Stratigraphic tests of cladistic hypotheses

Abstract

Cladograms predict the order in which fossil taxa appeared and, thus, make predictions about general patterns in the stratigraphic record. Inconsistencies between cladistic predictions and the observed stratigraphic record reflect either inadequate sampling of a clade's species, incomplete estimates of stratigraphic ranges, or homoplasy producing an incorrect phylogenetic hypothesis. A method presented in this paper attempts to separate the effects of homoplasy from the effects of inadequate sampling. Sampling densities of individual species are used to calculate confidence intervals on their stratigraphic ranges. The method uses these confidence intervals to test the order of branching predicted by a cladogram. The Lophospiridae (“Archaeogastropoda”) of the Ordovician provide a useful test group because the clade has a good fossil record and it produced species over a long time. Confidence intervals reject several cladistic hypotheses that postulate improbable “ghost lineages.” Other hypotheses are acceptable only with explicit ancestor-descendant relationships. The accepted cladogram is the shortest one that stratigraphic data cannot reject. The results caution against evaluating phylogenetic hypotheses of fossil taxa without considering both stratigraphic data and the possible presence of ancestral species, as both factors can affect interpretations of a clade's evolutionary dynamics and its patterns of morphologic evolution.