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Figure 8.2 Diversity curve for marine faunal families. Source: Adapted from Sepkoski (1993)

Figure 8.3 Phanerozoic diversity changes. (a) Diversity curves for terrestrial tetrapod families. The upper curve shows the total diversity with time. Drops in diversity indicate six apparent mass extinctions, following peaks numbered 1-6. Slightly elevated extinction rates and a reduced origination rate produced the mass extinctions. (b) Diversity curves for vascular plant species. Each group comprises plants sharing a common structural grade, a common reproductive grade, or both. Sources: (a) Adapted from Benton (1985); (b) Adapted from Niklas et al. (1983).

Age (million years)

Figure 8.3 Phanerozoic diversity changes. (a) Diversity curves for terrestrial tetrapod families. The upper curve shows the total diversity with time. Drops in diversity indicate six apparent mass extinctions, following peaks numbered 1-6. Slightly elevated extinction rates and a reduced origination rate produced the mass extinctions. (b) Diversity curves for vascular plant species. Each group comprises plants sharing a common structural grade, a common reproductive grade, or both. Sources: (a) Adapted from Benton (1985); (b) Adapted from Niklas et al. (1983).

available niches, diversity tracked changes in the disposition of the continents. Moderately high diversity was associated with moderately separated continents during the Palaeozoic; diversity dropped as the continents came together to form Pangaea; and diversity rose after the Permian as the continents broke up (but see Raup 1972). A more recent study of Phanerozoic diversity used five major and essentially independent estimates of lower taxa (trace fossil diversity, species per million years, species richness, generic diversity, and familial diversity) in the marine fossil record (Sepkoski et al. 1981). Strong correlations between the independent data sets indicated that there is a single underlying pattern of taxonomic diversity during the Phanerozoic.

Occasional unusual events may have caused spurts of evolution and biodiversity. An example of such an event is the late Palaeozoic oxygen pulse (mid Devonian, Carboniferous, and Permian). This involved a marked rise (possibly to a hyperoxic 35 per cent) and then fall (possibly to 15 per cent) in atmospheric oxygen and associated changes in atmospheric carbon dioxide. Bottlenecks in lignin cycling, and in the cycling of other refractory compounds synthesized by the newly evolved land plants, probably caused it (Robinson 1990, 1991). Its effect was to quicken the terrigenous organic-carbon cycle and to enable terrestrial production to increase with a concomitant rise in atmospheric oxygen levels. The oxygen pulse influenced diffusion-dependent features of organisms (including respiration and lignin biosynthesis), and may have fuelled diversification and ecological radiation, permitting greater exploitation of aquatic habitats and the newly evolving terrestrial biosphere (Graham et al. 1995).

The well-researched Cambrian 'explosion' is a pivotal diversification involving the emergence of large body-size, biomineralized skeletons, and complex ecological roles (e.g. predation). It established 'a wide range of metazoan designs, codified in orthodox terminology as phyla, and a corresponding occupation of marine ecologies' (Conway Morris 1998, 331). The triggers for the Cambrian diversification are debatable and uncertainties over when the diversification started (a very late Proterozoic origin is possible) render their elucidation problematic. Nonetheless, many researchers point to a starring role for external causes such as atmospheric oxygen levels and tectonic reconfiguration. To be sure, a supercontinent broke up into smaller landmasses during the Neoproterozoic, leading to an increase in the area of such habitats as the edges of continental shelves and shallow seas and so furnishing new niches and new opportunities for 'proto-metazoans' (e.g. Kirschvink et al. 1997). In addition, a Neoproterozoic glaciation may have caused oceanic upwelling, so boosting primary production and atmospheric oxygen levels, which may have become sufficiently high to allow animals to build large bodies without oxygen diffusion becoming a constraint and to synthesize biomineralized skeletons. Over the last decade, a possible internal cause of the Cambrian diversification has come to the fore. Hox genes, discovered in 1994, contain the code for controlling the development of basic structures within bilateral body plans, such as eyes, the same Hox gene controlling the development of eyes in fruit flies, mice, and humans (Quiring et al. 1994) (p. 104). Other Hox genes code for most of the structures in all metazoans, no matter how distant their relationships. Therefore, it is possible that the development or mutation of just one Hox gene in an ancestral metazoan could have led to a substantial morphological change in the animal. This mechanism might have enabled the rapid evolution of the wide range of body plans observed in the Cambrian fauna.

The subsequent mid-Ordovician radiations produced a greater diversity of organisms, including cnidarians, brachiopods, cephalopods, echinoderms, and ectoprocts (bryozoans), and that within a few million years, at least for important shelly groups (e.g. trilobites, brachiopods, and some molluscs). The causes of this spectacular diversification remain hard to pin down. The final surge in marine diversity started in the Jurassic and, apart from the blip associated with end-Cretaceous extinctions, it has continued rising to the Recent. Until very recently, the Earth housed the richest biotas it had ever seen (Conway Morris 1998, 331).

Communities and ecosystems, as well as individuals, have become more complex, insofar as they have come to contain more, and a greater variety of, species. In addition, they have become more diverse, mainly because the abiotic and biotic environments have become increasingly patchy - geodiversity and biodiversity have both increased. The actions of organisms, especially by those that, after their death, form the material of such sedimentary rocks as limestone, have increased geodiversity.

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