Since David Raup's seminal 1976 work, paleontologists have been aware of the relationship between outcrop area and diversity. By incorporating lithologic data derived from Alexander Ronov and coworkers into our own data on areas of mapped outcrops from worldwide geologic maps, we are able to establish a quantitative connection between area of sedimentary rock exposure and diversity from the same general depositional environments. Significant power-law relations are observed at both global and continental scales of consideration. The addition of data on areas of habitable area estimated from paleogeographic maps does not substantially affect these correlations between outcrop area and diversity, indicating that the relation between outcrop area and diversity is primarily a function of sampling and not a common cause such as sea level. We observe a significant diversity-area effect, first noted by Jack Sepkoski in the marine realm. Unlike Sepkoski's, however, our diversity-area effect appears to play a substantial role in influencing diversity through time; a true global diversity signal appears to be contained in the rock record despite the impacts of variable sampling.Greater outcrop area can serve to increase estimated diversity by increasing both the sample size and the range of habitats and biogeographic provinces sampled. After standardizing for pure sampling intensity by rarefying the number of taxon occurrences, outcrop area continues to explain a substantial portion of global marine diversity. This indicates that coverage, or sampling from multiple habitats and biogeographic provinces, is even more important than sampling intensity. If we remove the effect of outcrop area from our estimate of global biodiversity, we do not observe a net increase in diversity toward the present, lending support to other studies that have not supported a substantial, long-term global increase in biodiversity during the Phanerozoic.

The concept of coordinated stasis, manifest as a pattern of long intervals of concurrent taxonomic and ecologic persistence separated by comparatively abrupt periods of biotic change, has been challenged in recent studies that claim a lack of prolonged persistence of taxa and associations. A key problem has been the difficulty of distinguishing faunal change owing to localized, short-term environmental fluctuation or patchiness from that indicating regionally pervasive, long-term evolutionary or ecological change. Here, we use an extensive database from the Middle Devonian Hamilton Group of the Appalachian Basin to test for taxonomic and ecologic persistence within this ecological-evolutionary subunit, a succession of purported relative stability. Replicate samples collected from many localities and stratigraphic horizons over a wide geographic area allow us to address the effects of small-scale environmental variation and localized faunal patchiness while exploring basin-scale variation in faunal composition within and between the formations of the Hamilton Group.Observed stratigraphic distributions of fossils are consistent with a scenario in which all taxa are present from bottom to top of the Hamilton Group, and absences result only from sampling failure. Although small-scale variation in faunal composition indeed does occur, there is no more variation among formations than occurs within them. Assemblages from different formations, whether they are defined by taxonomic or ecologic composition, are statistically indistinguishable according to several independent metrics, including ANOSIM and a maximum likelihood estimation that evaluates stratigraphic turnover using Bayesian “Information Criterion.” Simulated data sets indicate that test results are most consistent with species-level extinction of 2.6% per Myr within the Hamilton Group, far lower than the Givetian rate of 11.5% per Myr generic extinction derived from a global database. Such faunal persistence over the ~5.5 Myr encompassed by this unit is consistent with the pattern of coordinated stasis. Earlier studies showing greater amounts of temporal turnover in Hamilton Group faunas are likely influenced by their smaller geographic scale of analysis, suggesting that regional studies done elsewhere may yield similar results.

Studies of biodiversity through time have primarily focused on the marine realm which is generally considered to have a more complete record than terrestrial environments. Recently, this assumption has been challenged, and it has been argued that the record of life on land is comparable or even more robust than that of the shallow oceans. Moreover, it has been claimed that terrestrial successions preserve an exponential rise in diversity, even when corrected for sampling biases such as changes in continental rock volume through time. We evaluate relations between terrestrial diversity and exposed areas of terrestrial sediments using our compiled data on areas of global continental outcrops and generic diversity from the Paleobiology Database. Terrestrial global generic diversity and terrestrial outcrop area are highly correlated following a linear relation. No significant correlation is observed between habitat area and either outcrop area or biodiversity, suggesting that the observed relation between diversity and outcrop area is not driven by a common cause, such as eustasy. We do find evidence for a small residual increase in diversity through time after removing the effect of outcrop area, but caution that this may be driven by an increased proportion of terrestrial fauna with high preservation potential.

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