Margaret L. Fraiser

Understanding mechanisms for the end-Permian mass extinction and the protracted Early Triassic aftermath and recovery

Modern study of the end-Permian mass extinction in the marine realm has involved intensive documentation of the fossil content, sedimentology, and chemostratigraphy of individual stratigraphic sections where the mass extinction interval is well preserved. These studies, coupled with innovative modeling of environmental conditions, have produced specific hypotheses for the mechanisms that caused the mass extinction and associated environmental stress. New paleobiological studies on the environmental distribution and ecological importance of brachiopods, benthic molluscs, and bryozoans support the hypothesis that stressful ocean conditions—primarily elevated H2S levels (euxinia) but also heightened CO2 concentrations—were the prime causes of the end-Permian mass extinction. These studies also further demonstrate that both the Late Permian interval preceding this mass extinction and the Early Triassic interval that followed were times of similar elevated environmental stress. In the low-diversity Early Triassic biosphere, huge numbers of benthic molluscs, in particular four cosmopolitan genera of bivalves, typically covered the seafloor. That a few marine genera thrived during this time indicates a greater than usual tolerance to some combination of marine anoxia, as well as elevated CO2 and/or increased H2S concentrations. Research focusing on experiments with modern organisms similar to those that died, as well as those that thrived, in microcosms where levels of O2, CO2, and H2S can be experimentally manipulated will enable an even more detailed understanding of the nature of this greatest Phanerozoic biotic crisis.

When bivalves took over the world

Abstract The end-Permian mass extinction is commonly portrayed not only as a massive biodiversity crisis but also as the time when marine benthic faunas changed from the Paleozoic Fauna, dominated by rhynchonelliform brachiopod taxa, to the Modern Fauna, dominated by gastropod and bivalve taxa. After the end-Permian mass extinction, scenarios involving the Mesozoic Marine Revolution portray a steady increase in numerical dominance by these benthic molluscs as largely due to the evolutionary effects of an “arms race.” We report here a new global paleoecological database from study of shell beds that shows a dramatic geologically sudden earliest Triassic takeover by bivalves as numerical dominants in level-bottom benthic marine communities, which continued through the Early Triassic. Three bivalve genera were responsible for this switch, none of which has any particular morphological features to distinguish it from many typical Paleozoic bivalve genera. The numerical success of these Early Triassic bivalves cannot be attributed to any of the well-known morphological evolutionary innovations of post-Paleozoic bivalves that characterize the Mesozoic Marine Revolution. Rather, their ability to mount this takeover most likely was due to the large extinction of rhynchonelliform brachiopods during the end-Permian mass extinction and aided by their environmental distribution and physiological characteristics that enabled them to thrive during periods of oceanic and atmospheric stress during the Permian/Triassic transition.

Opportunistic behaviour of invertebrate marine tracemakers during the Early Triassic aftermath of the end-Permian mass extinction

A regional field study of ichnocoenoses in the Griesbachian Dinwoody Formation and the Smithian Sinbad Limestone Member revealed that benthic invertebrate tracemaking populations exhibited aspects of opportunistic behaviour following the end-Permian mass extinction. This study represents the first documentation of population strategies of ichnocoenoses following a mass extinction event. These trace fossil assemblages are characterised by low-to-moderate ichnodiversity, low-to-moderate bioturbation, small burrow widths, non-specialised behaviour and shallow tiering. Our data combined with other published studies indicate that various ecological characteristics of ichnocoenoses differed by environment, paleolatitude and stage through the Early Triassic. The pattern of opportunistic behaviour is likely attributed to repeated intervals of deleterious environmental conditions similar to those that caused the end-Permian mass extinction.

