Publications by authors named "Seth Finnegan"

21 Publications

  • Page 1 of 1

A high-resolution record of early Paleozoic climate.

Proc Natl Acad Sci U S A 2021 Feb;118(6)

Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139.

The spatial coverage and temporal resolution of the Early Paleozoic paleoclimate record are limited, primarily due to the paucity of well-preserved skeletal material commonly used for oxygen-isotope paleothermometry. Bulk-rock [Formula: see text] datasets can provide broader coverage and higher resolution, but are prone to burial alteration. We assess the diagenetic character of two thick Cambro-Ordovician carbonate platforms with minimal to moderate burial by pairing clumped and bulk isotope analyses of micritic carbonates. Despite resetting of the clumped-isotope thermometer at both sites, our samples indicate relatively little change to their bulk [Formula: see text] due to low fluid exchange. Consequently, both sequences preserve temporal trends in [Formula: see text] Motivated by this result, we compile a global suite of bulk rock [Formula: see text] data, stacking overlapping regional records to minimize diagenetic influences on overall trends. We find good agreement of bulk rock [Formula: see text] with brachiopod and conodont [Formula: see text] trends through time. Given evidence that the [Formula: see text] value of seawater has not evolved substantially through the Phanerozoic, we interpret this record as primarily reflecting changes in tropical, nearshore seawater temperatures and only moderately modified by diagenesis. Focusing on the samples with the most enriched, and thus likely least-altered, [Formula: see text] values, we reconstruct Late Cambrian warming, Early Ordovician extreme warmth, and cooling around the Early-Middle Ordovician boundary. Our record is consistent with models linking the Great Ordovician Biodiversification Event to cooling of previously very warm tropical oceans. In addition, our high-temporal-resolution record suggests previously unresolved transient warming and climate instability potentially associated with Late Ordovician tectonic events.
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http://dx.doi.org/10.1073/pnas.2013083118DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8017688PMC
February 2021

How predictable is extinction? Forecasting species survival at million-year timescales.

Philos Trans R Soc Lond B Biol Sci 2019 12 4;374(1788):20190392. Epub 2019 Nov 4.

Department of Integrative Biology, University of California Berkeley, Berkeley, CA, USA.

A tenet of conservation palaeobiology is that knowledge of past extinction patterns can help us to better predict future extinctions. Although the future is unobservable, we can test the strength of this proposition by asking how well models conditioned on past observations would have predicted subsequent extinction events at different points in the geological past. To answer this question, we analyse the well-sampled fossil record of Cenozoic planktonic microfossil taxa (Foramanifera, Radiolaria, diatoms and calcareous nanoplankton). We examine how extinction probability varies over time as a function of species age, time of observation, current geographical range, change in geographical range, climate state and change in climate state. Our models have a 70-80% probability of correctly forecasting the rank order of extinction risk for a random out-of-sample species pair, implying that determinants of extinction risk have varied only modestly through time. We find that models which include either historical covariates or account for variation in covariate effects over time yield equivalent forecasts, but a model including both is overfit and yields biased forecasts. An important caveat is that human impacts may substantially disrupt range-risk dynamics so that the future will be less predictable than it has been in the past. This article is part of a discussion meeting issue 'The past is a foreign country: how much can the fossil record actually inform conservation?'
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http://dx.doi.org/10.1098/rstb.2019.0392DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6863491PMC
December 2019

Isotopes from fossil coronulid barnacle shells record evidence of migration in multiple Pleistocene whale populations.

Proc Natl Acad Sci U S A 2019 04 25;116(15):7377-7381. Epub 2019 Mar 25.

Department of Integrative Biology, University of California, Berkeley, CA 94720.

