EVOLUTION DURING THE MID-CRETACEOUS: THREE EXAMPLES OF RAPID TURNOVER

The iterative patterns of planktonic foraminiferal evolution during the Cretaceous, Paleogene, and Neogene are related in part to the complex interplay of changing physical, chemical, and biological variables of the upper water column. Times of rapid global change are catalysts for evolution. Here we present three examples of accelerated rates evolutionary turnover (speciation plus extinction) during the mid-Cretaceous likely associated with times of changing upper water column stratification, ocean chemistry, and productivity during Oceanic Anoxic Events (OAEs).

Planktonic foraminifers diversified greatly during the late Aptian (~119-113 Ma) as witnessed by increased numbers of species, test size, and morphologic variability, including the first appearance of a peripheral keel. Multispecies stable isotope evidence indicates that species were stratified in the upper water column (mixed layer and thermocline) similar to modern taxa. This radiation was followed by a diversity collapse across the Aptian-Albian boundary (~112 Ma) associated with changes in global carbonate chemistry during OAE 1b. This is the largest turnover (up to 92%) prior to the Cretaceous/Paleogene boundary.

Planktonic foraminiferal simple diversity, test size, calcification, and morphologic variability again increased rapidly during the late Albian (~105-99 Ma). This radiation parallels trends in rising sea level and global warming, and is associated with increased stratification of the water column and upper water column based on multispecies oxygen isotope evidence. The development of regional OAEs (1c, 1d) or OAE-like times of enhanced organic matter production during the late Albian, suggest that productivity variability may have also been a contributing factor for high rates of turnover at this time.

OAE 2 during the latest Cenomanian-earliest Turonian (~94-93 Ma) was one of the most widespread and best developed of the OAEs. The planktonic foraminifers display a significant turnover that mostly affected deep-dwelling taxa, including the extinction of the long-ranging Rotalipora and the radiation of Dicarinella, among others. Multispecies oxygen isotope data suggest that deep waters warmed to at least 18°C and upper water column stratification collapsed during OAE 2.