Gastropod evidence against the Early Triassic Lilliput effect: REPLY

Downloaded from geology.gsapubs.org on February 8, 2010 Gastropod evidence against the Early Triassic Lilliput effect Arnaud Brayard1*, Alexander Nützel2, Daniel A. Stephen3, Kevin G. Bylund4, Jim Jenks5, and Hugo Bucher6,7 1 UMR 5561 CNRS Biogéosciences, Université de Bourgogne, 21000 Dijon, France Bayerische Staatssammlung für Paläontologie und Geologie, Ludwig-Maximilians-University Munich, Department für Geo- und Umweltwissenschaften, Sektion für Paläontologie, 80333 München, Germany 3 Department of Earth Science, Utah Valley University, Orem, Utah 84058, USA 4 140 South 700 East, Spanish Fork, Utah 84660, USA 5 1134 Johnson Ridge Lane, West Jordan, Utah 84084, USA 6 Paläontologisches Institut und Museum, Universität Zürich, 8006 Zürich, Switzerland 7 Department of Earth Sciences, ETH Zürich, Switzerland 2 ABSTRACT Size reduction in the aftermath of the Permian-Triassic mass extinction event has repeatedly been described for various marine organisms, including gastropods (the Lilliput effect). A Smithian gastropod assemblage from Utah, USA, reveals numerous large-sized specimens of different genera as high as 70 mm, the largest ever reported from the Early Triassic. Other gastropods reported from Serbia and Italy are also as large as 35 mm. Size frequency distributions of the studied assemblages indicate that they were not unusually small when compared with later Mesozoic and modern faunas. The occurrence of large-sized gastropods less than 2 Ma after the Permian-Triassic mass extinction refutes the Lilliput hypothesis in this clade, at least for the last ~75% of the Early Triassic. INTRODUCTION It is generally assumed that several groups of marine organisms, including gastropods, were affected by a pronounced size reduction in the aftermath of the Permian-Triassic (P-T) mass extinction (Fraiser and Bottjer, 2004; Fraiser et al., 2005; Payne, 2005; Twitchett, 2007). The temporary size reduction of organisms inhabiting postcrisis biota has been termed the Lilliput effect (Urbanek, 1993). According to several authors (e.g., Fraiser and Bottjer, 2004; Payne, 2005), Early Triassic gastropod faunas are strongly dominated by microgastropods (defined as smaller than 10 mm) and larger sizes are rare, if not absent. Mass occurrences of microgastropods were identified as a typical Early Triassic biofacies (Fraiser et al., 2005). In contrast, a considerable number of large gastropod species have been reported from the Permian as well as from the Middle and Late Triassic. Compatible with a delayed recovery model (e.g., Alroy, 2008), a small body size in the aftermath of the P-T mass extinction has been interpreted as a consequence of potential recurrent or prolonged harsh environmental conditions including, for example, anoxia, productivity decline, or size selection during the mass extinction and its recovery, as well as low predation and competitive pressure or collapse of food webs (Fraiser and Bottjer, 2004; Fraiser et al., 2005; Payne, 2005; Twitchett, 2001, 2007). Only a few doubts have been raised as to whether the Early Triassic Lilliput effect is really such a significant phenomenon (Nützel, 2005a, 2005b; McGowan et al., 2009; Nützel et al., 2010). Small gastropods are common in *E-mail: arnaud.brayard@u-bourgogne.fr. many fossil and extant assemblages. However, an absence of very large specimens (~100 mm) seemed to be true for the Early Triassic time interval. Thus, most present-day gastropods are still Lilliputians and it appears that it is more the absence of Early Triassic Gulliver specimens that is remarkable (Nützel et al., 2010). Here we present a new Early Triassic gastropod assemblage from an unusual setting within a muddy facies of the Thaynes Group of western Utah, USA (see GSA Data Repository Fig. DR11). It yielded numerous large (>40 mm) specimens. Moreover, a relatively large gastropod is also reported from the Sinbad Formation of the Thaynes Group where it interfingers with the Moenkopi Group (Smithian, southeastern Utah). We also studied new gastropod faunas of the Upper Werfen Formation (Spathian, Serbia and Italy); their size distributions indicate that they are not unusually small. These new data as well as brief reports of normal-sized gastropods in the literature strongly suggest that the Lilliput phenomenon in the aftermath of the P-T mass extinction is equivocal. DATA AND RESULTS A gastropod collection from the Thaynes Group (Smithian, Utah) and two collections from the Upper Werfen Formation (Spathian, European Alps and adjacent areas) were measured. The studied material from western Utah comes 1 GSA Data Repository item 2010029, location map of the studied assemblages and a biostratigraphical scale with occurrences of large gastropods, is available online at www.geosociety.org/pubs/ft2010.