Early Triassic Gulliver gastropods: Spatio-temporal ...

Earth-Science Reviews 146 (2015) 31–64

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Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev

Early Triassic Gulliver gastropods: Spatio-temporal distribution and significance for biotic recovery after the end-Permian mass extinction Arnaud Brayard a,⁎, Maximiliano Meier b, Gilles Escarguel c, Emmanuel Fara a, Alexander Nützel d, Nicolas Olivier e, Kevin G. Bylund f, James F. Jenks g, Daniel A. Stephen h, Michael Hautmann b, Emmanuelle Vennin a, Hugo Bucher b a

UMR CNRS 6282 Biogéosciences, Université de Bourgogne, 6 boulevard Gabriel, 21000 Dijon, France Paläontologisches Institut und Museum, Universität Zürich, 8006 Zürich, Switzerland c UMR CNRS 5276, Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, Université Claude Bernard Lyon 1, 27-43 Boulevard du 11 novembre 1918, 69622 Villeurbanne Cedex, France d SNSB-Bayerische Staatssammlung für Paläontologie und Geologie, Department of Earth and Environmental Sciences, Palaeontology & Geobiology, GeoBio-Center LMU, Richard-Wagner-Str. 10, 80333 München, Germany e Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS, IRD, OPGC, 5 rue Kessler, 63038 Clermont Ferrand, France f 140 South 700 East, Spanish Fork, UT 84660, USA g 1134 Johnson Ridge Lane, West Jordan, UT 84084, USA h Department of Earth Science, Utah Valley University, 800 West University Parkway, Orem, UT 84058, USA b

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Article history: Received 19 October 2014 Accepted 18 March 2015 Available online 28 March 2015 Keywords: Gastropods Early Triassic Lilliput effect Body size Sampling effect Biotic recovery

a b s t r a c t A reduction in body size (Lilliput effect) has been repeatedly proposed for many marine organisms in the aftermath of the Permian–Triassic (PT) mass extinction. Specifically-reduced maximum sizes of benthic marine invertebrates have been proposed for the entire Early Triassic. This concept was originally based on observations on Early Triassic gastropods from the western USA basin and the Dolomites (N Italy) and it stimulated subsequent studies on other taxonomic groups. However, only a few studies have tested the validity of the Lilliput effect in gastropods to determine whether the paucity of large-sized gastropods is a genuine signal or the result of a poor fossil record and insufficient sampling. In combination with a review of the literature, we document numerous new, abundant, large-sized gastropods from the Griesbachian outcrops of Greenland and from the Smithian–early Spathian interval in the southwestern USA. We show that large-sized (“Gulliver”) gastropods (i) were present soon after the PT mass extinction, (ii) occurred in various basins, sedimentary facies and environmental contexts (from shallow to deeper settings), and (iii) belong to diverse higher-rank taxa. Focusing on the western USA basin, we investigate areas from which microgastropod shell-beds were previously presented as being typical. However, we show that Gulliver gastropods do occur in the very same areas. Insufficient sampling effort is probably the main reason for the rarity of reports of large Early Triassic gastropods, which is supported by preliminary rarefaction-based simulations. Finally, it appears that the recently documented middle to late Smithian climate shifts and the severe end-Smithian extinction of nekto-pelagic faunas did not reduce maximum shell sizes of gastropods. © 2015 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . Lilliput and Gulliver gastropods in living and fossil assemblages Previous reports of Early Triassic Gulliver occurrences . . . . . Newly sampled Gulliver collections: the fieldwork contribution Griesbachian Gullivers from Greenland . . . . . . . . . . . . 5.1. Geological and biostratigraphical settings . . . . . . . 5.2. Gulliver gastropods . . . . . . . . . . . . . . . . .

⁎ Corresponding author. E-mail address: [email protected] (A. Brayard).

http://dx.doi.org/10.1016/j.earscirev.2015.03.005 0012-8252/© 2015 Elsevier B.V. All rights reserved.

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6.

