Greenhouse−icehouse transition in the Late Ordovician marks a step change in extinction regime in the marine plankton

Abstract

Two distinct regimes of extinction dynamic are present in the major marine zooplankton group, the graptolites, during the Ordovician and Silurian periods (486−418 Ma). In conditions of "background" extinction, which dominated in the Ordovician, taxonomic evolutionary rates were relatively low and the probability of extinction was highest among newly evolved species ("background extinction mode"). A sharp change in extinction regime in the Late Ordovician marked the onset of repeated severe spikes in the extinction rate curve; evolutionary turnover increased greatly in the Silurian, and the extinction mode changed to include extinction that was independent of species age ("high-extinction mode"). This change coincides with a change in global climate, from greenhouse to icehouse conditions. During the most extreme episode of extinction, the Late Ordovician Mass Extinction, old species were selectively removed ("mass extinction mode"). Our analysis indicates that selective regimes in the Paleozoic ocean plankton switched rapidly (generally in <0.5 My) from one mode to another in response to environmental change, even when restoration of the full ecosystem was much slower (several million years). The patterns observed are not a simple consequence of geographic range effects or of taxonomic changes from Ordovician to Silurian. Our results suggest that the dominant primary controls on extinction throughout the lifespan of this clade were abiotic (environmental), probably mediated by the microphytoplankton.T he importance of the marine plankton in both the carbon cycle and in the food web that supports the diversity of marine life is undisputed. However, the evolutionary dynamics of planktic species and the factors controlling their diversity and evolutionary turnover are still poorly known (1, 2). This is particularly so for the Paleozoic, where problems of preservation and sampling bias, and poor time resolution, have precluded detailed analysis. How does the marine plankton respond to environmental perturbations arising from climate change over geological time? How does background extinction differ from episodic and mass extinction in the pelagic realm? Is the risk of plankton species extinction dependent on the amount of time since the species originated (3)? These questions have important implications for macroevolutionary process, stability of marine ecosystems, and modern biodiversity conservation (3-5). Here, using a new global data set of unparalleled temporal resolution, we attempt to answer these questions.The graptoloid clade (order Graptoloidea) constituted the main component of the early Paleozoic macrozooplankton from the beginning of the Ordovician to the Early Devonian (6). Graptoloids were colonial filter-feeding protochordates, generally ranging from a few millimeteres up to ∼200 mm in maximum dimension, which lived suspended in the water column in a range of depth zones. They have been used extensively for correlation and zonation (7-10), and the stratigraphic distributions of species are well docum...

Graptolite community responses to global climate change and the Late Ordovician mass extinction

“…Deposition of black shales, especially in regions of oceanic upwelling, was widespread (22), and global graptolite species diversity reached a Late Ordovician peak at this time (25,40). As sea surface temperatures subsequently declined into the Hirnantian glaciation, extinction rates also rose and graptolite species diversity declined (13,17). At the present study sites, most graptolite communities of the early P. pacificus interval are best fit by complex SAD models (Datasets S4 and S5).…”

“…The loss of graptolite biodiversity in the LOME, accompanied by the wholesale extinction of the previously dominant Diplograptina (taxonomic use follows ref. 11) and their replacement by the previously marginal, high-latitude Neograptina (12)(13)(14)(15)(16), provides an opportunity to study the impact of climate change on a macroplanktonic invertebrate fauna over several million years during an interval of unusual species turnover (17,18). A focus on climate change over geological timescales as a driver of extinction dynamics leads us to ask whether there is evidence of ecological community changes in the interval leading up to mass extinction.…”

“…Further, we ask whether those changes in community abundance structure provide novel insights into the mass extinction that differ from those suggested by the well-documented history of changes in the species composition of Late Ordovician faunas (17).…”

Five palaeobiological laws needed to understand the evolution of the living biota

“…In fact, this is probably an underestimate because: (1) the peak species richness in the Cenozoic may well have been higher than today's richness, based on their Cenozoic diversity trajectory 12 ; (2) it appears that the biome today that has the highest species richness (that is, the tropical moist forests 17 ) has shrunk considerably in the past dozen Myr 18 , which further supports the idea that the diversity of mammals may have been higher in the early to mid Miocene 12 (15)(16)(17)(18)(19)(20)(21)(22)(23) Myr ago) than today.…”

“…For example, among the few groups where mean species durations have been estimated, largely marine invertebrates, late-Palaeozoic crinoids (echinoderms) (1.38-2.72 Myr depending on the clade and time interval analysed 19 ), and extinct graptoloids (hemichordates) (0.65-1 Myr) 20 have similar durations. However, other groups have longer species lifetimes, for example, the extinct blastoids (echinoderms) (5.1 Myr) 21 , and among protists, planktic foraminifera (5-10 Myr, depending on their morphology) 22 .…”