Palaeoecology of the Spathian Virgin Formation (Utah, USA) and Its Implications for the Early Triassic Recovery

Palaeoecology of the Spathian Virgin Formation (Utah, USA) and its implications for the Early Triassic recovery RICHARD HOFMANN, MICHAEL HAUTMANN, MARTIN WASMER, and HUGO BUCHER Hofmann, R., Hautmann, M., Wasmer, M., and Bucher, H. 2013. Palaeoecology of the Spathian Virgin Formation (Utah, USA) and its implications for the Early Triassic recovery. Acta Palaeontologica Polonica 58 (1): 149–173. The Spathian (late Early Triassic) Virgin Formation of south−western Utah (USA) yields a comparatively diverse benthic fauna that flourished ~2 Ma after the end−Permian mass extinction. In this study, we present quantitative palaeoecological data, which are analysed in the context of depositional environments. This integrated approach helps to discriminate between effects of the end−Permian mass extinction event and local environmental factors on alpha diversity and ecological structure of the Virgin Fauna. Shallow subtidal environments yield the highest species richness and lowest dominance val− ues as recorded in two benthic associations, the Eumorphotis ericius Association and the Protogusarella smithi Association, both of which contain 20 benthic species (bivalves, gastropods, brachiopods, echinoderms, and porifers). Tidal inlet deposits yield a low diverse fauna (Piarorhynchella triassica Association) with a very high dominance of filter feeders adapted to high energy conditions. Another comparably low diverse fauna is recorded by the Bakevellia exporrecta Association, which occurs in deposits of the offshore transition zone, most likely reflecting unconsolidated substrates. A single sample contain− ing five bivalve species (Bakevellia costata Assemblage) is recorded from a marginal−marine setting. The Virgin fauna yields a bulk diversity of 30 benthic species (22 genera) of body fossils and 14 ichnogenera and, thus, represents the most di− verse marine bottom fauna known so far from the Early Triassic. Our results suggest that oceanographic conditions during the early Spathian enabled ecosystems to rediversify without major abiotic limitations. However, taxonomical differentia− tion between habitats was still low, indicating a time lag between increasing within−habitat diversity (alpha diversity) and the onset of taxonomical differentiation between habitats (beta diversity). We suggest that taxonomical habitat differentia− tion after mass extinction events starts only when within−habitat competition exceeds a certain threshold, which was not yet reached in the Spathian of the investigated area. This interpretation is an alternative to previous suggestions that the preva− lence of generalistic taxa in the aftermath of mass extinction events reflects protracted environmental stress. The onset of in− creasing beta diversity is a potential criterion for distinguishing two major recovery phases, the first ending with habitat satu− ration and the second ending with the completion of ecosystem differentiation. Key words: Biotic recovery, mass extinction, palaeoecology, diversity, Early Triassic, Spathian, Virgin Formation, USA. Richard Hofmann [[email protected]], Michael Hautmann [[email protected]], Martin Wasmer [[email protected]], and Hugo Bucher [[email protected]], Paläontologisches Institut und Museum, Universität Zürich, Karl Schmid−Strasse 4, CH−8006 Zürich, Switzerland. Received 27 April 2011, accepted 13 December 2011, available online 16 December 2011. Copyright © 2012 R. Hofmann et al. This is an open−access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Introduction An important but largely neglected tool in reconstructing recovery processes is that of quantitative palaeoecological The end−Permian mass extinction is widely accepted as rep− studies. Although the Virgin Formation (or Virgin Limestone resenting the most devastating biotic crisis in the history of Member in most other studies) and related strata of the western metazoan life (Erwin 1993). Besides the study of the causes USA have been studied in various aspects (e.g., Poborski and mechanism of the extinction itself, the ensuing recovery 1954; Schubert and Bottjer 1995; Boyer et al. 2004; Pruss and interval has attracted considerable attention since the early Bottjer 2004; Pruss et al. 2007; McGowan et al. 2009; Pruss 1990s. The paramount questions addressed in post−extinc− and Payne 2009; Mata and Bottjer 2011), very little palaeo− tion research are, in particular, how biota and ecosystems re− ecological community data were presented. The hitherto most spond to large environmental perturbations, how rapidly eco− comprehensive study of the Virgin palaeoecology is that of systems can recover and how evolutionary processes