Migration is an integral feature of modern mysticete whale ecology, and the demands of migration may have played a key role in shaping mysticete evolutionary history. Constraining when migration became established and assessing how it has changed through time may yield valuable insight into the evolution of mysticete whales and the oceans in which they lived. However, there are currently few data which directly assess prehistoric mysticete migrations. Here we show that calcite δO profiles of two species of modern whale barnacles (coronulids) accurately reflect the known migration routes of their host whales. We then analyze well-preserved fossil coronulids from three different locations along the eastern Pacific coast, finding that δO profiles from these fossils exhibit trends and ranges similar to modern specimens. Our results demonstrate that migration is an ancient behavior within the humpback and gray whale lineages and that multiple Pleistocene populations were undertaking migrations of an extent similar to those of the present day.
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http://dx.doi.org/10.1073/pnas.1808759116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6462050PMC
April 2019

Twelve thousand recent patellogastropods from a northeastern Pacific latitudinal gradient.

Sci Data 2018 01 9;5:170197. Epub 2018 Jan 9.

University of California, Department of Integrative Biology and Museum of Paleontology, Berkeley, CA 94720, USA.

Body size distributions can vary widely among communities, with important implications for ecological dynamics, energetics, and evolutionary history. Here we present a dataset of body size and shape for 12,035 extant Patellogastropoda (true limpet) specimens from the collections of the University of California Museum of Paleontology, compiled using a novel high-throughput morphometric imaging method. These specimens were collected over the past 150 years at 355 localities along a latitudinal gradient ranging from Alaska to Baja California, Mexico and are presented here with individual images, 2D outline coordinates, and 2D measurements of body size and shape. This dataset provides a resource for assemblage-scale macroecological questions and documents the size and diversity of recent patellogastropods in the northeastern Pacific.
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http://dx.doi.org/10.1038/sdata.2017.197DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5759373PMC
January 2018

Identifying the most surprising victims of mass extinction events: an example using Late Ordovician brachiopods.

Biol Lett 2017 Sep;13(9)

Palaeoecosystems Group, Department of Earth Sciences, Durham University, Durham, UK.

Mass extinction events are recognized by increases in extinction rate and magnitude and, often, by changes in the selectivity of extinction. When considering the selective fingerprint of a particular event, not all taxon extinctions are equally informative: some would be expected even under a 'background' selectivity regime, whereas others would not and thus require special explanation. When evaluating possible drivers for the extinction event, the latter group is of particular interest. Here, we introduce a simple method for identifying these most surprising victims of extinction events by training models on background extinction intervals and using these models to make per-taxon assessments of 'expected' risk during the extinction interval. As an example, we examine brachiopod genus extinctions during the Late Ordovician Mass Extinction and show that extinction of genera in the deep-water ' fauna' was particularly unexpected given preceding Late Ordovician extinction patterns.
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http://dx.doi.org/10.1098/rsbl.2017.0400DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5627174PMC
September 2017

Hierarchical complexity and the size limits of life.

Proc Biol Sci 2017 Jun;284(1857)

Department of Mathematics and Statistics, Swarthmore College, Swarthmore, PA 19081, USA.

Over the past 3.8 billion years, the maximum size of life has increased by approximately 18 orders of magnitude. Much of this increase is associated with two major evolutionary innovations: the evolution of eukaryotes from prokaryotic cells approximately 1.9 billion years ago (Ga), and multicellular life diversifying from unicellular ancestors approximately 0.6 Ga. However, the quantitative relationship between organismal size and structural complexity remains poorly documented. We assessed this relationship using a comprehensive dataset that includes organismal size and level of biological complexity for 11 172 extant genera. We find that the distributions of sizes within complexity levels are unimodal, whereas the aggregate distribution is multimodal. Moreover, both the mean size and the range of size occupied increases with each additional level of complexity. Increases in size range are non-symmetric: the maximum organismal size increases more than the minimum. The majority of the observed increase in organismal size over the history of life on the Earth is accounted for by two discrete jumps in complexity rather than evolutionary trends within levels of complexity. Our results provide quantitative support for an evolutionary expansion away from a minimal size constraint and suggest a fundamental rescaling of the constraints on minimal and maximal size as biological complexity increases.
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http://dx.doi.org/10.1098/rspb.2017.1039DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5489738PMC
June 2017

Increase in predator-prey size ratios throughout the Phanerozoic history of marine ecosystems.

