ROLE OF SAMPLING BIASES IN THE DIVERSITY DECLINE OF SPIRIFERIDE BRACHIOPODS FOUND IN THE IOWA BASIN AT THE FRASNIAN-FAMENNIAN EXTINCTION

The recent controversy spawned (Waters et al. 2001) over whether the Frasnian-Famennian (F-F) extinction event is truly biological in nature or is artificially inflated by sampling biases, and the lack of fine-scale approaches used to examine biodiversity questions, has inspired this investigation of spiriferide brachiopods in the Iowa Basin. This two part investigation examines whether pseudoextinction is inflating biodiversity decline by (1) conducting a phylogenetic analysis to examine evolutionary relationships of taxa present before and after the apparent F-F event, and (2) determining if taxa present in the basin before the F-F apparent event can be identified as having an environmental preference, with respect to lithology and relative depth. A case showing both a Devonian and a Mississippian taxon to be linked through a phylogenetic analysis, and that Devonian taxon having a preference for an environment that is lost from the record, is supportive of pseudoextinction. Fieldwork yielded 12,558 individual brachiopods from both carbonate and siliciclastic environments, and from relatively shallow and deep water depths. A probabilistic approach was used to ascertain if 12 Devonian taxa had a preference for any environments by determining whether the observed distributions from the field could be significantly replicated randomly. Analysis determined that five of eight victims of the F-F event preferred a carbonate environment, while two of four survivors preferred a siliciclastic environment. This possibly suggests that either the loss of a carbonate environment led to a true extinction or lack of carbonate preservation led to pseudoextinction. No pattern was discerned between victims, survivors, and a preference for relative water depth. A phylogenetic analysis conducted with 14 Devonian and 9 Mississippian taxa produced two equally parsimonious evolutionary trees. Mapping the results from the environmental preferences onto the cladograms shows that the possibility of pseudoextinction seems unlikely. Results support that extinction of spiriferide brachiopods in the Iowa Basin at the F-F event are real, and not the product of pseudoextinction. More work using fine-scale approaches is needed to further evaluate this ongoing controversy.