A STRATOCLADISTIC APPROACH FOR EVALUATING HYPOTHESES OF CLADOGENESIS, ANAGENESIS, AND BUDDING APPLIED TO THE MISSISSIPPIAN CRINOID BARYCRINUS
Gahn and Kammer (2002), in a strict cladistic analysis of Barycrinus, hypothesized that a single, abundant, spatiotemporally widespread, and morphological variable species, Barycrinus rhombiferus (Owen and Shumard, 1852), was directly ancestral to many Barycrinus species. Stratocladistics was used to test this hypothesis by: 1) reanalyzing their data matrix using time constrained lineage segments, rather than species, as operational taxonomic units, 2) minimizing total parsimony (morphologic and stratigraphic) debt on the phylogenetic trees by making time consecutive lineage segments ancestral within their respective species, 3) rearrangement or swapping of lineage segments and lineage segment clusters to further reduce total parsimony debt, 4) the identification of direct A-D pairs, and 5) evaluation of branching patterns in the most parsimonious phylogenetic hypothesis against models of cladogenesis, anagenesis, and budding.
The results of this analysis do not refute the hypothesis of B. rhombiferus being a direct ancestor of many Barycrinus species. In addition, branching patterns in the most parsimonious phylogenetic hypothesis are consistent with models of budding (71%) and anagenetic (28%) speciation, with only a single speciation event being consistent with strict cladogenesis. Another contribution of this study is recognition of the importance of taxa with highly inconsistent placement within a cladogram (such as B. rhombiferus). Although some workers may choose to eliminate such problematic taxa from additional research cycles, this work demonstrates that they may play a critical role in understanding speciation dynamics.