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. from the base of the Thaynes Group (Brayard et al., 2009a; Stephen et al., 2009). Based on a biostratigraphical zonation by ammonoids and conodonts, this assemblage is early and middle Smithian in age (below and within the Meekoceras beds; Fig. DR2). Calibration of the ammonoid zonation by means of U/Pb ages indicates that these gastropods postdate the P-T boundary by 1–2 Ma (Galfetti et al., 2007). Large gastropods from western Utah were found in both shale intervals and in limestone beds of the studied section (Brayard et al., 2009a; Stephen et al., 2009). The gastropod preservation is relatively poor, and steinkern preservation is common. The specimens represent at least three genera and are assigned tentatively to Naticopsis, Omphaloptycha, and “Polygyrina” sp. (Fig. 1), which represent common taxa during this time interval. The studied section is probably slightly older than the basal Sinbad Formation (Goodspeed and Lucas, 2007), which has produced a rich microgastropod fauna in southeastern Utah (Batten and Stokes, 1986; Fraiser and Bottjer, 2004; Nützel and Schulbert, 2005). The largest “Polygyrina” sp. specimens are 68 and 61 mm high, and several exceed a height of 40 mm (Figs. 1C–1H). The largest specimen of the ?Naticopsis-?Omphaloptycha group is 38 mm high and 28 mm wide (Fig. 1A); 71% are higher than 10 mm (Fig. 2). Most of the studied specimens are incomplete, so their original size was considerably larger. In addition, the discovery of a relatively large specimen from the Sinbad Formation (southeastern Utah, Smithian; Fig. DR1) is reported here. Tempestitic shell beds of the Sinbad Formation yielded a diverse and abundant gastropod fauna (Batten and Stokes, 1986). With a few exceptions, these gastropods usually do not exceed 10 mm and have been classified as microgastropods (e.g., Batten and Stokes, 1986; Fraiser and Bottjer, 2004). However, a teleoconch fragment of a high-spired gastropod from the Mexican Hat location (Nützel and Schulbert, 2005) suggests that this specimen was at least as high as 56 mm and as wide as 20 mm (Fig. 3). Both collections from the Upper Werfen Formation consist of two species: Natiria costata and Werfenella rectecostata, commonly found © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. GEOLOGY, February 2010 Geology, February 2010; v. 38; no. 2; p. 147–150; doi: 10.1130/G30553.1; 4 figures; Data Repository item 2010029. 147 Downloaded from geology.gsapubs.org on February 8, 2010 130 A 120 Sinbad 110 Omphaloptycha + Naticopsis 100 Frequency 90 80 Studied Thaynes assemblage n = 472 70 60 50 40 30 20 10 0 0 4 8 12 16 20 24 28 32 36 40 B Median = 11.4 Height (mm) Figure 2. A: Black is height distribution of sampled specimens belonging to?Naticopsis-?Omphaloptycha group from the early and middle Smithian Thaynes Group. Gray is previously published height histogram (Fraiser and Bottjer, 2004) of microgastropod specimens belonging to Naticopsis-Omphalopthycha group from almost time-correlative Sinbad Limestone of southeastern Utah. Note that height range is different and considerably enlarged for Thaynes Group of western Utah. B: Plot showing median, range, extremes, and 25 and 75 percentiles of sampled specimens of ?Naticopsis-?Omphaloptycha group from Thaynes Group of western Utah. Figure 1. A, B: Large-sized specimens (Université de Bourgogne Géologie Dijon, UBGD 277109 and UBGD277110, respectively) of ?Naticopsis-?Omphaloptycha group. C–H: ”Polygyrina” specimens (UBGD27111 to UBGD27116) from the early and middle Smithian (Early Triassic) Thaynes Group of western Utah (Fig. DR1; see footnote 1). Scale bars are 10 mm. in this area within low-diversity assemblages. Figure 4 shows size frequency distributions for these taxa from the two studied collections. Both species have a median size >10 mm and a maximum size >25 mm. For both species, a maximum size of ~35 mm is reported (Frech, 1912; Nützel, 2005a, 2005b; Nützel et al., 2010). The literature was reviewed for particularly large Early Triassic gastropod specimens. There are reports from different paleolatitudes and paleoenvironments of relatively large species of the genus Naticopsis, such as a species >20 mm from the Griesbachian of Oman (Wheeley and Twitchett, 2005). Naticopsis arctica from the Griesbachian of Greenland is also 30 mm high and wide (Spath, 1935). 