Mass Extinctions and Changing Taphonomic Processes
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Chapter 16 1 Mass Extinctions and Changing 2 Taphonomic Processes 3 Fidelity of the Guadalupian, Lopingian, 4 and Early Triassic Fossil Records 5 Margaret L. Fraiser, Matthew E. Clapham, and David J. Bottjer 6 Contents 1 Introduction .......................................................................................................................... 000 2 Previous Understanding of Biases in the Middle Permian to Early Triassic Fossil Record 000 2.1 End-Guadalupian Extinction and Lopingian Aftermath ............................................. 000 2.2 End-Permian Mass Extinction and Early Triassic Aftermath ..................................... 000 3 Methods ................................................................................................................................ 000 4 Results .................................................................................................................................. 000 4.1 Guadalupian–Lopingian Lazarus Effect ..................................................................... 000 4.2 Patterns in Permian Silicification ............................................................................... 000 4.3 Early Triassic Lazarus Effect ...................................................................................... 000 4.4 Patterns in Early Triassic Silicification ....................................................................... 000 5 Conclusions .......................................................................................................................... 000 References .................................................................................................................................. 000 Abstract The biotic crisis of the Middle Permian through Early Triassic is unmatched 7 in the Phanerozoic in terms of taxonomic diversity losses and paleoecological reor- 8 ganization. However, the potential taphonomic bias from post-mortem diagenesis for 9 this crucial time has not been evaluated. We assessed the quality of the fossil record 10 during this interval by quantifying the number of Lazarus taxa using – our own 11 database, data available in the Paleobiology Database and previous compilations. We 12 also quantitatively tested for paleoecological differences between silicified versus 13 M.L. Fraiser ( ) Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53203, USA e-mail: [email protected] M.E. Clapham Department of Earth and Planetary Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA e-mail: [email protected] D.J. Bottjer Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA e-mail: [email protected] P.A. Allison and D.J. Bottjer (eds.), Taphonomy, Second Edition: Process and Bias Through Time, Topics in Geobiology 32, DOI 10.1007/978-90-481-8643-3_16, © Springer Science+Business Media B.V. 2010 M.L. Fraiser et al. 14 non-silicified faunas. Herein we report that there is no major taphonomic bias due to 15 skeletal mineralogy or fossil preservation affecting the Middle and Late Permian fos- 16 sil record, but that aragonite-shelled molluscs may exhibit a significant Lazarus effect 17 during the Induan. We propose that a variety of mechanisms affected the fossil record 18 of the Paleozoic/Mesozoic transition, including ocean chemistry, paleobiology of the 19 examined groups, and human influences on taxonomic and sampling practices. 20 1 Introduction 21 Mass extinctions are geologically short intervals of time when biodiversity losses 22 are significantly elevated above background rates of extinction (e.g. Jablonski 23 1986a; Sepkoski 1986; Flessa 1990). They are a prominent feature of the fossil 24 record and, along with the rise and fall of the three great evolutionary faunas, 25 shaped the Phanerozoic biodiversity curve (Raup and Sepkoski 1982; Sepkoski 26 1981, 1984; Courtillot and Gaudemer 1996). Mass extinctions are also important 27 agents of macroevolutionary change because they eliminate successful groups of 28 organisms and create new evolutionary opportunities for previously minor groups 29 (Gould and Calloway 1980; Jablonski 1986a, b, 2001, 2005; Raup 1986, 1994; 30 Erwin 2001; Bambach et al. 2002). A complete understanding of the evolutionary 31 role of a mass extinction must include more than just an analysis of the taxonomic 32 crisis because the effects of mass extinctions extend beyond the biodiversity 33 losses: the aftermaths may be as important as the extinctions themselves because 34 the new ecological patterns arising from survivors that interact in new ways in less 35 crowded ecological niches (Droser et al. 1997, 2000; Erwin 2001; Bambach et al. 36 2002; Jablonski 2001, 2002). 