Sirenians are fairly common as fossils in many late Tertiary nearshore marine deposits of the West Coast. The general pattern of their evolution in the North Pacific from early Miocene to recent times is now known. With the exception of the early Miocene forms, all the known species are stratigraphically successive and seem to belong to a single, unbranching evolutionary sequence.

Their evolution was particularly rapid during the late Miocene and early Pliocene in response to changing climate, and the resulting morphologic changes were so profound that different evolutionary stages can be recognized, and rough stratigraphic correlations made, on the basis of quite fragmentary skeletal material. In at least two areas of California (Santa Cruz and Orange Counties), sea cows have been collected from several different zones near the Miocene-Pliocene boundary. These sequences of fossils well illustrate these rapid changes, and in the former case considerable evolution can be observed even within a single species. More detailed study of these and other sections may permit the use of sirenians in correlating widely separated marine deposits on the West Coast.

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