Patterns of Foraminiferal Micro-evolution and Enviromental Change in the Lower Chalk

Johnston, Russell (2005). Patterns of Foraminiferal Micro-evolution and Enviromental Change in the Lower Chalk. PhD thesis The Open University.



The research tests Sheldon's Plus ҫa change model by tracking a single fossil lineage through a succession of marine environments showing geological-scale differences in background stability. Orbitally driven cyclic sediments of Cenomanian age, predominantly recording a 20 kyr precessional shift, provide both the time-frame and the main engine of environmental variability, although transgressive pulses and other events are also superimposed. The cyclicity provides spectacular geographical and temporal control,, allowing a million-year sequence to be sampled at 100 kyr intervals at three laterally adjacent sites, and partially sampled at 20 kyr and 2 kyr intervals at one of these sites. The lineage of interest is a benthic agglutinated foraminiferan, Tritaxia pyramidata, which occurs in prolific numbers; additional ecological evidence comes from a large microfossil database recovered from the same samples.

The Lower Chalk benthic microfauna have very stable patterns of relative abundance, with the same species occurring in similar proportions for at least a million years. Analysis of Tritaxia's ecology identifies it as an r-selected generalist playing a keystone role in the community. During development, Tritaxia exhibits a persistent tendency to uncoil, but this tendency is strongly manifest only after average life expectancy, leading to its interpretation as a construction mistake rather than a product of design. This developmental quirk sheds light on the grain and texture of the morphospace through which Tritaxia is forced to navigate, significantly limiting its evolutionary potential. The result is a lineage that achieves a million years' worth of wobbly stasis, largely because it is boxed into a small corner of morphospace by the joint influence of a narrow developmental channel and competitive interference from other species.

The dynamics, of this process predominantly support the Plus ҫa change model, even though not all the predicted patterns are found.

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