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Ecological Polarities of Mid-Cenozoic Fossil Plants and Animals from Central Oregon

Gregory Retallack
Paleobiology
Vol. 30, No. 4 (Autumn, 2004), pp. 561-588
Published by: Paleontological Society
Stable URL: http://www.jstor.org/stable/4096911
Page Count: 28
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Ecological Polarities of Mid-Cenozoic Fossil Plants and Animals from Central Oregon
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Abstract

Ecological polarities are theoretical roles of organisms, reflected in evolved behaviors and characters. Ecological polarity includes what has been called life history strategies, functional types, habitat templates, and r and K selection. Three common ecological polarities emphasize reproduction, agonistic behavior, and withstanding harsh conditions. Such organisms can be called breeders, competitors, and tolerators, respectively. Polarities of ecospace can be envisaged graphically as apices of a triangular diagram within which each species occupies a particular region. Quantitative studies of ecological polarities rely on proxy measurements of specific morphological features, such as the proportional functional area of canines (for competitors), molars (for tolerators), and incisors (for breeders) among mammals. Such proxy measures of morphospace or chemospace are traditionally judged successful by the degree to which they reveal adaptive differences between species. This approach to approximating ecological polarity is here applied to modern soils, plants, snails, and mammals, as well as to comparable fossils of the Clarno and John Day Formations (Eocene and Oligocene) of central Oregon. An advantage of this approach is that adaptive similarities can be tested quantitatively, as shown here for Oligocene oaks and maples, rather than assuming that extinct species were comparable to related living plants. Paleosols that supported fossil creatures provide useful supporting evidence of past selection pressures for ecologically significant adaptations. Degree of hardship can also be quantified from paleosol features. For example, fossil snails had narrower apertures in paleosols of drier climates as revealed by their shallower calcic horizons, and leaves of extinct relatives of Meliosma and Oreomunnea were sclerophyllous in paleosols showing evidence of waterlogging, nutrient-deficiency, and metal toxicity. Evolutionary trends of ecological specialization revealed by this approach include molarization (interpreted as evolution toward the tolerator pole) in ungulates. Adaptive breakthroughs that initiated evolutionary radiations also can be reassessed by using these approximations of ecospace, for example, the convergent evolution by bears of degree of caninization previously evolved in an extinct creodont (Hemipsalodon). Ecological polarities provide new concepts and metrics for ordering morphological, chemical, and ecological characters of fossil and modern organisms, and for reassessing evolutionary trends.

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