ASSESSING THE ECOLOGICAL DOMINANCE OF PHANEROZOIC MARINE INVERTEBRATES

Abstract Ecological studies have revealed that the functional roles of dominant species in modern communities are often more important than overall diversity in governing community composition and functioning. Despite this recognition that abundance and diversity data are both required for a complete understanding of ecological processes, many paleoecological studies focus on presence-absence data, possibly because of concerns regarding the taphonomic fidelity of time-averaged fossil accumulations. However, the abundance of organisms in shell beds has been shown to provide a fairly accurate record of the living community, suggesting that the benefits of relative-abundance data should be reconsidered. Recognition of ecologically dominant species in local fossil assemblages should be based on counts of relative abundance and assessment of ecological role. Ecological dominance at larger spatial or temporal scales can be quantified using the mean rank order of a clade and the proportion of assemblages where the clade is present, providing unbiased, quantitative values for measuring the ecological importance of a clade. Their utility has been tested with three case studies encompassing a range of geographic and taxonomic scales, using a database of 1221 Ordovician–Paleogene quantitative fossil collections. The dominance metrics for rhynchonelliform brachiopods, bivalves, and gastropods broadly parallel anecdotal trends, even including some more detailed patterns documented by regional studies. An examination of substrate preferences for brachiopod and bivalve orders confirms the abundance of infaunal bivalves in siliciclastics and epifaunal bivalves in carbonates, but it also reveals intriguing patterns regarding substrate preferences among rhynchonelliform brachiopod orders. The final case study analyzed changes in dominance between early Mesozoic fossil assemblages from Tethys and Panthalassa, documenting significant geographic differences in the ecological importance of rhynchonelliform brachiopods and bivalves. These large-scale dominance patterns often approximately matched those inferred from diversity trends; however, there are also times when dominance was decoupled from diversity, indicating that further investigation of ecological dominance will provide additional insights into ecological influences on the Phanerozoic history of life. “Are most species simply passengers in ecosystems that are run basically by a few dominants?” (Worm and Duffy, 2003, p. 631)

Paleoecology and geochemistry of Early Triassic (Spathian) microbial mounds and implications for anoxia following the end-Permian mass extinction

Large microbialite mounds (1–2 m in height) have previously been reported from two units within the Spathian section of the Virgin Limestone Member of the Moenkopi Formation at Lost Cabin Spring, Nevada (United States). Previous investigations led to the interpretation that the mounds were formed under anoxic and alkaline conditions that suppressed metazoan grazers and delayed the biotic recovery from the end-Permian mass extinction. Here we report low organic carbon and total sulfur abundances throughout the section that suggest that anoxia was not prevalent during deposition. We also report that the upper mound-bearing unit contains stromatolitesponge patch reefs in which mutual encrustation between stromatolites and sponges contributed to the building of a reef framework. The stromatolite-sponge patch reefs contain discrete burrows within stromatolitic laminations, suggesting that there was suffi cient oxygen for grazing during the formation of the upper unit mounds. The enhanced ecological complexity of the upper unit mounds leads us to conclude that the mounds represent the transition to biotic recovery following the end-Permian mass extinction.

Gastropod evidence against the Early Triassic Lilliput effect: COMMENT

[Brayard et al. (2010)][1] assert that their study of late Early Triassic gastropods provides evidence against the post-extinction Lilliput effect. Regrettably, their data provide no such evidence. The Lilliput effect as defined by [Urbanek (1993)][2] is a quantifiable, temporary decrease in the

Sedimentology and palaeoecology of lonestone-bearing mixed clastic rocks and cold-water carbonates of the Lower Permian Basal Beds at Fossil Cliffs, Maria Island, Tasmania (Australia): Insight into the initial decline of the late Palaeozoic ice age

Abstract The middle Sakmarian Basal Beds on Maria Island were deposited during the initial decline of the Late Palaeozoic Ice Age following Late Pennsylvanian–Early Sakmarian maximum glaciation. At that time, Tasmania was located within the South Polar Circle between an apparent ice-free pole (Antarctica) and the mid- to high-latitude Sydney-Bowen-Gunnedah glacigenic basins in eastern Australia. The dropstone-bearing Basal Beds consist of: interstratified siltstone, conglomerate and cold-water limestones of the Lower Erratic Zone; siltstone and Eurydesma-rich cold-water carbonates of the Darlington Limestone; and siltstone and conglomerate of the Upper Erratic Zone. Interstratification of the coarse-clastic strata, siltstones and limestones within these units were previously attributed to glacial/non-glacial cycles. However, the interfingering of beds within each of these units and the occurrence of large, fossil bryozoans crossing and abutting lithological boundaries indicate that cyclicity was of shorter duration than that of Milankovitch-driven cycles. Within these intertidal and subtidal deposits, the occurrence of rounded dropstones derived from local basement rocks exposed along a rocky coastline and an absence of glacial indicators other than dropstones, along with other evidence, suggest that ice rafting was by sea ice rather than by icebergs. Study results confirm the spatial restriction of Middle Sakmarian to earliest Wuchiapingian glaciation.