Science 2017 06;356(6343):1178-1180

Department of Integrative Biology and Museum of Paleontology, University of California, Berkeley, 1005 Valley Life Sciences Building 3140, Berkeley, CA 94720, USA.

The escalation hypothesis posits that predation by increasingly powerful and metabolically active carnivores has been a major driver of metazoan evolution. We test a key tenet of this hypothesis by analyzing predatory drill holes in fossil marine shells, which provide a ~500-million-year record of individual predator-prey interactions. We show that drill-hole size is a robust predictor of body size among modern drilling predators and that drill-hole size (and thus inferred predator size and power) rose substantially from the Ordovician to the Quaternary period, whereas the size of drilled prey remained stable. Together, these trends indicate a directional increase in predator-prey size ratios. We hypothesize that increasing predator-prey size ratios reflect increases in prey abundance, prey nutrient content, and predation among predators.
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http://dx.doi.org/10.1126/science.aam7468DOI Listing
June 2017

Plate tectonic regulation of global marine animal diversity.

Proc Natl Acad Sci U S A 2017 05 15;114(22):5653-5658. Epub 2017 May 15.

Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706.

Valentine and Moores [Valentine JW, Moores EM (1970) 228:657-659] hypothesized that plate tectonics regulates global biodiversity by changing the geographic arrangement of continental crust, but the data required to fully test the hypothesis were not available. Here, we use a global database of marine animal fossil occurrences and a paleogeographic reconstruction model to test the hypothesis that temporal patterns of continental fragmentation have impacted global Phanerozoic biodiversity. We find a positive correlation between global marine invertebrate genus richness and an independently derived quantitative index describing the fragmentation of continental crust during supercontinental coalescence-breakup cycles. The observed positive correlation between global biodiversity and continental fragmentation is not readily attributable to commonly cited vagaries of the fossil record, including changing quantities of marine rock or time-variable sampling effort. Because many different environmental and biotic factors may covary with changes in the geographic arrangement of continental crust, it is difficult to identify a specific causal mechanism. However, cross-correlation indicates that the state of continental fragmentation at a given time is positively correlated with the state of global biodiversity for tens of millions of years afterward. There is also evidence to suggest that continental fragmentation promotes increasing marine richness, but that coalescence alone has only a small negative or stabilizing effect. Together, these results suggest that continental fragmentation, particularly during the Mesozoic breakup of the supercontinent Pangaea, has exerted a first-order control on the long-term trajectory of Phanerozoic marine animal diversity.
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http://dx.doi.org/10.1073/pnas.1702297114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5465924PMC
May 2017

Formation of the Isthmus of Panama.

Sci Adv 2016 08 17;2(8):e1600883. Epub 2016 Aug 17.

Smithsonian Tropical Research Institute, Box 0843-03092, Balboa, Republic of Panama.; Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843, USA.

The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.
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http://dx.doi.org/10.1126/sciadv.1600883DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4988774PMC
August 2016

Biogeographic and bathymetric determinants of brachiopod extinction and survival during the Late Ordovician mass extinction.

Proc Biol Sci 2016 04;283(1829)

Palaeoecosystems Group, Department of Earth Sciences, Durham University, Durham DH1 3LE, UK Department of Geology, University of Lund, Lund, Sweden.

