Figure 1. Techniques for assessing the quality of the fossil record. Comparisons are made between branching order in cladograms and stratigraphic data (a–e), and between the relative amount of gap and the known record (e). The example is a cladogram with nine terminal branches (A–I). For comparisons of clade order and age order, cladistic rank is determined by counting the sequence of primary nodes in a cladogram (a). Nodes are numbered from 1 (basal node) upwards to the ultimate node. In cases of nonpectinate cladograms (a), the cladogram is reduced to pectinate form (b), and groups of taxa that meet the main axis at the same point are combined and treated as a single unit. The stratigraphic sequence of clade appearance is assessed from the earliest known fossil representative of sister groups, and clade rank and stratigraphic rank may then be compared (c). Matching of clade rank and stratigraphic rank may be tested by Spearman rank correlation (SRC). SRC coefficients may range from 1.0 (perfect correlation) through 0 (no correlation) to –1.0 (perfect negative correlation). For assessing the proportion of ghost range, or minimum implied gap (MIG), and known stratigraphic range, the whole cladogram is used (e). MIG (diagonal rule) is the difference between the age of the first representative of a lineage and that of its sister, as oldest known fossils of sister groups are rarely of the same age. The proportion of MIG to known range is assessed using the relative completeness index (RCI), according to the formula RCI=(1–((å(MIG))/(å(SRL)))×100%. RCI values may range from 100% (no ghost range) through 0 (ghost range=known range) to high negative values (ghost range >> known range). Stratigraphic consistency is assessed (d, e) as a comparison of the ratio of nodes that are younger than, or of equal age to, the node immediately below (consistent), compared with those that are apparently older (inconsistent). The stratigraphic consistency index (SCI) is assessed on the full cladogram (d, e). SCI values range from 1.0 (all nodes stratigraphically consistent) to 0 (no nodes stratigraphically consistent). Based on data in Benton and Hitchin (1997).

Figure 2. Comparisons of the patterns of diversification of nonmarine tetrapods (a) and marine animals (b) based on data sets of different vintage. The diversity patterns are broadly similar, showing gradual increases in diversity through time, interrupted by various declines, corresponding to mass extinction events (Late Cambrian, Late Ordovician, Late Devonian, end-Permian, late Triassic, end-Cretaceous and Late Eocene). The main change, in both cases, is that overall diversity has increased. Abbreviations: V, Vendian; Cam, Cambrian; Ord, Ordovician; S, Silurian; D, Devonian; Carb, Carboniferous; P, Permian; Tr, Triassic; Jur, Jurassic; Cret, Cretaceous; Tert, Tertiary. Based on data in Maxwell and Benton (1990) and Sepkoski (1993).

Figure 3. Relative improvement in our knowledge of the fossil record quality from 1967 to 1993. During these 26 years, gaps in the record were filled, and there is a clear shift in the distribution of RCI (relative completeness index) values to the right from 1967 to 1993, indicating improvement in palaeontological knowledge (significant shift at P < 0.05; t-test and nonparametric signs and Wilcoxon-signed ranks tests). Based on data in Benton and Storrs (1994).