Smithian–early Spathian Gullivers from western USA . . . . . . . . . . . . . . . . . . . 6.1. Geological and biostratigraphical settings . . . . . . . . . . . . . . . . . . . . . 6.2. Gulliver gastropods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1. General observations . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2. Gullivers as micro-habitat for epibionts . . . . . . . . . . . . . . . . . . 6.2.3. Local lateral variations . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Sampling and preservation effects . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1. Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.2. Preservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Distribution within the western USA basin: local and regional controls . . . . . . . . 7.3. Early Triassic spatio-temporal distributions . . . . . . . . . . . . . . . . . . . . . 7.4. Gulliver gastropods vs. global Early Triassic environmental changes . . . . . . . . . . 7.5. Is there an Early Triassic Lilliput effect? . . . . . . . . . . . . . . . . . . . . . . 7.5.1. Lilliput effect sensu stricto . . . . . . . . . . . . . . . . . . . . . . . . 7.5.2. Lilliput effect sensu lato . . . . . . . . . . . . . . . . . . . . . . . . . 7.5.3. The paradox of the western USA basin and the risk of across-scale extrapolation 8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Outlook: research challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Recovery from the devastating end-Permian mass extinction is mostly assumed to be a delayed process, spanning at least the entire Early Triassic (~ 5 Myr). More precisely, the crisis aftermath is portrayed as a time of high ecological stress characterized by changes in water temperature (Sun et al., 2012; Romano et al., 2013), large-scale fluctuations of the global carbon cycle and harsh marine conditions, including a combination of ocean acidification, anoxia, euxinia, and fluctuating productivity (e.g., Payne et al., 2004, 2010; Galfetti et al., 2007a,b; Horacek et al., 2007; Payne and Kump, 2007; Hinojosa et al., 2012; Clarkson et al., 2013; Grasby et al., 2013). This suggests tight links between these environmental variables and the restructuring of ecosystems, but the actual drivers of this long-term process still remain elusive. Associated with the scenario of a delayed recovery, which was mainly based on global diversity patterns of benthic organisms such as bivalves, gastropods, brachiopods, crinoids or corals, several other Early Triassic globalscale paradigms such as a proposed “reef gap” or a “Lilliput effect” (see Erwin, 2006) have been discussed intensively. Contrasting with these phenomena, recent analyses of nekto-pelagic taxa such as ammonoids and conodonts document a non-delayed, explosive Early Triassic re-diversification (Orchard, 2007; Brayard et al., 2009b). Similarly, metazoan reefs, commonly acknowledged not to have been re-established until the Middle Triassic, have been recently reported from the Early Triassic of the western USA, suggesting a fast reef rebuilding wherever permitted by environmental conditions (Brayard et al., 2011b; Marenco et al., 2012; Olivier et al., in press; Vennin et al., 2015), although large metazoan reefs were certainly not as widespread and frequent in the Early Triassic than in the Late Triassic. The contention that Early Triassic benthic faunas were generally depauperate has become controversial over the past few years, as shown by the diversified assemblages recently described from the Griesbachian of various latitudes in South China (Kaim et al., 2010; Hautmann et al., 2011), Oman (Twitchett et al., 2004; Wheeley and Twitchett, 2005) and Italy (Hofmann et al., 2011, in press); the Griesbachian–Dienerian interval of the Canadian Arctic (Zonneveld et al., 2007; Beatty et al., 2008); the Griesbachian–Smithian interval of South Primorye (Far East Russia; Shigeta et al., 2009); and the Griesbachian–early Spathian interval of the western USA (McGowan et al., 2009; Hautmann et al., 2011; Hofmann et al., 2013a,b, 2014).