The Late Ordovician mass extinction (LOME) coincided with dramatic climate changes, but there are numerous ways in which these changes could have driven marine extinctions. We use a palaeobiogeographic database of rhynchonelliform brachiopods to examine the selectivity of Late Ordovician-Early Silurian genus extinctions and evaluate which extinction drivers are best supported by the data. The first (latest Katian) pulse of the LOME preferentially affected genera restricted to deeper waters or to relatively narrow (less than 35°) palaeolatitudinal ranges. This pattern is only observed in the latest Katian, suggesting that it reflects drivers unique to this interval. Extinction of exclusively deeper-water genera implies that changes in water mass properties such as dissolved oxygen content played an important role. Extinction of genera with narrow latitudinal ranges suggests that interactions between shifting climate zones and palaeobiogeography may also have been important. We test the latter hypothesis by estimating whether each genus would have been able to track habitats within its thermal tolerance range during the greenhouse-icehouse climate transition. Models including these estimates are favoured over alternative models. We argue that the LOME, long regarded as non-selective, is highly selective along biogeographic and bathymetric axes that are not closely correlated with taxonomic identity.
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http://dx.doi.org/10.1098/rspb.2016.0007DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4855380PMC
April 2016

Marine extinction risk shaped by trait-environment interactions over 500 million years.

Glob Chang Biol 2015 Oct 18;21(10):3595-607. Epub 2015 Jul 18.

Department of Biology, Dalhousie University, 1355 Oxford Street, P.O. Box 15000, Halifax, NS, B3H 4R2, Canada.

Perhaps the most pressing issue in predicting biotic responses to present and future global change is understanding how environmental factors shape the relationship between ecological traits and extinction risk. The fossil record provides millions of years of insight into how extinction selectivity (i.e., differential extinction risk) is shaped by interactions between ecological traits and environmental conditions. Numerous paleontological studies have examined trait-based extinction selectivity; however, the extent to which these patterns are shaped by environmental conditions is poorly understood due to a lack of quantitative synthesis across studies. We conducted a meta-analysis of published studies on fossil marine bivalves and gastropods that span 458 million years to uncover how global environmental and geochemical changes covary with trait-based extinction selectivity. We focused on geographic range size and life habit (i.e., infaunal vs. epifaunal), two of the most important and commonly examined predictors of extinction selectivity. We used geochemical proxies related to global climate, as well as indicators of ocean acidification, to infer average global environmental conditions. Life-habit selectivity is weakly dependent on environmental conditions, with infaunal species relatively buffered from extinction during warmer climate states. In contrast, the odds of taxa with broad geographic ranges surviving an extinction (>2500 km for genera, >500 km for species) are on average three times greater than narrow-ranging taxa (estimate of odds ratio: 2.8, 95% confidence interval = 2.3-3.5), regardless of the prevailing global environmental conditions. The environmental independence of geographic range size extinction selectivity emphasizes the critical role of geographic range size in setting conservation priorities.
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http://dx.doi.org/10.1111/gcb.12963DOI Listing
October 2015

Extinctions. Paleontological baselines for evaluating extinction risk in the modern oceans.

Science 2015 May;348(6234):567-70

Australian Research Council Centre of Excellence for Coral Reef Studies, School of Biological Sciences, University of Queensland, St. Lucia, QLD 4072, Australia.

Marine taxa are threatened by anthropogenic impacts, but knowledge of their extinction vulnerabilities is limited. The fossil record provides rich information on past extinctions that can help predict biotic responses. We show that over 23 million years, taxonomic membership and geographic range size consistently explain a large proportion of extinction risk variation in six major taxonomic groups. We assess intrinsic risk-extinction risk predicted by paleontologically calibrated models-for modern genera in these groups. Mapping the geographic distribution of these genera identifies coastal biogeographic provinces where fauna with high intrinsic risk are strongly affected by human activity or climate change. Such regions are disproportionately in the tropics, raising the possibility that these ecosystems may be particularly vulnerable to future extinctions. Intrinsic risk provides a prehuman baseline for considering current threats to marine biodiversity.
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http://dx.doi.org/10.1126/science.aaa6635DOI Listing
May 2015

A signature of transience in bedrock river incision rates over timescales of 10(4)-10(7) years.

Nature 2014 Jan;505(7483):391-4

Department of Integrative Biology, University of California Berkeley, Berkeley, California 94720, USA.

