Apparatus Architecture as a Key to Conodont Phylogeny—a Natural Experiment in Cladistics

Euconodonts comprise an extinct clade of primitive vertebrates or a sister group that mineralized a pharyngeal feeding apparatus composed of 13-17 individual elements. Component elements have different morphologies, which evolved at different relative rates. The apparatuses for most conodont taxa are divided into an anterior set of structurally similar elements comprised of morphologies designated as S elements, a related set with a different structure designated as M elements, and a posterior set comprised of more robust P elements. The S, M, and P elements are further subdivided on more subtle differences among the individual components within each of the three groups. Rapid evolution of the P₁ elements has resulted in the traditional widely-utilized biostratigraphy based on detailed changes in P₁ element ornamentation. The P₂ elements evolved more slowly, followed in turn by the even more conservative components of the anterior S and M elements.

The differential evolution among the elements in a single apparatus provides a natural experiment in cladistic systematics. Synapomorphies in the P₂ elements can be used to characterize derived genera from a conservative family-level stock recognized by similar S elements, and sequential apomorphic P₁ elements show different species evolving within a genus. What these relationships demonstrate is that conodont clades often evolved iteratively from relatively conservative stock, with individual genus-level clades radiating to fill particular niches. Because P₁ elements commonly evolved through heterochrony, a major complication for conodont biostratigraphy is homeomorphy. Reincorporating the entire apparatus into conodont taxonomy helps resolve these problems. Examples from the Late Paleozoic are presented to illustrate the utility of this systematic approach and to reconstruct otherwise difficult to understand phylogenetic relationships. The next step is to apply these methods to recognize paleobiogeographic variation around Pangea, where traditional form taxonomy, based solely on P₁ element morphology, has failed to produce reliable biostratigraphic results.