaic characteristics. Perhaps conditions in the Antarctic or in the Southern Ocean generally work in some way to maintain these old-fashioned community structures, they suggest.
-NSF-
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