Domning, D. P. (Museum of Paleontology, University of California, Berkeley, California 94720). 1976. An ecological model for late Tertiary sirenian evolution in the North Pacific. Syst. Zool. 25:352–362.—The known sirenians of the North Pacific comprise one halitheriine species, which became extinct about 12 million years ago, and an unbranching lineage (the Hydrodamalinae) of two genera and possibly six species, ending in Steller's sea cow (Hydrodamalis gigas). On the basis of the temporal and spatial distribution of these forms, their functional morphology, and several basic generalizations about the biology of sirenians and their food, the causal factors in North Pacific sirenian evolution may be reconstructed. Late Tertiary climatic cooling and marine regression on the west coast of North America restricted and changed the character of food resources and habitat, intensifying competition between sirenians and desmostylians and leading the hydrodama-lines toward evolution of novel morphological adaptations in a new adaptive zone. Explanation of the latter event, to which the term “quantum evolution” may be applicable, does not seem to require recourse to the concept of “punctuated equilibria.”

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