148 DISCUSSION The abundance and predominance of small gastropods have been identified as a global phenomenon covering the entire Early Triassic time interval as the consequence of prolonged unfavorable conditions (e.g., Fraiser and Bottjer, 2004; Payne, 2005). Such size variations are commonly interpreted as a proxy for changes in environmental conditions during and after the P-T mass extinction (e.g., He et al., 2007). Hautmann and Nützel (2005) pointed out that the potential Early Triassic size reduction could have contributed to the Lazarus effect; they reported the first Early Triassic heterodont bivalve, Sinbadiella, an unusually small bivalve that had escaped previous discovery. (Heterodonts had been known from the Paleozoic and the MiddleLate Triassic to present day; the description of Sinbadiella thus closed this Lazarus gap.) With a shell height of ~70 mm the Smithian gastropods from Utah studied here represent the largest gastropods ever reported from the Early Triassic. These and other reports of relatively large specimens ask the question whether the Early Triassic Lilliput effect is a real phenomenon, at least for Smithian and Spathian gastropods. Rare doubts were raised as to whether the Early Triassic Lilliput effect is really such a significant phenomenon in gastropods (Nützel, 2005a, 2005b; Nützel et al., 2010), especially because most modern gastropods are smaller than 10 mm. For example, ~60% of ~2600 shelled present-day mollusk species from tropical New Caledonia have an adult size smaller than 10 mm, and ~90% are smaller than 40 mm (Bouchet et al., 2002). Small specimens are common and even dominant in many fossil gastropod assemblages. In Nützel et al. (2010), it will be argued that it is rather the absence or scarcity of large Early Triassic gastropod GEOLOGY, February 2010 Downloaded from geology.gsapubs.org on February 8, 2010 38 A A 5 36 4 32 Frequency Werfenella rectecostata Natiria costata 28 3 24 20 n = 13 16 2 n = 143 12 1 8 4 0 0 56 mm 0 4 8 12 16 20 24 28 B 32 0 4 8 12 16 20 24 28 32 B Median = 11.4 Width (mm) Median = 17.1 Width (mm) Figure 4. A: Size distributions of studied specimens from Spathian Upper Werfen Formation, belonging to species Natiria costata and Werfenella rectecostata. These specimens come from Bletterbach Gorge at Mount Weißhorn, Italian Alps (housed in the Bayerische Staatssammlung für Paläontologie, BSPG) and from western Serbia (Humboldt Museum, Berlin, MB; Ga.3809.1–3809.14). B: Plot showing median, range, extremes, and 25 and 75 percentiles of same studied specimens from Upper Werfen Formation. Figure 3. Teleoconch fragment of large highspired gastropod from Sinbad Limestone (Mexican Hat, southeastern Utah). specimens (see also Payne, 2005) than the presence of minute gastropods that is unusual for this time interval. Thus, Gulliver absence seems to be more accurate than Lilliput presence, as most living gastropods are still Lilliputians (Nützel et al., 2010). The new data presented here corroborate the view that the significance of Early Triassic gastropod size reduction has been overestimated. The cited suggestions that Early Triassic gastropods are particularly small (i.e., microgastropods) are in conflict with the present reports of relatively large or normal-sized gastropods from Utah, Serbia, and Italy. Our large specimens from western Utah can be considered as Gullivers compared to most other described Early Triassic gastropods. Maximum size, size frequency distribution, and a mean size >10 mm (Fig. 2) all suggest that this is not a microgastropod fauna (sensu Fraiser and Bottjer, 2004; Payne, 2005; Twitchett, 2007). Older reports of Griesbachian GEOLOGY, February 2010 Naticopsis species that are larger than 20 mm (Spath, 1935; Wheeley and Twitchett, 2005) and gastropods from the Upper Werfen Formation as large as 35 mm (Frech, 1912; Neri and Posenato, 1985; Nützel, 2005a) also reinforce the hypothesis that the Lilliput phenomenon is equivocal. Moreover, the size frequency distributions (Figs. 2 and 4) of these new Early Triassic gastropod assemblages suggest a body size that is even larger than, for example, that of a gastropod collection from the late Carnian Cassian Formation, which is very diverse and well structured and was therefore probably not environmentally stressed (Nützel et al., 2010). The assumption that Early Triassic gastropods (and other organisms) were generally smaller than during other periods therefore needs more substantiation (see McGowan et al., 2009). The Early Triassic diversification patterns of benthic organisms are less well documented than for conodonts or