Figure 8.7 Cycles in Phanerozoic genus diversity. Source: Reprinted by permission from Macmillan Publishers Ltd: Nature (Rohde and Muller 2005), copyright © 2005.

(2005) discovered these periods using Sepkoski's compendium of the first and last strati-graphic appearances of 36,380 marine genera. The 62-million-year cycle is plain to see in the diversity of the 13,682 'short-lived' genera (those that endured for 45 million years or less) (Figure 8.9). The short-lived genera represent, on average, 44 per cent of the diver sity at any instant in the geological record, but they are responsible for 86 per cent of the amplitude in Figure 8.7(e); by contrast, the long-lived genera show few significant variations and only strongly participate in the end-Permian extinction (Rohde and Muller 2005). The five big mass extinctions may be an aspect of the 62-million-year cycle, which itself may be connected with the 26-32-million-year cycle reported by Fischer, Sepkoski, Prokoph, and others. This is because, although the 62-million-year cycle is the dominant cycle in Sepkoski's diversity data, the existence of secondary features in the middle of some cycles (Silurian, Upper Carboniferous, Lower Jurassic, and Eocene) might have influenced previous reports of an approximate 30-million-year cyclicity (Rohde and Muller 2005).

The cause of the 62-million-year cycle is not clear. It might be the outcome of largely biological processes, though it is difficult to see how biological processes could function over time-spans of such magnitude, or it could be an artefact of the integrity of the fossil record. Then again, it might result from the action of cosmic and geophysical processes, of which Rohde and Muller considered seven possibilities:

1 It is possible that the rate of comet impacts on the Earth varies owing to sporadic perturbations of the Oort Cloud as the Solar System periodically passes through molecular clouds, galactic arms, or some other structure (Matese et al. 2001).

2 Periodic volcanism is a credible candidate for driving a 62-million-year cycle. The problem is to find a geological cause for periodic volcanism. There is substantial evidence for a 30-million-year geological driver. Alfred G. Fischer (1981) thought that the 30-million-year diversity cycle might involve carbon dioxide fluctuations. To be sure, a link with periodic convection in the mantle seems possible, and a model based on fluctuating temperatures at the core-mantle boundary furnished an explanation of the correlation in 30-million-year periodicities of magnetic-field reversals, climate, and

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