37 Proper interpretation of the duration, magnitude, and causes of mass extinctions 38 and the nature of the survival and recovery of organisms during their aftermaths is 39 contingent upon accurate reconstruction of taxonomic and ecological changes. 40 Artifacts of sampling methods or taxonomic practice can obscure the real trends 41 (Sepkoski 1986; Flessa 1990), whereas taphonomic biases inherent in the geologic 42 record, such as mode of organism preservation (Schubert et al. 1997), rock volume 43 (e.g., Crampton et al. 2003), and preferential loss of organisms with aragonitic shell 44 mineralogy (e.g. Cherns and Wright 2000) may also influence observed patterns. 45 Such taphonomic biases may have obscured the true biotic patterns during the 46 Permian–Triassic extinction and its aftermath. These potentially confounding 47 effects have been inferred from the abundance of Lazarus taxa – taxa that temporar- 48 ily disappear from the fossil record but reappear later unchanged (Flessa and 49 Jablonski 1983) – and from a decrease in preservation by silicification (Erwin and 50 Pan 1996; Schubert et al. 1997; Twitchett 2001). Lazarus taxa may be an indicator 51 of the quality of the fossil record if the phenomenon reflects a failure of certain 52 organisms to be preserved through taphonomic effects such as the Signor–Lipps 53 effect, outcrop area bias, paleolatitudinal sampling bias, or reduced preservation 54 quality (Signor and Lipps 1982; Allison and Briggs 1993; Erwin and Pan 1996; 16 Mass Extinctions and Changing Taphonomic Processes Smith and McGowan 2007). Lazarus taxa abundance may also be due to biological 55 factors such as reduced population size (which may also affect the chance of sam- 56 pling a taxon) or reduced geographic range and migration to refugia. (Jablonski 57 1986a,b; Kauffman and Harries 1996; Wignall and Benton 1999; Twitchett 2001; 58 Rickards and Wright 2002). Taxonomic uncertainty can cause an apparent Lazarus 59 effect (Wheeley and Twitchett 2005). 60 Herein we test two aspects of the quality of the fossil record during the 61 Guadalupian, Lopingian, and Early Triassic. The end-Guadalupian and end-Permian 62 extinctions marked the end of the Paleozoic (Fig. 1) and heralded major changes in 63 benthic marine ecology (e.g. Fraiser and Bottjer 2007; Clapham and Bottjer 2007a, b), 64 but several studies have proposed that taphonomic processes make it difficult to 65 extract real ecological patterns during these key intervals in evolutionary history 66 (e.g. Erwin and Pan 1996; Twitchett 2001). First, we quantified the number of Lazarus 67 taxa among several key taxonomic groups, as an increased number of Lazarus taxa 68 may indicate reduced preservation quality. Second, a potential source of bias in the 69 fossil record for this interval, changes in preservation via silicification, was tested 70 by quantifying the proportion of silicified fossil collections, comparing the alpha 71 diversity of silicified and non-silicified (preserved as molds and casts) collections, 72 and assessing the number of taxa exclusive to silicified collections. Silicification is 73 important because it allows fossils to be acid-etched and freed from calcareous 74 matrix, often preserving very fine morphological details and improving ease of iden- 75 tification by taxonomists (e.g. Holdaway and Clayton 1982). It can also preserve a 76 more faithful record of the original diversity and abundance within an assemblage 77 (Cherns and Wright 2000; Wright et al. 2003, Butts and Briggs, this volume). Results 78 of this test will reveal any temporal trends in silicification and the extent to which 79 silicified faunas preserve a higher fidelity record. Together these tests document the 80 Fig. 1 Geologic timescale of Middle Permian (Guadalupian), Late Permian (Lopingian), and Early Triassic stages. Ch = Changhsingian, Ind = Induan. The lower panel shows the per-capita extinction rates (Foote 2000) for rhynchonelliform brachiopods, bivalves, and gastropods in each stage based on data from Clapham et al. (2009) (Permian invertebrates), Chen et al. (2005) (Early Triassic brachiopods), Gastrobase (Early Triassic gastropods), and the Paleobiology Database (Early Triassic bivalves and gastropods). The per-capita extinction for rhynchonelliform brachio- pods is undefined in the Induan because no genera cross both bottom and top boundaries of the stage M.L. Fraiser et al. 81 taphonomic quality of the Permian–Triassic fossil record in greater detail and elucidate 82 the impact of temporal trends of taphonomic bias on the records of the end-Guadalupian 83 extinction, the end-Permian extinction, and their aftermaths. 84 2 Previous Understanding of Biases in the Middle 85 Permian to Early Triassic Fossil Record 86 2.1 End-Guadalupian Extinction