Diversity cycles

The diversity life has fluctuated through the Phanerozoic with the fossil record yielding up tantalizing suggestions of periodicity. Researchers claim to have teased out cycles of varying length using varied information, including diversity, first and last appearances, and extinction and origination rates. Early work on marine data hinted at a 30-million-year cycle (Fischer 1984) that expresses itself in the global diversity of planktonic and nektonic taxa, including globigerinacean foraminifers and ammonites, and in the episodic development of superpredators with body lengths of 10-18 m (Fischer 1981). Long-lasting phases of increasing diversity ('oligotaxic' phases), when oceans were cool, were punctuated roughly every 30 million years by high-diversity crises ('polytaxic' phases), when the oceans were warm. Each polytaxic pulse brought a new group of superpredators - ichthyosaurs, pliosaurian plesiosaurs, mosasaurs, whales, and sharks successively filled the superpredator niche opened up after each biotic crisis. Later work benefited from improved compilations of data. Jack Sepkoski (1989), using his data on marine genera covering the last 270 million years, found peaks where the extinction rate has risen above background levels 12 times, with eight of these peaks matching well-known extinction events, and nine being roughly periodic, following a 26-million-year timetable (Figure 8.6). Marine families follow the same schedule (Sepkoski 1989; see also Rampino and Caldeira 1993).

Reasons offered to explain this seemingly strong 26-million pulse centred on bouts of widespread volcanism or episodes of heavy bombardment. However, many of the periodic extinction events occurred over several stratigraphical stages or substages. Generic extinctions followed two patterns, both of which suggested a gradualistic mechanism behind the periodicity (Sepkoski 1989). First, most of the extinction peaks, especially for filtered data, had almost identical amplitudes, well below the amplitudes of the three major events. Second, the widths of most of the peaks spanned several stages. Taken with the three massive extinction events, this implied two distinct causes of mass extinctions. The first cause is a 26-million-year oscillation of the Earth's oceans, or climates, or both, that leads to higher extinction rates over long periods, either continuously or in high-frequency, stepwise episodes. The second cause involves independent agents or constraints upon extinction, including bombardment episodes, volcanic episodes, and sea-level changes, that boost the periodic oscillation of extinction rate when they happen to occur at times of increasing extinction. In other words, large impacts and massive outpourings of lava may trigger mass

Table 8.2 Examples of palaeontological studies showing evidence of persistence and punctuated change in terrestrial palaeocommunities of vertebrates, invertebrates, and plants.

Location and Study and age Duration Taxa Temporal patterns source (million years)

Duration and resolution

Potwar Plateau, Siwalik mammals, northern Pakistan Middle to Upper

Southern Ethiopia, Omo Shungura northern Turkana Formation (4.0-1.5) Basin (Bobe et cA. 2002)

Southern Illinois Basin, USA (DiMichele and Phillips 1996; DiMichele et at. 2002) Green River and Unita Basins, North America (Wilf et at. 2001)

North central Texas, USA (Olson 1952, 1958)

Coal beds, Pennsylvanian: Desmoinesian-Westphalian D (310-306) and Missourian-Stephanian A (306-303)

Latest Palaeocene to early Middle Eocene (-56-43)

Early Permian (Clear Fork Group) (-275)

5 Myr 115 mammal taxa;

100 Kyr insectivores, tree shrews, primates, Tubilidentata, Prosboscidea, Lagomorpha, Perissodactyla, Artiodactlya, Rodentia, Pholidota 2.5 Myr Mammals: bovids, 3-100 Kyr suids, primates

4 Myr plus Peat-forming plants 3 Myr preserved in coal balls;

lycopsids, ferns, sphenopsids, pteridosperms, cordiates

13 Myr Insect damage: 40 types <1 kyr documented on 2,435 leaf fossils from 58 host species

3-5 Myr Fish, amphibians, reptiles 100s- (21 genera and 32 species)

1,000s yr

Background turnover relatively high (50-60%) but not correlated with changes fluvial palaeoenvironments; three turnover events within 100,000-300,000 year intervals account for 44% of the faunal change; extinctions and appearances are not coincident in time; latter two or three turnover events seem to correlate with climate change Persistence of most taxa with species turnover at 2.8 million years ago and after 2.0 million years ago, new dominant subsequently; large mammal community as a whole stable for 300,000-year interval followed by 100,000-year cyclicity in taxonomic abundances

Recurrent intraswamp community patterns among successive coal beds during the Westphalian; patterns ended by a major extinction at the close of Westphalian; replacement of Desmoinesian communities by new kinds of persistent assemblages in the Missourian

Three sample levels based upon six quarry sites across the Eocene Continental Thermal Maximum interval showing persistence in feeding types but change in intensity and distribution of damage based on host-plant antiherbivore strategies

Faunal assemblages associated with different environments (upland, stream, pond margin, and pond) traceable through time and persisting through periods of environmental change with some turnover but overall continuity in the taxonomic and ecomorphic character of the chronofaunas

Source: Adapted from DiMichele et al. (2004).

Figure 8.6 Extinction rate per genus for 49 sampling intervals from the mid-Permian (Leonardian) to Recent. A periodicity of 26 million years is indicated by the vertical lines. (a) Time series for the entire data set of 17,500 genera. (b) 'Filtered' time series for a subset of 11,000 genera, from which genera confined to single stratigraphical intervals are excluded. Source: Adapted from Sepkoski (1989).

Age (million years)

Figure 8.6 Extinction rate per genus for 49 sampling intervals from the mid-Permian (Leonardian) to Recent. A periodicity of 26 million years is indicated by the vertical lines. (a) Time series for the entire data set of 17,500 genera. (b) 'Filtered' time series for a subset of 11,000 genera, from which genera confined to single stratigraphical intervals are excluded. Source: Adapted from Sepkoski (1989).

extinctions under some circumstances (cf. Yabushita 1994). A recent statistical analysis on high-resolution records of calcareous plankton diversity, global sea-level, ratios of marine isotopes (oxygen, carbon, and strontium), large igneous province eruptions, and dated impact craters over the last 230 million years pointed to astrophysical or geophysical pacemakers of diversity change with periods in the range 25 to 33 million years (Prokoph et al. 2004). The results suggested that since the early Mesozoic debut of plankton, long-term cyclical changes in global environmental conditions and periodic large volcanic and impact events have modulated their diversity (Prokoph et al. 2004).

Although the arguments for periodicity in various aspects of species diversity are generally persuasive if not watertight, the latest analysis teases out a strong 62-million-year cycle with high statistical significance, particularly evident in shorter-lived genera, with a secondary peak at 140 million years (Figures 8.7 and 8.8). Robert A. Rohde and Richard A. Muller

4,000

2,000

1,000

4,000

2,000

1,000

300 200

Age (million years)

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