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Although several relatively diverse Early Triassic marine invertebrate faunas have been reported during the recent years, the diversity of Late Triassic faunas such as assemblages from the early Carnian Cassian Formation is much higher than that of any known Early Triassic fauna (e.g., Hausmann and Nützel, 2015). One of most prominent Early Triassic paradigms is the “Lilliput effect”, i.e., a temporary body-size reduction within survivor clades. It was first proposed for Silurian graptolites at the species level by Urbanek (1993) and later suggested for other time intervals and taxa, including several marine Early Triassic clades: foraminifers (Song et al., 2011; Rego et al., 2012), bivalves (Hautmann and Nützel, 2005; Twitchett, 2007; Posenato, 2009; Metcalfe et al., 2011), gastropods (see references below), brachiopods (He et al., 2007, 2010, 2015; Peng et al., 2007; Leighton and Schneider, 2008; Chen et al., 2009; Posenato, 2009; Metcalfe et al., 2011; Posenato et al., 2014), ostracods (Forel, 2013), ophiuroids (Twitchett et al., 2005), fishes (Mutter and Neuman, 2009), sponges (Liu et al. 2013) and trace fossils (e.g., Twitchett and Barras, 2004; Twitchett, 2007). Luo et al. (2008) and Chen et al. (2013) also recorded a body-size reduction for some conodont lineages in South China. However, these are restricted to a few conodont zones during the immediate Permian–Triassic (PT) extinction aftermath and the following end-Smithian crisis, respectively. Gastropods were among the first organisms used as a model for the Lilliput effect, mainly based on faunas from the western USA basin (e.g., Batten, 1973; Batten and Stokes, 1986; Schubert and Bottjer, 1995; Fraiser and Bottjer, 2004). Later, it was proposed that the Lilliput effect in Early Triassic gastropods was a global phenomenon spanning the entire Early Triassic (e.g., Fraiser and Bottjer, 2004; Fraiser et al., 2005; Payne, 2005). Several “microgastropod” (arbitrarily defined as adult specimens smaller than 1 cm; Fraiser and Bottjer, 2004) assemblages have been reported from the Early Triassic (e.g., Batten and Stokes, 1986; Schubert and Bottjer, 1995; Twitchett and Wignall, 1996; Lehrmann et al., 2003; Boyer, 2004; Fraiser and Bottjer, 2004; Fraiser et al., 2005; Nützel and Schulbert, 2005; Sano et al., 2012). Up to now, most of the studies dealing with the Lilliput effect in Early Triassic gastropods have focused on the proliferation of microgastropods and especially on the absence of large specimens, i.e., a reduction of the maximum size in comparison to Permian and Middle to Late Triassic faunas. However, Brayard et al. (2010, 2011a) documented numerous large-sized specimens (up to an estimated maximum size of ~ 10 cm) representing several gastropod genera and higher taxa from Smithian (i.e., ~1 Myr after the PT extinction; Galfetti et al., 2007b) outcrops of


