The writer studied 13 fossil assemblages from the Pleistocene Millerton Formation of Tomales Bay, California, to evaluate criteria involved in paleoecological analysis. He lists a fauna of 46 extant and 1 extinct species, including 18 not previously reported from the Millerton Formation. A modern fauna of the same species composition would be expected farther south.

The writer used the modern environmental distributions of the species to infer the sources of the 13 fossil assemblages. Most of the Millerton fossil assemblages contain specimens exotic with respect to the majority of species, to the site of deposition, or both. Four assemblages are composed entirely of organisms that appear to have been transported to the site of deposition. Nine of the fossil assemblages include remains of organisms from two or more environments. Such hybrid accumulations were not derived solely from habitats adjacent to the site of deposition.

The writer concludes that assemblages that were largely transported to the site of deposition can be recognized by their physical aspects without recourse to the bulk of the ecological data. The sources of foreign elements cannot be identified, however, on paleontological evidence alone. Ecological evaluation of a fossil assemblage is necessary, therefore, to reconstruct the mode of formation. The study of recurrent combinations of species in assemblages that underwent minimal disturbance should provide the basis for such an evaluation in ancient sediments.

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