My dissertation research, conducted in the Holland Lab
at the University of Georgia, focused on
the patterns and processes of paleocommunity assembly during the middle Carboniferous, a time that saw the onset of one
of the Phanerozoic's major ice ages.
Field collections from Arkansas and Oklahoma of
brachiopod assemblages across the
Mississippian/Pennsylvanian boundary (~ 318 Million years ago) reveal a taxonomic turnover event in which the
overall diversity structure is conserved. Diversity structure is conserved even though 63% of the latest Mississippian
genera go regionally extinct and 50% of the earliest Pennsylvanian genera are either immigrants or newly
originated. Taxonomic richness and evenness (Simpson's D) are indistinguishable between the latest
Mississippian and earliest Pennsylvanian stages. Additive diversity partitioning shows that the within-collection,
between-collections (i.e., within bed), and between-bed diversity components do not change across the
Mississippian/Pennsylvanian boundary for richness or evenness. Rank-abundance distributions of genera are the
same for both stages. However, for the taxa present in both stages, there is no correlation between their
Mississippian abundance and their abundance in the Pennsylvanian. An estimation of spatio-temporal heterogeneity
of taxonomic composition (Morisita-Horn index) within each stage reveals that the earliest Pennsylvanian was
significantly more homogeneous. These results suggest that middle Carboniferous brachiopod assemblages from
tropical shallow-water carbonate platform settings were organized by some factor that was independent of the
specific taxa present. Furthermore, the increased homogeneity in taxonomic composition in the earliest
Pennsylvanian did not affect the overall diversity structure. Strong interspecific interactions among taxa do not
appear to be important in determining the taxonomic compositions and abundances of brachiopod assemblages at the
temporal scale of this study.
- Heim, N.A. 2009. Stability of regional brachiopod diversity structure across the
Mississippian/ Pennsylvanian boundary. Paleobiology 35: 393-412.