Abstract

There is a marked, possibly stepwise, extinction of marine taxa across the Cenomanian-Turonian boundary. Across the boundary in southwestern Utah, there is only minor species-level extinction of brackish-water taxa, and an actual increase in diversity of fully terrestrial organisms; significant family-level extinctions are restricted to aquatic taxa such as fishes and turtles.

It is not possible in the nonmarine setting to determine if this is a gradual, stepwise, or instantaneous extinction, or to what degree it correlates to marine extinction events. Nonmarine faunas underwent no major change during the transgressive phase of the Greenhorn cycle, and the loss of aquatic taxa along with displacement (but not extinction) of brackish-water vertebrates and some marsupial mammals is first apparent in rocks deposited during regression in the Turonian. The loss of flood-plain habitat at maximum transgression may have caused the extinction of some of the aquatic taxa. The absence but not extinction of certain taxa on flood plains during the Greenhorn regression suggests that there may be some significant difference in transgressive and regressive flood plains. Drawdown increases the gradients of rivers and results in incision along coastal margins. This restricts the extent of brackish-water environments and may have had an impact on faunal compositions of riverine systems and contributed to extinction within aquatic communities.

This pattern is quite different from that at the Cretaceous-Tertiary (K-T) boundary. Aquatic taxa underwent relatively minor losses at that boundary, whereas terrestrial organisms underwent major extinction. It appears that much of the Late Cretaceous aquatic community was restructured (mostly by exclusion of many taxa rather than extinction) and reduced in diversity during large-scale regression in the middle of the Maastrichtian before the end of the Cretaceous. This aquatic community was living in a rapidly expanding environment (overall regression of marine waters) at the K-T boundary. The extinction of terrestrial taxa at the boundary is unlike the pattern observed at the Cenomanian-Turonian boundary and suggests that some mechanism other than eustatic change played a significant role in the extinction.

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