Competitive replacement has probably played a minor role in the history of tetrapods. In an assessment of the origins of 840 families of amphibians, reptiles, birds, and mammals, fewer than 26%, and probably fewer than 13%, were identified as candidate competitive replacements (CCR's). This is the pool of families that shared their geographic area of origination and their broadscale adaptations with a pre-existing family, and is thus a maximum measure of families that might have competitively displaced another. Most tetrapod families arose by expansion of the ecospace and geographic areas previously occupied by other members of the clade Tetrapoda. The numbers of CCRS through time are roughly in proportion to numbers of originations, but particular peaks in CCRS occurred after mass extinctions. Pure expansions, with no possibility of competitive replacement, were prevalent during the Late Palaeozoic (radiation of early amphibian and reptile groups, and broadening of habitats occupied and diets) and during the mid Cretaceous (radiations of frogs, turtles, lizards, snakes, and birds).