A STRATOCLADISTIC APPROACH FOR EVALUATING HYPOTHESES OF CLADOGENESIS, ANAGENESIS, AND BUDDING APPLIED TO THE MISSISSIPPIAN CRINOID BARYCRINUS

Deciphering modes of speciation in the fossil record is critical to understanding the dynamics of diversity generation and evolutionary radiations. Herein, I present a stratocladistic approach for evaluating modes of speciation, specifically: cladogenesis, anagenesis, and budding. Stratocladistics permits explicit testing of speciation hypotheses as it facilitates the recognition of ancestor-descendant (A-D) pairs and incorporates time.

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.