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Arabidopsis thaliana (plant - thale cress)

~30

~24,000

Drosophila melanogaster (fruit fly)

~50

13,601

Zebra fish, human

~120

80,000-100,000

Source: Adapted from Carroll (2001).

Source: Adapted from Carroll (2001).

at the end of the Permian Period and the end of the Cretaceous Period - Palaeozoic life, Mesozoic life, and Cenozoic life. Not until over a century later did modern workers take an interest in this history of diversity. Palaeontologists have fleshed out the record of past life to the extent that a trustworthy picture has emerged.

No one questions the fact that, since life first evolved, the Earth has become an increasingly biodiverse planet. Less certain is the nature of the biodiversity increase - is it continuous and monotonic or does it occur in short bursts? Current opinion points to the latter view, with episodic mass extinctions precipitating marked reductions global diversity. However, a caveat is necessary here: the history of biodiversity is based on those groups of organisms that have left a long and rich fossil record; the historical biodiversity of small organisms and organisms made of soft tissues is unknown.

An increase in biodiversity has paralleled the evolutionary burgeoning of life forms, as is seen in rising number of fossil taxa recorded in successive stages during the Phanerozoic (Figure 8.2). The pattern is low diversity during the Cambrian; a higher but not steadily increasing diversity through the Ordovician, Silurian, Devonian, Carboniferous, and Permian; low diversity during the early Mesozoic, notably in the Triassic; and increasing diversity through the Mesozoic culminating in a maximum diversity during the Cenozoic (see Signor 1994). Congruent patterns have been discerned in the record of marine vertebrates (Raup and Sepkoski 1982), non-marine tetrapods (Figure 8.3(a)), and vascular land plants (Figure 8.3(b)), and insects (Labandeira and Sepkoski 1993). Superimposed upon this overall pattern are three episodes of noteworthy diversification so clearly displayed in the marine fossil record: the early Cambrian, the mid Ordovician, and the Cenozoic.

The causes of long-term Phanerozoic diversity changes are illusive (see Benton 1990). One suggestion is that after the Cambro-Ordovician explosion, which might have filled the

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