THE CRETACEOUS MARINE NUTRIENT INCREASE AND THE INITIAL DIVERSIFICATION OF TURRITELLINE GASTROPODS

The concept of “taxonomic uniformitarianism” -- the hypothesis that a fossil organism lived in a similar fashion, and in a similar range of environmental conditions as its closest living relative -- is central to most paleobiological investigation, yet we know that it is not always true, because evolution happens. Exceptions to taxonomic uniformitarianism are important to document when they can be specified as they lead to improved understanding of both paleoenvironmental conditions and evolutionary change. Recent turritelline gastropods are most abundant and diverse within a relatively narrow range of environmental conditions. They are most abundant in relatively shallow, cool, nutrient-rich marine environments and fossil turritellines have frequently been used as paleoenvironmental indicators for these conditions. The paleoenvironmental distribution of Cretaceous and early Tertiary turritelline- dominated assemblages (“TDAs”), however, suggests that turritellines may not always have displayed this pattern of environmental occurrence. A compilation of TDAs for the entire history of the group shows that, although they have occurred in siliciclastics throughout this history, they have occurred in warm-water carbonate environments only in the Cretaceous and Paleocene. Consideration of this pattern in the context of Cretaceous and early Tertiary paleoceanography suggests that 1) turritellines have changed their range of environmental occurrence over time; specifically they have always been most abundant in conditions of high nutrients, but have narrowed their thermal distribution since the early Tertiary; and 2) Cretaceous and perhaps also early Tertiary carbonate environments may have been significantly different from such environments today; specifically they may have been characterized by higher nutrient levels. Combining these conclusions with a database of all described species of turritellines supports the hypothesis that increased nutrient levels in Early Cretaceous seas were an important contributing factor to the initial diversification of the group.