| Summary: | Increasingly detailed stratigraphic work in intermontane basins from Montana to Texas has revealed significant differences between northern and southern early Tertiary terrestrial faunas of western North America. Paleontological data from southern California, Mexico, New Mexico, and west Texas allows analysis of the effects of topography and climate on southern faunal distribution. Endemism in the Paleocene and Eocene of southern California and the Paleocene of west Texas can be attributed in part to geographic isolation. Observed differences between northern and southern faunas are tied to climatic differences, which are traceable from locality to locality using paleopedology. For example, Paleocene fluvial mudstones in west Texas and New Mexico show prominent red and black banding. Red layers are rich in soil-formed calcite nodules. Red color-banding and nodules are not widespread in northern localities until the early Eocene, indicating a northward spread of warm, variable climate. Migrations of animals, triggered by such climatic changes, may be responsible for abrupt faunal changes m the northern intermontane basins. Early Tertiary fossil vertebrate sites are scarce east of the Big Bend region of Texas now but within a decade lignite strip mining in east Texas, Louisiana, and Arkansas will be producing outcrops. The best hope for finding extensive early Tertiary terrestrial faunas in eastern North America lies in the Gulf Coast. The early Tertiary Gulf Coast may have served as a source for new forms migrating to the west and north. The Paleocene and early Eocene faunas of the southeastern U. S. can be expected to have strong European affinities, and the late Eocene to have close ties to western North America. The Paleocene faunas of west Texas show less resemblance to those of France, which were similar in latitude, than the high degree of similarity of early Eocene animals of Europe and western North America would have suggested. Evidently, neither the slow-to-open North Atlantic nor the long distances ...
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