Measured rates of river incision into bedrock are commonly interpreted as proxies for rates of rock uplift (see refs 1 and 2, for example) and indices of the strength of climatic forcing of erosion over time (see refs 3 and 4, for example). This approach implicitly assumes that river incision rates are in equilibrium with external forcings over a wide range of timescales. Here we directly test this assumption by examining the temporal scaling of bedrock river incision from 155 independent measurements of river incision compiled from 14 sites. Of these sites, 11 exhibit a negative power-law dependence of bedrock river incision rate on measurement interval, a relationship that is apparent over timescales of 10(4)-10(7) years and is independent of tectonic and geomorphic setting. Thus, like rates of sediment accumulation, rates of river incision into bedrock exhibit non-steady-state behaviour even over very long measurement intervals. Non-steady-state behaviour can be explained by episodic hiatuses in river incision triggered by alluvial deposition, if such hiatuses have a heavy-tailed length distribution. Regardless of its cause, the dependence of incision rate on measurement interval complicates efforts to infer tectonic or climatic forcing from changes in rates of river incision over time or from comparison of rates computed over different timescales.
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http://dx.doi.org/10.1038/nature12913DOI Listing
January 2014

Climate change and the past, present, and future of biotic interactions.

Science 2013 Aug;341(6145):499-504

School of Natural Sciences, University of California, Merced, Merced, CA 95343, USA.

Biotic interactions drive key ecological and evolutionary processes and mediate ecosystem responses to climate change. The direction, frequency, and intensity of biotic interactions can in turn be altered by climate change. Understanding the complex interplay between climate and biotic interactions is thus essential for fully anticipating how ecosystems will respond to the fast rates of current warming, which are unprecedented since the end of the last glacial period. We highlight episodes of climate change that have disrupted ecosystems and trophic interactions over time scales ranging from years to millennia by changing species' relative abundances and geographic ranges, causing extinctions, and creating transient and novel communities dominated by generalist species and interactions. These patterns emerge repeatedly across disparate temporal and spatial scales, suggesting the possibility of similar underlying processes. Based on these findings, we identify knowledge gaps and fruitful areas for research that will further our understanding of the effects of climate change on ecosystems.
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http://dx.doi.org/10.1126/science.1237184DOI Listing
August 2013

Extinctions in ancient and modern seas.

Trends Ecol Evol 2012 Nov 10;27(11):608-17. Epub 2012 Aug 10.

National Evolutionary Synthesis Center, Durham, NC 27705, USA.

In the coming century, life in the ocean will be confronted with a suite of environmental conditions that have no analog in human history. Thus, there is an urgent need to determine which marine species will adapt and which will go extinct. Here, we review the growing literature on marine extinctions and extinction risk in the fossil, historical, and modern records to compare the patterns, drivers, and biological correlates of marine extinctions at different times in the past. Characterized by markedly different environmental states, some past periods share common features with predicted future scenarios. We highlight how the different records can be integrated to better understand and predict the impact of current and projected future environmental changes on extinction risk in the ocean.
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http://dx.doi.org/10.1016/j.tree.2012.07.010DOI Listing
November 2012

Climate change and the selective signature of the Late Ordovician mass extinction.

Proc Natl Acad Sci U S A 2012 May 17;109(18):6829-34. Epub 2012 Apr 17.

Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA.

Selectivity patterns provide insights into the causes of ancient extinction events. The Late Ordovician mass extinction was related to Gondwanan glaciation; however, it is still unclear whether elevated extinction rates were attributable to record failure, habitat loss, or climatic cooling. We examined Middle Ordovician-Early Silurian North American fossil occurrences within a spatiotemporally explicit stratigraphic framework that allowed us to quantify rock record effects on a per-taxon basis and assay the interplay of macrostratigraphic and macroecological variables in determining extinction risk. Genera that had large proportions of their observed geographic ranges affected by stratigraphic truncation or environmental shifts at the end of the Katian stage were particularly hard hit. The duration of the subsequent sampling gaps had little effect on extinction risk, suggesting that this extinction pulse cannot be entirely attributed to rock record failure; rather, it was caused, in part, by habitat loss. Extinction risk at this time was also strongly influenced by the maximum paleolatitude at which a genus had previously been sampled, a macroecological trait linked to thermal tolerance. A model trained on the relationship between 16 explanatory variables and extinction patterns during the early Katian interval substantially underestimates the extinction of exclusively tropical taxa during the late Katian interval. These results indicate that glacioeustatic sea-level fall and tropical ocean cooling played important roles in the first pulse of the Late Ordovician mass extinction in Laurentia.
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http://dx.doi.org/10.1073/pnas.1117039109DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3345012PMC
May 2012

