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, 276 (1673), 3629-34

Environmental Change Drove Macroevolution in Cupuladriid Bryozoans

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Environmental Change Drove Macroevolution in Cupuladriid Bryozoans

Aaron O'Dea et al. Proc Biol Sci.

Abstract

Most macroevolutionary events are correlated with changes in the environment, but more rigorous evidence of cause and effect has been elusive. We compiled a 10 Myr record of origination and extinction, changes in mode of reproduction, morphologies and abundances of cupuladriid bryozoan species, spanning the time when primary productivity collapsed in the southwestern Caribbean as the Isthmus of Panama closed. The dominant mode of reproduction shifted dramatically from clonal to aclonal, due in part to a pulse of origination followed by extinction that was strongly selective in favour of aclonal species. Modern-day studies predict reduced clonality in increasingly oligotrophic conditions, thereby providing a mechanistic explanation supporting the hypothesis that the collapse in primary productivity was the cause of turnover. However, whereas originations were synchronous with changing environments, extinctions lagged 1-2 Myr. Extinct species failed to become more robust and reduce their rate of cloning when the new environmental conditions arose, and subsequently saw progressive reductions in abundance towards their delayed demise. Environmental change is therefore established as the root cause of macroevolutionary turnover despite the lag between origination and extinction.

Figures

Figure 1.
Figure 1.
Stratigraphic ranges of cupuladriid species in the southwestern Caribbean ordered by their last then first occurrences with species richness, origination and extinction rates per million years. Open squares indicate actual occurrences in different faunules.
Figure 2.
Figure 2.
(a) Life histories and (b) relative abundances of 24 common cupuladriid bryozoan species responded dramatically to environmental change (horizontal yellow bar) in the SWC. As planktonic primary productivity collapsed, species that survived significantly reduced their levels of clonality and maintained steady abundances (blue), while newly originated species were all predominantly aclonal and showed progressive increases in abundance (green). In contrast, species that failed to reduce levels of clonality showed synchronous reductions in abundance leading up to their final extinction 1–2 Myr after the environment changed (red). Thickness of the vertical range bar for each species represents the percentage of (a) clonal colonies and (b) percentage abundance derived from LOWESS from individual faunules (represented by circles). Data for all species combined (grey) were calculated as the percentage of all clonal colonies irrespective of species. Two closed circles are estimated data. Note that 0 per cent clonality is shifted slightly to the right to give entirely aclonal values some thickness, and percentage abundance is log-scaled.
Figure 3.
Figure 3.
Mechanistic correlates of rates of clonal reproduction. (a) Rates of cloning for entire faunules (all species combined) are positively correlated with mean annual range of temperature (MART). (b,c) Rates of cloning decrease with increased calcification for (b) extant species but not for (c) extinct species. In (b) and (c), dots represent individual species in faunules and heavy black lines the regression of all species in all faunules. Coloured lines represent regressions for individual species.

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