Sea level changes in the early Tertiary affected mammalian

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distribution and speciation by affecting paleogeography, climate, and deposition. Intercontinental links were broadened during regressions. Transgressions had moderating effects on climate, produced by lessening continentality. A middle Paleocene major regression probably marked the withdrawal of the North American interior seaway, and its absence was linked to Paleocene-Eocene transition climatic warming and drying. Shifts in sea level shifted loci of deposition, affecting rates of animal burial and diagenesis.

Sea level effects on shape and disjunction of ranges on the Gulf Coast were of particular importance because southern sources were likely for the wave of new forms, many representing the first appearance of modern mammal orders, which marks the Paleocene-Eocene transition in northern sites. Gulf Coast regressions exposed a broad continental shelf producing terrestrial conditions analogous to those of the broad, stable epicontinental seas produced by major transgressions. An embayment in Texas at the location of the Cretaceous interior seaway could have functioned to produce eastern and western Gulf Coast terrestrial provinces as the Mississippi embayment did in the Pleistocene. Transgressions reduced the area of lowlands, constricted ranges, and promoted speciation by isolating demes n highlands. Regressions could also promote speciation, by lowering water tables, increasing the extent of savannas, and thus fragmenting the habitats of forest dwellers. The effects of sea level changes are important in the burst of mammalian speciation that characterizes the early Tertiary, just as they are in marine evolution.

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