Global Climate Change and the Evolutionary Ecology of Antarctic Mollusks in the Late Eocene
This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a paleoecological and paleoenvironmental study of Seymour Island. Global climate change late in the Eocene epoch had an important influence in Antarctica. This was the beginning of the transition from a cool-temperate climate in Antarctica to the polar climate that exists there today. The cooling trend strongly influenced the structure of shallow-water, Antarctic marine communities, and these effects are still evident in the peculiar ecological relationships among species living in modern Antarctic communities. Cooling late in the Eocene reduced the abundance of fish and crabs, which in turn reduced skeleton-crushing predation on invertebrates. Reduced predation allowed dense populations of ophiuroids (brittlestars) and crinoids (sea lilies) to appear in shallow-water settings at the end of the Eocene. These low-predation communities appear as dense fossil echinoderm assemblages in the upper portion of the late Eocene La Meseta Formation on Seymour Island, off the Antarctic Peninsula. Today, dense ophiuroid and crinoid populations are common in shallow-water habitats in Antarctica but generally have been eliminated by predators from similar habitats at temperate and tropical latitudes; their persistence in Antarctica to this day is an important ecological legacy of climatic cooling in the Eocene. Although the influence of declining predation on Antarctic ophiuroids and crinoids is now well documented, the effects of cooling on the more abundant mollusks have not been investigated. This study will examine the evolutionary ecology of gastropods (snails) and bivalves (clams) in the late Eocene.
A series of hypotheses will be tested in the La Meseta Formation, based on the predicted responses of mollusks to declining temperature and changing levels of predation. The shapes of gastropod shells, the activities of gastropods that prey on other mollusks by drilling holes in their shells, and the effects of predation on the thickness of mollusk shells should have changed significantly through late Eocene time. First, defensive features of gastropod shells, such as spines and ribbing, should decline as temperature and, therefore, the activity of skeleton-crushing predators declined. Second, drilling of bivalve prey by predatory gastropods should increase with time since the drillers should themselves have been subject to lower predation pressure as temperature declined. Drilled shells, therefore, should become more common through time. Third, patterns in the thickness of shells through time will make it possible to separate the direct, physiological effects of declining temperature (shells are more difficult to produce at cooler temperatures, and so should be thinner) from the indirect effects of temperature on evolving biological interactions (increased drilling predation should result in thicker shells).
Seymour Island contains the only fossil outcrops readily accessible in Antarctica from this crucial period in Earth history. The La Meseta Formation on Seymour Island thus provides a unique opportunity to learn how climate change affected Antarctic marine communities. In practical terms, global climate change will probably increase upwelling over the next few decades to centuries in some temperate coastal regions. Recent ecological evidence suggests that the resultant lowering of sea temperatures could lower predation in those areas. Understanding the response of the La Meseta faunas to global cooling in the late Eocene will provide direct insight into the rapidly changing structure of modern benthic communities.
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