The SRC has been criticized regarding its appropriateness (Huelsenbeck 1994), its
contingency upon temporal spacing (Benton and Storrs 1994; Hitchin and Benton 1996),
and the procedural requirement of altering the cladogram before analysis (trees must be
pectinate for analysis and so more balanced trees must be pruned a priori precluding the full
analysis; Huelsenbeck 1994). Neither are the SCI and RCI metrics free of potential
artefact. The SCI is handicapped by tree balance such that only fully imbalanced trees can
achieve the full theoretical range of values 0.00–1.00; perfectly balanced trees also have
theoretical maximum SCI score of 1.00, but the minimum value achievable is 0.50
(Siddall 1996, 1997; Wills 1999). The SCI is also affected by the temporal distribution of
first occurrences such that if they are all contemporaneous the SCI will equal 1.00,
regardless of tree balance; if no first appearances are contemporaneous, the range of SCI
Figure 10.3 Stratigraphic range data for the major extinct and extant groups of vertebrates.
Observed stratigraphic range represented by thick black bars; large gaps within observed range
represented by dashed bars; 95 per cent confidence limit on first appearance represented by medium
thickness dark grey bars; 99 per cent confidence limit on first appearance represented by thin light
grey bars. These data are also presented in Table 10.1.
PHILIP C.J.DONOGHUE ET AL. 203