Telling time from fossils: a phylogeny-based approach to chronological ordering of paleobiotas

The potential for using fossils for temporal ordering of sedimentary rocks is as old as historical geology itself. In spite of this, however, most current biostratigraphic and biochronologic techniques do not make use of phylogenetic information, but rely instead on some measure of species' presence or absence or their turnover in the fossil record. A common phylogenetic approach to biochronology has been to use “stage of evolution” arguments, whereas more rigorous, cladogram‐based methods have been proposed but have seen little use. Cladistic biochronologic analysis (CBA) is developed here as a new method for determining biochronologic order between paleobiotas based on the phylogenetic relationships of their constituent taxa. CBA is adapted from Brooks' parsimony analysis, and analyzes syntaxon information from clades that transcend a number of paleobiotas to determine relative temporal order among these paleobiotas. Because CBA is based on phylogenetic information, it is suited to problems where a good fossil record is available, but where stratigraphic or chronologic relationships are poorly constrained, such as the terrestrial vertebrate record. A practical example, based on the Cenozoic fossil record of North America, pits CBA against a test case in which the correct temporal order of biotas is known beforehand. The method successfully recovers correct temporal order between paleobiotas with reasonable levels of support, and is also shown to outperform a previously proposed cladistic biochronologic method. In a second example, CBA is used to achieve the first empirical temporal ordination for several Late Cretaceous localities in the Gobi Desert that produce fossils crucial to the understanding of modern amniote clades, but which have poorly resolved temporal relationships. CBA is sensitive to large amounts of extinction and poor sampling of the fossil record, but problems such as gaps in the fossil record (Lazarus taxa) can be dealt with efficiently through a number of a priori and a posteriori scoring techniques. CBA offers a novel approach for biochronologic analysis that is independent of, but complementary to and readily combinable with other chronologic/stratigraphic methods. © The Willi Hennig Society 2007.