ORGANISM-ENVIRONMENT INTERACTIONS DURING THE PERMIAN-TRIASSIC MASS EXTINCTION AND ITS AFTERMATH

Multicellar life came closest to complete annihilation during the ca.252 Ma Permian–Triassic mass extinction (PTME), which resulted inthe largest crash in global biodiversity since the Cambrian explosion(Alroy et al., 2008). This largest biocrisis in the Phanerozoic has alsodramatically redirected the course of biotic evolution during theMesozoic and Cenozoic, and is responsible for much of the structure ofmarine and terrestrial ecosystems today (Chen and Benton, 2012).However, many aspects of the biotic recovery following the PTME havebeen puzzling, including its tempo and mechanism (Erwin, 1994, 2001;Benton and Twitchett, 2003). Growing evidence shows that stressedenvironments and multiple disaster events that further devastatedterrestrial and marine ecosystems during the Early Triassic may haveaccounted for the prolonged delay in biotic recovery following thePTME (Algeo et al., 2011).Three decades of paleontological effort have made late Permian toEarly Triassic marine ecosystems among the most thoroughly studiedfossil communities in Earth history (Chen and Benton, 2012). Changesin biodiversity of all fossil groups through this critical interval are wellknown today. Among the clades most severely affected were corals,brachiopods, foraminifera, radiolarians, bryozoans, echinoderms,gastropods, bivalves, and ammonoids (Erwin, 1994). Individual cladesexhibited dramatically different diversity changes during the crisis,however. For example, brachiopods were among the commonestanimals in Permian oceans but experienced a sharp decline in diversityduring the Early Triassic, and their diversity did not reboundsignificantly until the early Middle Triassic (Chen et al., 2005). Coralssuffered a major diversity loss in the PTME and did not reappear untilthe middle Anisian (Sepkoski, 1984). This is also true for radiolarians, aclade that suffered a large depletion in diversity during the EarlyTriassic and early Anisian (O’Dogherty et al., 2010). In contrast,ammonoid faunas underwent a rapid albeit punctuated diversityrebound in the Early Triassic, reaching a higher diversity by theSmithian than prior to the PTME (Brayard et al., 2009; Stanley, 2009).Foraminifera also show a strong recovery during the Early Triassic,passing their precrisis diversity by the Smithian (Song et al., 2011). ThePTME had a lesser effect on conodonts, which showed a stepwiseincrease in diversity throughout the Early Triassic (Orchard, 2007).Among echinoderms, crinoids disappeared for most of the EarlyTriassic and rebounded during the end-Spathian (Erwin, 2001), whileophiuroids experienced a diversity increase and geographic expansionimmediately after the PTME (Chen and McNamara, 2006). As a result,the recovery of some animal clades (e.g., ammonoids, foraminifera) andtrace fossils occurred during the Olenekian, i.e., within 2–3 myr of thePTME, whereas that of others was delayed by 5–10 myr, lasting untilthe late Anisian stage of the Middle Triassic (Chen and Benton, 2012).The tempo of biotic recovery in the Triassic remains disputed becauseof these differences among individual clades.Since its inception in 2008, the goal of IGCP Project 572 (with .130members worldwide) has been to document the rebuilding of marineecosystems following the PTME in different environmental settingsworldwide. The IGCP572 community published a thematic issue onPermian–Triassic ecosystems in 2011 (Algeo et al., 2011). The presentthematic issue follows up on this earlier work, focusing on organism-environment interactions during the biocrisis and the postextinctionrecovery. The overall goal of this thematic issue is to provide a betterunderstanding of the causes of the prolonged devastation of marineecosystems following the PTME and its significance in terms of thelong-term coevolution of life and the physical environment in the Earthsystem. The studies contributed to this special issue provide strati-graphic, sedimentologic, paleontological, paleoecologic, and geochem-ical insights from diverse locations around the world. In particular,these studies address the role of protracted environmental devastationon the recovery of Early Triassic marine ecosystems.Analysis of the PTME at a global scale depends heavily ondevelopment of detailed biozonation schemes that are based largelyon the fossil record of conodonts (Mei et al., 1998; Henderson and Mei,2007; Orchard, 2007). High-resolution biostratigraphic frameworkshave been developed for the Meishan GSSP (Yin et al., 2001; Jianget al., 2007; Zhang et al., 2009) as well as other sections in South China(e.g., Nicoll et al., 2002; Zhao et al., 2007; Metcalfe and Nicoll, 2007;Jiang et al., 2011) and globally (e.g., Perri and Farabegoli, 2003; Kozur,2004, 2005). In this volume, two studies contribute to this effort byproviding detailed analyses of conodont biostratigraphy through theuppermost Permian to Lower Triassic from sections in GuizhouProvince (Yan et al., 2013) and Hubei Province (Zhao et al., 2013). Thestudied sections represent two very different environmental settings.Yan et al. (2013) examine the flanks of the Permian–Triassic GreatBank of Guizhou, an isolated carbonate platform in the NanpanjiangBasin, southwest China. This isolated platform has yielded detailedbiotic and geochemical records that have provided insights into marineecosystem perturbations and climatic extremes during the PTME andits aftermath (Payne et al., 2004; Song et al., 2011; Sun et al., 2012).Zhao et al.’s (2013) fine conodont biostratigraphic study of LowerTriassic strata in the Three Gorges area helps not only to defineprecisely the PTB and Lower Triassic stages and their boundaries, butalso to detect a much thicker PTB succession than that of the MeishanGSSP. The Lower Triassic succession studied by Zhao et al. (2013) hasthe potential to yield high-resolution geochemical records of environ-mental and climatic change following the end-Permian crisis.The other studies of the present thematic issue offer additionalinsights regarding organic-environment interactions and protracted orrecurrent environmental stresses during the PTB interval and EarlyTriassic. The paper by Brookfield et al. (2013) explores environmentalchanges through the Permian–Triassic transition at Guryul Ravine,Kashmir, a classic PTB site that has received detailed sedimentologicand isotopic study in the past (Brookfield et al., 2003; Algeo et al.,2007). The authors present a new interpretation of this section as arecord of possible extreme storm and/or earthquake and tsunamideposits around the PTB. They propose that at least two largeearthquakes and several tsunamis coincided with and/or rapidlyfollowed the PTME, although it is unclear whether these events were

