TESTING THE TIME AXIS OF PHYLOGENIES

Fossils are not perfect materials for phylogenetic analysis because of problems of missing characters and missing taxa. However, fossils have three major advantages: (1) they give the only direct evidence of the order of acquisition of characters, (2) they frequently present character combinations not found in modern forms, and (3) they may allow the coding of characters that have been overwritten by subsequent evolution within a clade. There are three independent sources of evidence about sequences of historical events in evolution - morphological, molecular and stratigraphic - and these may be mutually cross-tested. Tests of the quality of the fossil record against morphological cladistic data show that (1) age and clade data on branching sequences generally agree, (2) knowledge of the fossil record has improved by 5 % over the past 26 years of research, and (3) the fossil record of continental vertebrates is as good as that of(marine) echinoderms. Hence, systematists and evolutionary biologists may use fossil data with confidence in phylogeny reconstruction and to calibrate the time axis of phylogenies.