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• First known fossil

* Key transitions

Figure 10.14 Stratigraphie distribution of Late Precambrian and Early Paleozoic metazoan taxa, some key morphological transitions and the carbon isotope record (S13C). PDB, Vienna Pee Dee beleminite, the standard material for relative carbon isotope measurements. (Based on various sources.)

record, together with a revised molecular clock (see p. 133), have suggested an alternative hypothesis. The current Lower to Middle Cambrian fossil record displays the sequential and orderly appearance of successively more complex metazoans (Budd 2003), albeit rather rapidly (Fig. 10.16), and the timing is closely matched by revised molecular time scales (see p. 235; Peterson et al. 2004). Nevertheless there is some suggestion from the biogeo-graphic patterns of trilobites that the divergence of many metazoan lineages may have already begun 30-70 myr earlier (Meert & Lieberman 2004) and speciation rates during the explosion were not in fact so incredible compared with those of other diversifications preserved in the fossil record (Lieberman 2001).

Much of our knowledge of the Cambrian explosion is derived from three spectacular, intensively-studied Lagerstätte assemblages: Burgess (Canada), Chengjiang (China) and Sirius Passet (Greenland). The diversities of the Cambrian "background" faunas are generally much lower and arguably contain less morphologically different organisms. Reconstructions of these seafloors are possible (Fig. 10.17). But whereas the Cambrian explosion provided higher taxa, in some diversity, the Ordovician radiation generated the sheer biomass, biodiversity and biocomplexity that would fill the world's oceans.

Box 10.6 Roughness landscapes

There have been a number of explanations for the rapid explosion of life during the Early and Mid Cambrian involving all sorts of developmental (genetic), ecological and environmental factors. Why, too, was this event restricted to the Cambrian? Was there some kind of developmental limitation, an ecological saturation, or were there simply no further ecological opportunities left to exploit? One interesting model that may help explain the ecological dimension of the event involves the use of fitness landscapes. The concept is taken from genetics but can be adapted to morphological information (Marshall 2006). Biotas can be plotted against two axes, each representing morphological rules that can generate shapes. The Ediacara fauna has only three recognizable bilaterians, so the landscape is relatively smooth with only three peaks. On the other hand the Cambrian explosion generated at least 20 bilaterian body plans and a very rough landscape rather like the Alps or the Rockies (Fig. 10.15). What roughened the landscape, or why were there more bilaterians in the Cambrian fauna? Much of the bilaterian genetic tool kit was already in place in the Late Proterozoic and the environment was clearly conducive to their existence. The "principle of frustration" (Marshall 2006), however, suggests that different needs will often have conflicting solutions, ensuring that the best morphological design is rarely the most optimal one. Is it possible that, with the rapid development of biotic interactions such as predation, many morphological solutions were developed, some less than optimal but nevertheless driving a roughening of the fitness landscape. Thus "frustration", the multiplication of attempted solutions to new opportunities, led to the roughening of the Cambrian landscape and may have been an important factor in the Cambrian explosion.

Figure 10.15 Comparison of Ediacara and Cambrian landscapes: (a) fitness landscapes; (b) locally optimal morphologies (Nicklas' plants); and (c) locally optimal morphologies (bilaterian animals). (Based on Marshall 2006.)

Budd &

Traditional Gould 1989 Fortey et al. 1996 Jensen 2000

Recent-

Relative disparity

Cambrian-

Figure 10.16 Modes of the Cambrian explosion. (Based on Budd & Jensen 2000.)

Ordovician radiation_

During an interval of some 25 myr, during the Mid to Late Ordovician, the biological component of the planet's seafloors was irreversibly changed. A massive hike in biodiversity was matched by an increase in the complexity of marine life (Harper 2006). The event witnessed a three- to four-fold increase in, for example, the number of families, leveling off at about 500; these clades would dominate marine life for the next 250 myr. Nevertheless the majority of "Paleozoic" taxa were derived from Cambrian stocks. With the exception of the bryozoans (see p. 313), no new phyla emerged during the radiation, although more crown groups emerged from the stem groups generated during the Cambrian explosion.

The great Ordovician radiation is one of the two most significant evolutionary events in the history of Paleozoic life. In many ways the Ordovician Period was unique, enjoying unusually high sea levels, extensive, large epicontinental seas, with virtually flat seabeds, and restricted land areas, many probably represented only by archipelagos. Magmatic and tectonic activity was intense with rapid plate movements and widespread volcanic activity. Island arcs and mountain belts provided sources for clastic sediment in competition with the carbonate belts associated with most of the continents. Biogeographic differentiation was extreme, affecting plankton, nekton and benthos, and climatic zonation existed, particularly in the southern hemisphere.

Finally, during the Mid Ordovician, the Earth was bombarded with asteroids that appear in some way also to be linked to the biodiversification (Schmitz et al. 2008). Taken together, these conditions were ideal for all kinds of speciation processes and the evolution of ecological niches. Most significant was the diversification of skeletal organisms, including the brachiopods, bryozoans, cephalopods, con-odonts, corals, crinoids, graptolites, ostra-codes, stromatoporoids and trilobites that we will read about later.

Whereas the Cambrian explosion involved the rapid evolution of skeletalization and a range of new body plans, together with the extinction of the soft-bodied Ediacara biota and the appearance of the Bilateria, the Ordo-vician diversification generated few new higher taxa, for example phyla, but witnessed a staggering increase in biodiversity at the family, genus and species levels. This taxo-nomic radiation, which included members of the so-called "Cambrian", "Paleozoic" and "Modern" evolutionary biotas (see p. 538), set the agenda for much of subsequent marine life on the planet against a background of sustained greenhouse climates. Although many outline analyses have been made, there are relatively few studies of the ecological and environmental aspects of the Ordovician diversification (Bottjer et al. 2001). Moreover the causes of the event, and its relationship to both biological and environmental factors, are far from clear. Evolution of the plankton, however, may have been a primary factor (Box 10.7).

Figure 10.17 The Cambrian (a) and Ordovician (b) seafloors. (Based on McKerrow 1978.)

Figure 10.17 The Cambrian (a) and Ordovician (b) seafloors. (Based on McKerrow 1978.)

Box 10.7 Larvae and the Ordovician radiation

Many factors, mainly ecological and environmental, have been invoked to explain the great Ordovician biodiversification or Ordovician radiation. Did the diversification have its origins in the plankton? Most early bilaterians probably had benthic lecithotrophic larvae (see p. 241). But the Cambrian oceans, relatively free of pelagic predators, offered great possibilities. Exploitation of the water column by larvae occurred a number of times independently, turning the clear waters of the Early Cambrian into a soup of planktonic organisms in the Ordovician. The fossil record and molecular clock data suggest that at least six different feeding larvae developed from non-feeding types between the Late Cambrian and Late Silurian (Peterson 2005). In addition to planktotrophic larvae, the oceans were rapidly colonized by diverse biotas of other microorganisms such as the acritarchs (see p. 216). The dramatic diversification of the suspension-feeding benthos coincides with the evolution of planktotrophy in a number of different lineages (Fig. 10.18). These factors had an undoubted effect on the diversification of Early Paleozoic life, which reached a plateau of diversity during the Ordovician.

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