Fossil Record: Quality

Abstract

There are divergent opinions about the quality of the fossil record and its ability to give a useful representation of the history of life. Extensive testing, by comparisons of the congruence of morphological and molecular phylogenies, suggests that the fossils do tell the story relatively well.

Keywords: fossils; quality; completeness

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−((∑(MIG))/(∑(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 .

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 and Sepkoski .

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 .

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References

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Further Reading

Benton MJ (1994) Palaeontological data, and identifying mass extinctions. Trends in Ecology and Evolution 9: 181–185.

Benton MJ (2003) The quality of the fossil record. In: Donoghue PCJ and Smith MP (eds) Telling the Evolutionary Time: Molecular Clocks and the Fossil Record, pp. 66–90. New York: CRC Press.

Donovan SK and Paul CRC (eds) (1998) The Adequacy of the Fossil Record. New York: Wiley.

Norell MA (1992) Taxic origin and temporal diversity: the effect of phylogeny. In: Novacek MJ and Wheeler QD (eds) Extinction and Phylogeny, pp. 89–118. New York: Columbia University Press.

Patterson C (1981) Significance of fossils in determining evolutionary relationships. Annual Review of Ecology and Systematics 12: 195–223.

Raup DM (1972) Taxonomic diversity during the Phanerozoic. Science 215: 1065–1071.

Smith AB (1994) Systematics and the Fossil Record. Oxford: Blackwell Scientific.

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Benton, Michael J(Sep 2005) Fossil Record: Quality. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0004144]