In an attempt to understand the timing and structure of post-Paleozoic adaptive radiation of bivalves, temporal patterns of origination of extant heteroconch families were analyzed, based on the normalized number of originated families per 6 My (NNFO). The mean value through the Mesozoic and Cenozoic is 1.34. The origination occurred temporally unevenly. Three intervals and two events which are characterized by higher rate of origination of families were recognized; Carnian to Hettangian, Mid-Cretaceous, and Paleocene/Eocene Origination Intervals, and Campanian and Lower Miocene Origination Events. Each origination interval or event appears to be characteristic of a salinity regime. Freshwater families originated only in the Late Triassic and Neogene. In contrast, nearly all the families that originated in the Late Cretaceous were marine. This appears to match the Wilson Cycle corresponding to continental aggregation and separation. The actual rather complex pattern of heteroconch family origination recognized in this study is probably a composite of such effects of the long-term pattern associated with the Wilson Cycle, change in nutrient level, predation pressure, and seawater chemistry.
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