INFERRING EVOLUTIONARY ORDER AND DURATIONS IN A FOSSIL LINEAGE USING BOTH STRATIGRAPHY AND CLADISTICS

Both stratigraphic and cladistic orders of appearance of specimens of the Ordovician bryozoan genus Peronopora are significantly correlated, indicating the sequential development of both patterns in geological time. Therefore, it is legitimate to predict first appearance positions of taxa from the available stratigraphic benchmarks extended downward to positions necessitated by cladistic order. The stratigraphic first appearances of 5 species of Peronopora match cladistic order, but 11 species required downward range extensions to provide matches. Corrections based on reduced major axis regressions and ghost range extensions represent two alternative modifications of first appearance data with the latter more strongly supported by stratigraphic congruence indices. Tests of the robustness of observed first appearances use the density of sampled horizons and the magnitudes of stratigraphic gaps. Based on this test, 8 of 15 hypothesized ancestor-descendant species pairs have first appearances supporting a pattern of descendants appearing stratigraphically above their ancestors. Seven ancestor-descendant pairs are not supported by stratigraphic appearance data, indicating that stratigraphic sampling densities provide no special support for those phylogenetic hypotheses. A significant sampling hiatus is present below the base of the Brannon Member of the Lexington Limestone. Nine species of Peronopora have first appearances or ranges extended down to this horizon. A test for the presence of a uniform distribution of occurrence probabilities indicates that 7 of the 9 could have originated below the base of the Brannon, reducing the “punctuated equilibrium” aspect of this cluster of first appearances. The early branching rate within Peronopora is 1.05 nodes/meter of strata. The rate of clade production within the same interval is 0.73 clades/meter of strata. Extrapolating downward using both rates indicates that the root of the generic clade lies between levels 973 and 981 in the Conodont Composite Standard Section. The speciation rate in Peronopora is 6.59 species/million years, and the average time for speciation is 161,813 years. The formation rate of intraspecific clades, that may include cryptic species, is 21.84 clades/million years, and the average waiting time per clade is 42,362 years.