the western USA basin. These assemblages also indicate that large-sized specimens were not unusual and that size––frequency distributions are comparable to later Mesozoic and modern gastropod faunas (Brayard et al., 2010, 2011a). As argued by Fraiser and Bottjer (2004), Payne (2005) and Nützel et al. (2010), it is more the absence or scarcity of large-sized gastropods, i.e., “Gulliver” specimens, that until recently appeared to be remarkable for the Early Triassic. Occurrence of large-sized gastropods in the western USA basin, combined with other worldwide records (Brayard et al., 2010, 2011a) thus questioned the Lilliput hypothesis for this clade. Is the paucity of large-sized gastropods a genuine signal or the outcome of a still inadequately sampled, and thus poorly known fossil record? Answering this question is crucial to test the validity of the Lilliput effect in the context of the Early Triassic biotic recovery. The present study documents numerous new, abundant, large-sized gastropods from Griesbachian outcrops of Greenland and from the Smithian–early Spathian interval in the western USA basin. In addition, reports of large Early Triassic gastropods from the literature are analyzed. We show that Gulliver gastropods (i) were present soon after the PT mass extinction, (ii) occurred in various basins, sedimentary facies and environmental contexts (from shallow to deeper settings), and (iii) belong to different genera and higher-rank taxa. Because Smithian–early Spathian specimens from the western USA basin are abundant and well dated, they provide an appropriate basis for a discussion of the potential factors influencing their spatial distribution and sampling. Focusing on this basin, we also investigated areas from which microgastropod shell-beds were previously reported to be typical (Batten and Stokes, 1986; Schubert and Bottjer, 1995; Boyer, 2004; Fraiser and Bottjer, 2004; Fraiser et al., 2005; Nützel and Schulbert, 2005; Pietsch et al., 2014), but our observations suggest that size distributions are much broader and comparable to other Phanerozoic assemblages. Taken together with other worldwide reports describing Gulliver faunas, they indicate that the Lilliput effect on gastropods was based on incomplete size spectra and cannot be used to characterize post-extinction deleterious environmental conditions. Furthermore, these data show that miniaturization is inapplicable to the most common taxonomically-identified gastropod genera in the aftermath of the PT extinction and does not represent a trend at the clade level. 2. Lilliput and Gulliver gastropods in living and fossil assemblages Published shell size–frequency distributions based on specimens measured individually for entire, local or regional, fossil and modern gastropod assemblages are rather scarce (e.g., Beck, 2000; Bouchet et al., 2002; Fraiser and Bottjer, 2004; McClain et al., 2005; Brayard et al., 2010; Finnegan et al., 2011; Waite and Strasser, 2011; Nawrot, 2012; Pietsch et al., 2014). This understandably results from the overly excessive time required to collect and measure all sampled specimens. Another difficulty when comparing fossil and modern gastropod assemblages relates to potential biases affecting the original size–frequency distribution (e.g., Webber, 2005; Zuschin et al., 2005, 2006; Cooper et al., 2006; Finnegan et al., 2011; Nawrot, 2012). These sources of bias are the result of: (i) environmental (e.g., bathymetric or latitudinal gradient, local habitat heterogeneity), (ii) biological/behavioral (e.g., adult vs. juvenile spatio-temporal distribution, habitat/feeding preferences), (iii) taphonomic (e.g., size-selective transport, lateral variations, post-mortem reworking, lithification, shell preservation), (iv) sampling (e.g., surface collecting vs. bulk sampling, discarding of the smallest specimens, main effort on largest or best preserved specimens, spatial effort), and (v) analytical effects (e.g., choice of size metrics). The dominance of small-sized species in natural communities is a well-known macroecological pattern (Brown and Maurer, 1989; Brown, 1995; Blackburn and Gaston, 1998; Kozłowski and Gawelczyk, 2002; Morand and Poulin, 2002). Extent mollusk communities are no exception, as shown by the only exhaustive survey by Bouchet et al. (2002) in Koumac, New Caledonia. This pattern is found at multiple

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scales: (i) individuals within a population, (ii) species within a community and, consequently, (iii) individuals within a community (e.g., Jonsson et al., 2005). Fig. 1 illustrates an example for points (ii) and (iii). In the Early Triassic fossil record, the recognition of fullygrown individuals and the taxonomic assignment of gastropod specimens are difficult: only few specimens have shell preservation. Consequently, size distributions can only be assessed at the scale of individuals within a community, regardless of their ontogenetic age. Small adult individuals and species are largely dominant in tropical modern marine faunas, with for instance 74.7% of the New Caledonian gastropod species smaller than 2 cm, and only 9% larger than 4 cm (Bouchet et al., 2002; see below and Fig. 1). Preliminary data obtained from extensive collections of the “normal-sized” and exceptionally well-preserved tropical middle Eocene gastropod assemblage from Grignon (Paris Basin, France) show that ~60% of the 161 sampled genera have a maximum shell size b2 cm and ~20% have a maximum size ≥ 4 cm (Thomas et al., unpublished data). In summary, a majority of fossil and extant gastropod taxa have a maximum adult size smaller than ~2 cm. We use this conservative value as a threshold and refer to “Gullivers” as all gastropod specimens with sizes larger than 2 cm. Because larger gastropod individuals and species are much rarer than smaller ones at all spatial resolutions, size–frequency distributions are highly right-skewed. The probability of sampling large-sized specimens and species is consequently very low, although their preservation potential and good visibility in sediments may compensate to some extent for this statistical expectation. To further investigate the impact of sample size on the sampled shell-size interval for a given species or individual assemblage, we build here on Brayard et al.'s (2011a) preliminary simulation approach. Based on Bouchet et al.'s (2002) extensive sampling, including 77,481 measured adult gastropods belonging to 1941 species, we developed rarefaction-based simulations to separately evaluate the effect of species and individual random sub-sampling upon the sampled range of shell size. The observed distribution of species abundance in the dataset from Bouchet et al. (2002) non-significantly departs from a Log-normal Model (p = 0.16 NS), even if an excess of very rare species suggests that a Zero-Sum Multinomial Model (ZSM) may actually better describe it. On the other hand, this distribution departs highly significantly from a Fisher's logseries distribution, as expected for a local community assemblage. Indeed, under a neutral assumption and point-mutation model, a logseries distribution is expected only at the regional metacommunity level, whereas a Log-normal-like ZSM distribution is expected at the local community level (Hubbell, 2001). In this large sample of an extant New Caledonian gastropod community, adult shell-size (estimated as the largest shell width or height) ranges from 0.4 mm to 30 cm, with an individual median size of 6 mm (inter-quartile range: 3.5–20.5 mm) and a species median size of 7.3 mm (inter-quartile range: 3.5–20 mm) (Fig. 1). While shell-size distributions in raw size-unit for both specimens (Fig. 1a) and species (Fig. 1c) show expected highly right-skewed shapes (with only 206/ 77,481 [0.27%] specimens and 29/1941 [1.49%] species showing an adult shell size ≥ 100 mm; see insets in Fig. 1a and c), the frequency distributions actually correspond to mixtures of two Log-normal distributions (Fig. 1b, d). Log-normal rather than normal (or any other type) distributions are logically expected here due to the geometric (i.e., multiplicative, not arithmetic/additive) nature of biological variation (Gingerich, 2000). Mixture analyses (Titterington et al., 1985; Harper, 1999) of these two empirical distributions using PAST v. 2.17 (Hammer et al., 2001) returned the following maximum-likelihood parameters (in Log10-mm): – Individuals (Fig. 1b): ○ small-sized group (61%): mean = 0.552, standard-deviation = 0.2949 (untransformed mean size [1σ confidence interval]: 3.6 [1.8–7.0] mm);

