Palaeogeography, Palaeoclimatology, Palaeoecology
Permian marine paleoecology and its implications for large-scale decoupling of brachiopod and bivalve abundance and diversity during the Lopingian (Late Permian)
Introduction
The end-Permian mass extinction, the most severe crisis in the Phanerozoic (Erwin, 2006), and its associated faunal shift from the brachiopod-rich Paleozoic Fauna to the molluscan Modern Fauna (Gould and Calloway, 1980, Sepkoski, 1981) represent a fundamental change in the taxonomic structure and ecological architecture of marine animal ecosystems. The dramatic shift in relative global richness at the genus or family level, from Paleozoic brachiopod-rich assemblages to post-Paleozoic mollusc-rich assemblages, was also accompanied by a significant increase in alpha diversity (Bambach, 1977), a shift towards mobile and infaunal organisms (Stanley, 1968, Bambach et al., 2002), and an increased occupation of ecospace in the marine realm (Bambach, 1983). This iconic “brachiopod–bivalve transition” has been attributed to the severe effects of the end-Permian biotic crisis (Gould and Calloway, 1980), differing physiological tolerances between brachiopods and molluscs (Steele-Petrovic, 1979), or competitive interactions between the two groups (Sepkoski, 1996). However, because of the close association of the faunal transition with the end-Permian mass extinction, it has generally been accepted that the severe effects of the biotic crisis forcibly dismantled brachiopod-rich assemblages that had dominated for 250 m.y. of the Paleozoic, opening benthic ecospace and new niches for molluscs and ultimately giving rise to modern-style marine ecosystems.
Detailed compilations of genus richness at the global scale indicate that the transition between the Paleozoic Evolutionary Fauna and the Modern Fauna was an abrupt switch at the Permian–Triassic boundary, coincident with the end-Permian mass extinction (Gould and Calloway, 1980, Sepkoski, 1996). The large magnitude and abrupt timing of this diversity shift has fueled a tacit assumption that the ecological shift in relative abundance between rhynchonelliform brachiopods and molluscs was synchronous with taxonomic changes and that it was also caused by the severe effects of the end-Permian biotic crisis. Although ecological dominance (as measured by relative abundance) does appear broadly correlated with global taxonomic richness at large temporal scales (Clapham et al., 2006, Madin et al., 2006), documented instances of decoupled taxonomic and ecological behavior over shorter timescales (e.g., McKinney et al., 1998) caution against the assumption of synchroneity during the end-Permian crisis. The ecology of the Permian–Triassic transition has been comprehensively investigated only in Early Triassic fossil assemblages (e.g., Schubert and Bottjer, 1995, Boyer et al., 2004, Fraiser and Bottjer, 2004, Fraiser and Bottjer, 2005), whereas the ecological structure of Permian fossil assemblages, and Late Permian (Lopingian) assemblages in particular, remains largely unknown.
This comparative lack of Middle and Late Permian abundance data, with only a single Middle Permian study (Waterhouse, 1987) and one from nearshore Late Permian environments (Hollingworth and Pettigrew, 1988), has troubling implications for the assumed abrupt Permian–Triassic ecological transition given the recognition of potentially important environmental and biotic crises in the Middle and Late Permian (Guadalupian–Lopingian interval). The end-Guadalupian extinction, occurring at the end of the Capitanian Stage of the Middle Permian about 8 m.y. prior to the end-Permian crisis, may have eliminated as many as 60% of marine invertebrate genera and perhaps preferentially affected members of the Paleozoic Fauna (Stanley and Yang, 1994). There were additional environmental changes during the Guadalupian–Lopingian interval (e.g., Isozaki, 1997) that may also have influenced the ecological transition between rhynchonelliform brachiopods and molluscs.
