Phanerozoic Diversity and Mass Extinctions

Transcription

1 Phanerozoic Diversity and Mass Extinctions Measuring Diversity John Phillips produced the first estimates of Phanerozoic diversity in 1860, based on the British fossil record Intuitively it seems simple to count the number of taxa, but there are actually quite a few complications Measuring Diversity Diversity was revisited with a more quantitative approach in the late 1970s and 1980s The number of described species had increased ~25 times, providing much more data for analysis New analyses Phillips, 1860

2 Measuring Diversity Jack Sepkoski spent 15 years compiling a database to produce the classic diversity curve Phanerozoic Diversity The Sepkoski curve revealed a number of interesting patterns in the history of animal diversity Ordovician Radiation Paleozoic Plateau Meso-Cenozoic Diversification Cambrian Explosion Sepkoski wanted to analyze fossil record for patterns Are there groups of taxa with coherent diversity trends? # Families per class Time Intervals Coherent Patterns? Classes A B C Used factor analysis to extract variables that explain variation in original data set

3 Factors are termed evolutionary faunas Primarily controlled by changes in the diversity of four groups Brachiopods w/ hinged shells Evolutionary Faunas Sepkoski grouped clades into evolutionary faunas based on the times during which they were most diverse EFs are not coherent biological units, but rather are composed of different taxa that happen to share similar diversity patterns

4 Bias and Diversity Patterns How closely does this curve actually match the true history of Phanerozoic biodiversity? What if fossil preservation or the extent of sedimentary rocks was lower in the past?

5 Rock Volume Bias There is a broad correspondence between diversity and the exposed area of sedimentary rocks More sampling = more taxa! Rock Volume Bias Changes in diversity correlate strongly with changes in the number of named geological formations (in the USA) # Formations Diversity Pull of the Recent Because the living fauna is so well known, the ranges of many taxa are artificially pulled to the Recent Diversity Diversity is inflated in younger time intervals Time

6 Standardization with the Paleobiology Database Standardization with the Paleobiology Database 1. Sampling standardization accounts for variable rock area 2. Excluding unlithified sediments (Neogene) compensates for Pull of the Recent No Paleozoic Plateau Cenozoic only slight more diverse than Paleozoic Mass Extinctions Raup, Sepkoski, and others realized that extinction and origination rates could be calculated from diversity compilations Five major mass extinctions have been recognized: Late Ordovician, Late Devonian, End-Permian, End-Triassic, End-Cretaceous

7 Ecological Effects In addition to reduced diversity and taxonomic losses, mass extinctions alter community and/or ecosystem structure Ecological impacts not always predictable from taxonomic severity Were mass extinctions gradual or abrupt? Signor-Lipps Effect Because of the imperfect fossil record we never find the true last occurrence of a species Tempo of Extinction Extinctions are smeared backwards and appear to be more gradual than they were K-Pg ammonite ranges with confidence intervals abrupt extinction

8 Abrupt Extinction End Cretaceous Abrupt Extinction End Permian Timing: Very abrupt (<<100 kyr) single event just before P/T boundary Gradual Extinction Late Devonian Timing: At least three major events in the Givetian, Frasnian, and Famennian stages Hangenberg Event Kellwasser Event Taghanic Event Kellwasser event was the most severe and virtually eliminated stromatoporoid reef ecosystems

9 Post-Extinction Recovery Biotic recovery after extinction can be divided into survival and recovery phases Survival phase: 1) Possible dead zone 2) Early crisis interval with extinction survivors and disaster taxa 3) Later interval with opportunists + some Lazarus taxa Recovery phase: 1) Reappearance of many Lazarus taxa 2) More rapid diversity increase, radiation of new lineages Disaster Taxa Specific type of opportunistic organisms that expand to become abundant in a wider range of habitats after a biotic crisis Early Triassic disaster taxa: Lingulid Brachiopods Microbes Post-Extinction Recovery Post-extinction communities often exhibit signs of stress 1. Fewer individuals, dominance by opportunists or disaster taxa 2. Increase in generalists (deposit feeders, etc.) 3. Reduction in body size Typically lasts <500 ka

10 Delayed Early Triassic Recovery 4-5 Myr long Reduced bioturbation and tiering, plus disaster taxa and opportunists Causes of Extinctions End-Cretaceous: Largest impact of the Phanerozoic (180 km-diameter Chicxulub crater) Contributions from environmental stress from Deccan trap volcanism? Kill curve to predict impact-caused extinction But Chicxulub is only impact associated with extinction Impacts and Extinctions Manicouagan, 214 Ma

11 Flood Basalt Volcanism Mass extinctions occurred during times of major flood basalt eruptions Flood Basalt Volcanism Three largest flood basalts associated with three largest extinctions, but no correlation beyond that level CAMP was probably larger than Siberian Traps, but P/T extinction is much more severe than T/J Siberian Traps P/T extinction CAMP T/J extinction Importance of Target Rock Chicxulub hit carbonate platform with abundant sulfates Siberian traps erupted into West Siberian high-sulfur coal basin Additional CO2, SO2 (+other toxic chemicals) more important than actual impact or volcanism

12 Volcanism (effects of reducing molecules like SO 2 ) leads to anoxia + euxinia (H 2 S) in surface ocean, ocean acidification Kellwasser Event: Frasnian Taghanic Event: Givetian Devonian extinctions also coincide with marine anoxia May actually be triggered by land plant evolution, not by flood basalt volcanism Climate cooling also hypothesized for Devonian crises (Hangenberg Event) Late Ordovician Glaciation Timing: Two short-lived events in the latest Ordovician Probable Cause: Climate change, short-lived Hirnantian glaciation Contrasts with extreme climate warming, caused by flood basalt eruptions, during T/J and probably P/T extinctions Hirnantia fauna

13 Extinction Causes Extinction Late Ordovician Late Devonian Climate Change? COOLING / WARMING Anoxia? Large Igneous Province? COOLING YES?? Impact? End Permian WARMING YES YES End Triassic WARMING YES YES End Cretaceous Yes? YES Diversity / Extinction Summary 1. Cenozoic diversity is greater than in the Paleozoic, but not as much as previously thought 2. Diversity compilations allow recognition of five major mass extinctions and many smaller extinction events 3. The Big Five had differing taxonomic and ecological consequences (but why is not clear) 4. Only the end Cretaceous was associated with an impact, but the correlations between anoxia and extinctions, and large igneous provinces and extinctions, are strong