Palaeoecology and sedimentology of carboniferous glacial and post-glacial successions in the Paganzo and Río Blanco basins of northwestern Argentina

Abstract Recent studies show that Late Palaeozoic Ice Age (LPIA) climate change broadly affected marine invertebrate faunas: glaciations decreased origination and extinction and long-term gradual global warming during the final deglaciation altered palaeocommunity composition. Carboniferous stratigraphy and palaeoecology in the Paganzo and Río Blanco basins of southwestern Gondwana (present-day Argentina) were studied in ordered to further determine how palaeocommunities were influenced by glacial and post-glacial processes during the LPIA. In the Paganzo Basin, the Guandacol Formation consisted of an ice-proximal to very ice-distal glaciomarine succession, while the Tupe Formation represented continental fluvial lowstand sedimentation interrupted by periodic marine incursions (estuary setting). In the Río Blanco Basin, the Río del Peñon Formation represents a shallowing-upwards coastal marine facies under the influence of wave and storm activity. Diversity and abundance data from northwestern Argentina reflects that ice-proximal environments proved physiologically stressful to organisms, limiting colonization to only opportunistic fauna. Following glaciation, the fauna of northwestern Argentina diversified and became increasingly ecologically complex during two marine transgressions in the Río Blanco Basin. It is interpreted that palaeocommunity establishment, composition and diversification in western Argentina during the LPIA was mostly dependent upon localized environmental conditions and palaeogeographic location.

Mass Extinctions and Changing Taphonomic Processes

The biotic crisis of the Middle Permian through Early Triassic is unmatched in the Phanerozoic in terms of taxonomic diversity losses and paleoecological reorganization. However, the potential taphonomic bias from post-mortem diagenesis for this crucial time has not been evaluated. We assessed the quality of the fossil record during this interval by quantifying the number of Lazarus taxa using our own database, data available in the Paleobiology Database and previous compilations. We also quantitatively tested for paleoecological differences between silicified versus non-silicified faunas. Herein we report that there is no major taphonomic bias due to skeletal mineralogy or fossil preservation affecting the Middle and Late Permian fossil record, but that aragonite-shelled molluscs may exhibit a significant Lazarus effect during the Induan. We propose that a variety of mechanisms affected the fossil record of the Paleozoic/Mesozoic transition, including ocean chemistry, paleobiology of the examined groups, and human influences on taxonomic and sampling practices.