The magnitude and duration of Late Ordovician-Early Silurian glaciation.

Science 2011 Feb 27;331(6019):903-6. Epub 2011 Jan 27.

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.

Understanding ancient climate changes is hampered by the inability to disentangle trends in ocean temperature from trends in continental ice volume. We used carbonate "clumped" isotope paleothermometry to constrain ocean temperatures, and thereby estimate ice volumes, through the Late Ordovician-Early Silurian glaciation. We find tropical ocean temperatures of 32° to 37°C except for short-lived cooling by ~5°C during the final Ordovician stage. Evidence for ice sheets spans much of the study interval, but the cooling pulse coincided with a glacial maximum during which ice volumes likely equaled or exceeded those of the last (Pleistocene) glacial maximum. This cooling also coincided with a large perturbation of the carbon cycle and the Late Ordovician mass extinction.
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http://dx.doi.org/10.1126/science.1200803DOI Listing
February 2011

The evolutionary consequences of oxygenic photosynthesis: a body size perspective.

Photosynth Res 2011 Jan 7;107(1):37-57. Epub 2010 Sep 7.

Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg. 320, Stanford, CA 94305, USA.

The high concentration of molecular oxygen in Earth's atmosphere is arguably the most conspicuous and geologically important signature of life. Earth's early atmosphere lacked oxygen; accumulation began after the evolution of oxygenic photosynthesis in cyanobacteria around 3.0-2.5 billion years ago (Gya). Concentrations of oxygen have since varied, first reaching near-modern values ~600 million years ago (Mya). These fluctuations have been hypothesized to constrain many biological patterns, among them the evolution of body size. Here, we review the state of knowledge relating oxygen availability to body size. Laboratory studies increasingly illuminate the mechanisms by which organisms can adapt physiologically to the variation in oxygen availability, but the extent to which these findings can be extrapolated to evolutionary timescales remains poorly understood. Experiments confirm that animal size is limited by experimental hypoxia, but show that plant vegetative growth is enhanced due to reduced photorespiration at lower O(2):CO(2). Field studies of size distributions across extant higher taxa and individual species in the modern provide qualitative support for a correlation between animal and protist size and oxygen availability, but few allow prediction of maximum or mean size from oxygen concentrations in unstudied regions. There is qualitative support for a link between oxygen availability and body size from the fossil record of protists and animals, but there have been few quantitative analyses confirming or refuting this impression. As oxygen transport limits the thickness or volume-to-surface area ratio-rather than mass or volume-predictions of maximum possible size cannot be constructed simply from metabolic rate and oxygen availability. Thus, it remains difficult to confirm that the largest representatives of fossil or living taxa are limited by oxygen transport rather than other factors. Despite the challenges of integrating findings from experiments on model organisms, comparative observations across living species, and fossil specimens spanning millions to billions of years, numerous tractable avenues of research could greatly improve quantitative constraints on the role of oxygen in the macroevolutionary history of organismal size.
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http://dx.doi.org/10.1007/s11120-010-9593-1DOI Listing
January 2011

Two-phase increase in the maximum size of life over 3.5 billion years reflects biological innovation and environmental opportunity.

Proc Natl Acad Sci U S A 2009 Jan 23;106(1):24-7. Epub 2008 Dec 23.

Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, CA 94305, USA.