pro− Schubert and Bottjer (1995), which, however, focused on ceed in vacated ecospaces. broad patterns throughout the entire Early Triassic of the west− Acta Palaeontol. Pol. 58 (1): 149–173, 2013 http://dx.doi.org/10.4202/app.2011.0060 150 ACTA PALAEONTOLOGICA POLONICA 58 (1), 2013 ern USA and was based on genus identification only. Al− depositional environments. This integrated approach helps to though their Spathian samples exhibit the highest generic di− distil unequivocal recovery signals from “noise−variation” versity and most advanced trophic structure among all Early merely reflecting the environmental heterogeneity of shal− Triassic faunas analysed, Schubert and Bottjer (1995) still low−marine environments and local environmental stress un− concluded that the entire Early Triassic of the western USA related to actual recovery trajectories. Additionally, it allows was dominated by depauperate benthic communities reflect− the identification of the degree of faunal heterogeneity be− ing a considerably delayed recovery. Schubert and Bottjer tween adjacent habitats along environmental gradients and, (1995: 28) suggested that this pattern largely results from the thereby, the assessing of the role of beta diversity in the extraordinary magnitude of the end−Permian mass extinction post−extinction diversity increase. and from intrinsic, ecological effects. Another mechanism that has been invoked to explain the putatively protracted Early Institutional abbreviations.—PIMUZ, Paläontologisches Triassic recovery is that adverse environmental conditions Institut und Museum collection, University of Zürich, Swit− persisted well beyond the actual extinction interval and inhib− zerland. ited re−diversification in the marine realm (Hallam 1991). This model was subsequently advocated by many investigators studying Lower Triassic sections from the USA (e.g., Wignall Material and methods and Hallam 1992; Boyer et al. 2004; Pruss and Bottjer 2004; Mata and Bottjer 2011) as well as other palaeogeographic re− Fieldwork was conducted by in October 2009 (by RH) and gions (e.g., Twitchett and Wignall 1996). However, recent August 2010 (RH and MW) in the area of St. George and Hur− studies have challenged several key aspects of this model, ricane in south−western Utah, USA. Bed−by−bed logging and such as the universality of small body sizes in the Early Trias− documentation was undertaken at three sections in the Beaver sic (Brayard et al. 2010, 2011a), the existence of global shal− Dam Mountains area west of St. George and at five sections low marine anoxia (Heydari et al. 2008), and a delayed recov− along the “Honeymoon Trail”, east of Hurricane (Fig. 1). This ery in general (Brayard et al. 2009, 2011b; Hautmann et al. main body of data was complemented by additional observa− 2011, Hofmann et al. 2011). tions from some single outcrops in the research area. At the The Virgin Formation has been repeatedly studied for studied sections, fossiliferous beds were chosen for quantita− more or less representative palaeoecological proxies such as tive sampling of macrofossils. The greater part of fossil mate− trace fossils (Pruss and Bottjer 2004; Mata and Bottjer 2011), rial was identified and counted in the field but reference sam− shell bed architecture (Boyer et al. 2004), and microbially in− ples were taken for lab preparation and further taxonomic duced sedimentary structures (Pruss et al. 2005). All of these identification. Benthic fossils from Lower Triassic succes− studies conclude that strata from this time interval in the sions are notorious for their poor preservation (Schubert and western USA record unusual and harsh environmental condi− Bottjer 1995; Fraiser and Bottjer 2005, Hautmann and Nützel tions, such as alkaline and anoxic waters permanently (Boyer 2005) with mostly internal and re−crystallised moulds being et al. 2004) or repeatedly (e.g., Pruss and Bottjer 2004) af− preserved. However, the material investigated herein is excel− fecting shallow marine habitats and, thereby, inhibiting eco− lently preserved by Early Triassic standards and poses few systems from taxonomical and ecological restoration. problems with its taxonomic identification. Crucial taxonomic A study to assess the faunal heterogeneity and the recovery criteria are readily observable on well preserved, primarily status of the Virgin fauna was recently provided by McGowan calcitic shells of epifaunal bivalves. Internal moulds of mostly et al. (2009). Their data suggest that Virgin communities actu− aragonitic infaunal bivalves are morphologically distinct and ally lack typical traits of post−extinction faunas such as pro− show no evidence of notable deformation. nounced small shell

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