Stratocladistics and evaluation of evolutionary modes in the fossil record: An example from the ammonite genus semiformiceras

At least two predominating modes of evolution have been proposed for the Early Tithonian oppeliid ammonite genus Semiformiceras, including phyletic transformation of a single lineage (S. darwini - S. semiforme - S. fallauxi) and, most recently, a bifurcating or cladogenetic model of speciation. We discuss methodological obstacles in past studies that have focused on specific modes of evolution, and offer a reanalysis of the morphological data first presented by Cecca and Rouget [ Palaeontology, 49, 1069-1080] using the stratocladistic software StrataPhy. The present analysis utilizes 11 ammonite taxa and 15 characters (14 morphological and one stratigraphic) and assesses all previous phylogenetic hypotheses, including those that recruit OTUs in ancestral or 'nodal' positions, without excluding evolutionary modes. The results cast doubt on the monophyly of S. darwini, S. semiforme and S. fallauxi, but do not follow completely the direct anagenetic progression proposed by stratophenetic hypotheses. We conclude that stratocladistics is a helpful tool for elucidating the extent of anagenesis and cladogenesis in extinct lineages owing to its capacity to reconstruct phylograms in their temporal framework, and to assess the distinctness and monophyly, not just of clades but of the OTUs themselves. Ultimately, this study addresses the novel utility of computer-assisted stratocladistic analysis in assessing evolutionary modes beyond the reach of traditional cladistic-based methodologies.