NICHE DYNAMICS IN LATE CRETACEOUS PLANKTIC FORAMINIFERA: AN EXAMPLE FROM THE WESTERN INTERIOR SEAWAY

A thin time-slice of sediments spanning the Cenomanian-Turonian boundary was sampled for planktic foraminifera. Eight localities distributed from Arizona to Manitoba were collected, sampled, and quantified by adhering to a standardized counting protocol. The goal was not to assign specific environmental interpretations to the data but to characterize the temporal and spatial distribution of the organisms as the sole proxy for the consummate effect of limiting variables, be they biological or physical factors, and to summarize that distribution in terms of niche breadth.

Using Levins’ metric, niche breadth is calculated for each species. Species with the greatest niche breadth have dispersal and abundance attributes that exceed those of other species. The distribution of the niche breadths for all species approximates a log-normal distribution, typically truncated and with a modal average that includes a group of species of somewhat narrow niche breadth. Characteristically then, the distribution is skewed towards species with broad niches. In addition and notably, the distribution of niche breadths shifts stratigraphically at all localities. The relationship between niche breadth and dominance for each species suggests that the maximum relative abundance of a species generally predicts its niche breadth, although outliers in that relationship may be useful in also suggesting uniquely impacting variables or a unique species’ response.

Finally, a mean community niche breadth is calculated for each sample reflecting niche breadth and species richness. By plotting these values stratigraphically for each locality, trends in community composition are recognizable. Whether the changes are evidence of water mass effects, changes in fertility, or changes in some other grouping of variables, they summarize the temporal and spatial distribution of the planktic foraminiferal biota during this time slice. The results suggest that mean niche breadths are consistently broad during the early late Cenomanian Metiococeras mosbyense Zone, decrease uniformly during the late Cenomanian Sciponoceras gracile Zone and then substantially increase again during the latest Cenomanian Neocardioceras juddi and earliest Turonian Watinoceras devonense Zones, coincident with Oceanic Anoxic Event 2.