The maximum size of organisms has increased enormously since the initial appearance of life >3.5 billion years ago (Gya), but the pattern and timing of this size increase is poorly known. Consequently, controls underlying the size spectrum of the global biota have been difficult to evaluate. Our period-level compilation of the largest known fossil organisms demonstrates that maximum size increased by 16 orders of magnitude since life first appeared in the fossil record. The great majority of the increase is accounted for by 2 discrete steps of approximately equal magnitude: the first in the middle of the Paleoproterozoic Era (approximately 1.9 Gya) and the second during the late Neoproterozoic and early Paleozoic eras (0.6-0.45 Gya). Each size step required a major innovation in organismal complexity--first the eukaryotic cell and later eukaryotic multicellularity. These size steps coincide with, or slightly postdate, increases in the concentration of atmospheric oxygen, suggesting latent evolutionary potential was realized soon after environmental limitations were removed.
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http://dx.doi.org/10.1073/pnas.0806314106DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2607246PMC
January 2009

The effect of geographic range on extinction risk during background and mass extinction.

Proc Natl Acad Sci U S A 2007 Jun 11;104(25):10506-11. Epub 2007 Jun 11.

Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA.

Wide geographic range is generally thought to buffer taxa against extinction, but the strength of this effect has not been investigated for the great majority of the fossil record. Although the majority of genus extinctions have occurred between major mass extinctions, little is known about extinction selectivity regimes during these "background" intervals. Consequently, the question of whether selectivity regimes differ between background and mass extinctions is largely unresolved. Using logistic regression, we evaluated the selectivity of genus survivorship with respect to geographic range by using a global database of fossil benthic marine invertebrates spanning the Cambrian through the Neogene periods, an interval of approximately 500 My. Our results show that wide geographic range has been significantly and positively associated with survivorship for the great majority of Phanerozoic time. Moreover, the significant association between geographic range and survivorship remains after controlling for differences in species richness and abundance among genera. However, mass extinctions and several second-order extinction events exhibit less geographic range selectivity than predicted by range alone. Widespread environmental disturbance can explain the reduced association between geographic range and extinction risk by simultaneously affecting genera with similar ecological and physiological characteristics on global scales. Although factors other than geographic range have certainly affected extinction risk during many intervals, geographic range is likely the most consistently significant predictor of extinction risk in the marine fossil record.
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http://dx.doi.org/10.1073/pnas.0701257104DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1890565PMC
June 2007

The Ordovician Radiation: A Follow-up to the Cambrian Explosion?

Integr Comp Biol 2003 Feb;43(1):178-84

Department of Earth Sciences, University of California, Riverside, California 92521.

There was a major diversification known as the Ordovician Radiation, in the period immediately following the Cambrian. This event is unique in taxonomic, ecologic and biogeographic aspects.While all of the phyla but one were established during the Cambrian explosion, taxonomic increases during the Ordovician were manifest at lower taxonomic levels although ordinal level diversity doubled. Marine family diversity tripled and within clade diversity increases occurred at the genus and species levels. The Ordovician radiation established the Paleozoic Evolutionary Fauna; those taxa which dominated the marine realm for the next 250 million years. Community structure dramatically increased in complexity. New communities were established and there were fundamental shifts in dominance and abundance.Over the past ten years, there has been an effort to examine this radiation at different scales. In comparison with the Cambrian explosion which appears to be more globally mediated, local and regional studies of Ordovician faunas reveal sharp transitions with timing and magnitudes that vary geographically. These transitions suggest a more episodic and complex history than that revealed through synoptic global studies alone.Despite its apparent uniqueness, we cannot exclude the possibility that the Ordovician radiation was an extension of Cambrian diversity dynamics. That is, the Ordovician radiation may have been an event independent of the Cambrian radiation and thus requiring a different set of explanations, or it may have been the inevitable follow-up to the Cambrian radiation. Future studies should focus on resolving this issue.
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http://dx.doi.org/10.1093/icb/43.1.178DOI Listing
February 2003