NONMARINE EXTINCTION ACROSS THE CENOMANIAN-TURONIAN BOUNDARY, SOUTHWESTERN UTAH, WITH A COMPARISON TO THE CRETACEOUS-TERTIARY EXTINCTION EVENT

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 brackishwater t axa, and an actual increase in diversity of fully terrestrial organism s; significant family-level extinctions are restricted to aquatic taxa such as fishes and turtles. It is not possible in the nonmarine setti ng to determine if this is a gradual, stepwise, or instantaneous extin ction, or to what degree it correlates to marine extinction events, No nmarine faunas underwent no major change during the transgressive phas e of the Greenhorn cycle, and the loss of aquatic taxa along with disp lacement (but not extinction) of brackish-water vertebrates and some m arsupial mammals is first apparent in rocks deposited during regressio n in the Turonian, The loss of flood-plain habitat at maximum transgre ssion 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 diffe rence 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 hav e had an impact on faunal compositions of riverine systems and contrib uted to extinction within aquatic communities. This pattern is quite d ifferent from that at the Cretaceous-Tertiary (K-T) boundary, Aquatic taxa underwent relatively minor losses at that boundary, whereas terre strial organisms underwent major extinction, It appears that much of t he Late Cretaceous aquatic community was restructured (mostly by exclu sion of many taxa rather than extinction) and reduced in diversity dur ing large-scale regression in the middle of the Maastrichtian before t he end of the Cretaceous. This aquatic community was living in a rapid ly expanding environment (overall regression of marine waters) at the K-T boundary, The extinction of terrestrial taxa at the boundary is un like the pattern observed at the Cenomanian-Turonian boundary and sugg ests that some mechanism other than eustatic change played a significa nt role in the extinction.