Congruence between parsimony and stratigraphy: Comparisons of three indices

Use of quantitative statistical tests can show that there is generally good congruence between estimated cladistic hypotheses of relationship and observed stratigraphy. A data set of 376 cladograms of fishes, continental tetrapods, and echinoderms was tested using three metrics, Spearman Rank Correlation (SRC), Relative Completeness Index (RCI), and Stratigraphic Consistency Index (SCI), to explore the relationships between the indices and differences in results among the three groups of organisms. There is a strong relationship between SCI and SRC, since both tests measure the same aspect of the fossil record. There is no relationship between RCI and either SCI or SRC. There is a highly significant relationship, as expected, between SRC coefficients and the number of taxa in a cladogram, but no such relationship for RCI or SCI (except in fishes). There is no significant relationship between any of the indices and either the number of internal nodes or tree balance. Echinoderms show the best stratigraphic consistency of nodes, while continental tetrapods have the best matching of stratigraphic age and cladistic node order. Fishes have the worst match of age and clade ranks, but they do have the most complete fossil record as measured by the RCI. They are followed by echinoderms, and then continental tetrapods, which have the least complete record. This seems to show that life in an aquatic environment leads, in general, to a more complete fossil record.