There is some circumstantial evidence to suggest that the Permian–Triassic ecological transition between the Paleozoic and Modern faunas may have been more protracted than previously assumed from global diversity metrics. Late Paleozoic onshore–offshore diversity patterns (Fig. 1) indicate that bivalves were becoming more diverse before the end-Permian mass extinction and expanding their range into a wider spectrum of shelf environments through the Early and Middle Permian (Sepkoski and Miller, 1985, Miller, 1988). This increased offshore diversification among bivalves may also be correlated with a Permian increase in their abundance and ecological importance relative to rhynchonelliform brachiopods. If true, the correlation may imply that at least part of the ecological transition in relative abundance between the Paleozoic and Modern faunas may also have occurred before the end-Permian mass extinction. There was also a gradual decrease in gastropod size through the Guadalupian–Lopingian interval (Payne, 2005), culminating in the predominance of small individuals in the Permian–Triassic extinction interval and the Early Triassic (Twitchett, 2001, Price-Lloyd and Twitchett, 2002, Fraiser and Bottjer, 2004). This long-term size diminution may have resulted from increasing but low levels of environmental stress, potentially a precursor to the environmental crisis that caused the end-Permian mass extinction and prolonged Early Triassic recovery.
This study reports quantitative counts and size data from silicified Permian fossil assemblages in order to constrain the nature and timing of the ecological transition from the brachiopod-rich Paleozoic Fauna to the molluscan Modern Fauna. Integration of this new Middle and Late Permian data with previously published Early Triassic quantitative information will indicate whether the shift in relative abundance was synchronous with the taxonomic change, as previously assumed, or if the two were decoupled and the ecological switch began earlier and was more protracted. In particular, the ecological structure of Late Permian assemblages is compared to Early and Middle Permian assemblages as well as Early and Middle Triassic assemblages to examine the correlation between taxonomic and ecological change, investigate the ecological consequences of the end-Guadalupian extinction, and test for environmental or ecological deterioration prior to the end-Permian biotic crisis.
Section snippets
Methods
Quantitative counts of silicified fossil abundance were made from bulk samples collected from individual, thin (< 20 cm) limestone (primarily lime mudstone or wackestone) shell beds. Sample size varied from 1 to 15 kg, depending on the size and density of fossils, in order to yield an average of 200–400 specimens per locality. Where possible, replicate samples were collected along the extent of the outcrop (Bennington, 2003). The depositional environment (nearshore, inner shelf — above fair
Early Permian onshore–offshore abundance patterns
Although onshore–offshore diversity patterns imply increasing molluscan importance through the Permian (Sepkoski and Miller, 1985, Miller, 1988), it is not clear that environmental trends in relative abundance parallel those diversity trends. Onshore–offshore abundance patterns were investigated from eight Early Permian fossil assemblages, spanning an environmental range from nearshore peritidal to outer shelf, near the shelf-slope break, in conjunction with previously published information
Decoupling of local and global processes
The substantial change in the relative abundance of rhynchonelliform brachiopods and molluscs occurred between the Capitanian (Middle Permian) and mid-Wuchiapingian (Late Permian), broadly coincident with the end-Guadalupian extinction (Stanley and Yang, 1994). However, a new global diversity compilation for the Middle and Late Permian, taking advantage of the large amount of published data on Lopingian fossils in the past decade and including data for most marine invertebrate groups (Table 1),
Causes of the ecological shift
Decoupling of local ecological processes from global taxonomic change during the Guadalupian–Lopingian transition from abundant brachiopods to dominant molluscs indicates that the ecological change was not associated with severe taxonomic effects during an extinction event. The marked shift in relative abundance is not a taphonomic artifact, as all samples are well-silicified during early diagenesis and no unsilicified shells or molds of dissolved specimens were observed in the field or in thin
Conclusions
Changes in relative abundance in offshore benthic communities reveal that the first stage in the ecological transition from the rhynchonelliform brachiopod-dominated Paleozoic Fauna to the molluscan Modern Fauna occurred between the Middle and Late Permian, in contrast to the abrupt diversity switch at the Permian–Triassic boundary. The dramatic increase in molluscan abundance, from 1% to 65% in these offshore assemblages, was paralleled by a similar increase in the abundance of motile
Acknowledgements
We thank AAPG, AMNH, The Evolving Earth Foundation, GSA, The Paleontological Society, Sigma Xi, USC Earth Sciences, and the Western Society of Malacologists for financial support of this research, S.Z. Shen for assistance with brachiopod data, and W.Z. Li and C. Powers for field logistics and assistance. M. Florence assisted with the collections at the Smithsonian National Museum of Natural History. We are grateful to D.A. Caron, F.A. Corsetti, and A.G. Fischer for stimulating discussion and P.
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