ammonoids and have directly influenced the classical view of a delayed recovery after the P-T mass extinction. However, provided that the delayed character of the Early Triassic diversification is real (Brayard et al., 2009b), the large size of the Smithian specimens studied here could represent an early recovery phenomenon: an improvement of environmental conditions could have allowed a size increase. In this case, large taxa must have evolved from smaller Griesbachian or Dienerian ancestors. Sampled sections from western Utah do not show any sign of anoxia or oxygen-deficient levels corresponding to the gastropod intervals pertaining to this discussion. Nevertheless, it is currently impossible to state that these apparent nonanoxic conditions primarily influenced and favored large-sized gastropods, as suggested for other faunas (Twitchett, 2007). Another possibility is that sampling and taphonomic biases influenced the record of gastropod size. The Early Triassic time interval is notorious for its poor gastropod preservation and its sparse overall fossil gastropod record (Nützel, 2005b). Nearly half of the known named gastropod species occur within just two formations, the Sinbad and the Werfen (Nützel and Schulbert, 2005). Many of these gastropod occurrences are from well-sorted tempestitic shell beds that contrast with the muddy facies of the new assemblages from Utah. These kinds of tempestitic deposits involve transport and size sorting. At least some of the Gulliver gastropods from western Utah could represent fully grown specimens of species that were only present as juveniles in the microgastropod fauna of the Sinbad Formation. The Sinbad Formation gastropod fauna occurs in tempestites (Nützel and Schulbert, 2005), and proximity of such deposits produces size-sorting effects. However, the large gastropods from the Thaynes Group are not preserved well enough to make comparisons with species found in the neighboring Sinbad Formation. The possibilities of ecological size limitation and sampling or taphonomic biases are not exclusive; both could contribute to Early Triassic gastropod size distributions: anoxia, abnormal salinity, turbidity, and substrate type on the one hand, and preservation and size sorting during transportation on the other. 149 Downloaded from geology.gsapubs.org on February 8, 2010 CONCLUSIONS With median shell heights >10 mm and maximum sampled size up to to 70 mm, gastropods from the Thaynes Group and the Sinbad Formation of Utah are the largest gastropods ever reported from the Early Triassic. Other reports also suggest that not all Early Triassic gastropods were microgastropods. The abundance of gastropods smaller than 10 mm is not only typical of the Early Triassic; many present-day and fossil gastropod assemblages show this small size distribution. Based on these observations, proposals that median sizes of Early Triassic gastropods are generally smaller than 10 mm and maximum sizes do not exceed 20–40 mm can be rejected. The large specimens reported here suggest that the Lilliput effect is an artifact or, at least, not as significant as previously thought. If the Lilliput effect (or Gulliver absence) nevertheless represents a real biological phenomenon, then the rebound of gastropod size occurred earlier than previously assumed. In that case, this could support the hypothesis that recovery was rapid after the P-T mass extinction (Brayard et al., 2009b). Numerous Lazarus taxa among bivalves, gastropods, and ammonoids reappeared during the Smithian (e.g., Hautmann and Nützel, 2005; Nützel, 2005b; Brayard et al., 2007). This short time interval also shows a major diversification of ammonoids and conodonts (Brayard et al., 2006, 2009b; Orchard, 2007). Indeed, the simultaneous reappearance of large gastropods suggests that recovery in the marine realm was well under way during this time, less than 2 Ma after the mass extinction. ACKNOWLEDGMENTS This article is a contribution to the team “Forme, Evolution, Diversité” of the UMR-5561 CNRS Biogéosciences (Brayard). Nützel acknowledges a research grant from the Deutsche Forschungsgemeinschaft (NU 96/6-1, 6-2). D. Stephen acknowledges research grant support for this project from the Scholarly Activities Committee, Utah Valley University. This work was supported by the Swiss National Science Foundation project 200020-113554 (Bucher). We thank N. Goudemand (Zurich) for his help with conodont determinations. We are grateful to both reviewers and G. Escarguel for their constructive comments, which helped us to improve the manuscript. 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