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○ large-sized group (39%): mean = 1.408, standard-deviation = 0.2023 (untransformed mean size [1σ C.I.]: 25.6 [16.1–40.8] mm); – Species (Fig. 1d): ○ small-sized group (52%): mean = 0.561, standard-deviation = 0.3074 (untransformed mean size [1σ C.I.]: 3.6 [1.8–7.4] mm); ○ large-sized group (48%): mean = 1.298, standard-deviation = 0.3625 (untransformed mean size [1σ C.I.]: 19.9 [8.6–45.8] mm).

Optimum cut-off values (sensu Favre et al., 2008) between the two shell-size groups are 10.8 mm for individuals (making 96% of all sampled individuals correctly assigned to their size-group) and 8.3 mm for species (making 86% of all sampled species correctly assigned to their size-group). Incidentally, it is worth noting that more than half of the species (52%) and individuals (61%) from this extant gastropod assemblage thus fall within the micro-gastropod category (b10 mm defined by Fraiser and Bottjer, 2004, for fossil gastropods). Starting from this large extant New Caledonian sample, we performed rarefaction-based simulations by randomly sampling without replacement sub-sets of individuals or species, observing the changes of the minimum, median and maximum sampled shell sizes with decreasing sampling effort (Fig. 2). Since Sanders (1968), interpolationbased rarefaction is a standard technique routinely used in population genetics, ecology and paleobiology to estimate the variation (usually decrease) of a diversity metric of interest (e.g., allelic or species

richness) with decreasing sampling effort (e.g., Hurlbert, 1971; Heck et al., 1975; Smith and Grassle, 1977; Simberloff, 1979; Coleman, 1981; Kalinowski, 2004, 2005; Hammer and Harper, 2006; Gotelli and Colwell, 2011; Colwell et al., 2012; among many others). On the one hand, by randomly selecting sub-sets of individuals from the full extant sample (each individual having the same sampling probability regardless of its shell size), individual-based rarefaction (Fig. 2a) generates a null model of shell size-independent random loss of information from the parent assemblage due to taphonomic causes and/or less than optimum sampling efforts. On the other hand, by randomly selecting subsets of species from the full extant sample (each species having the same sampling probability regardless of its relative abundance and average shell size in the parent assemblage), species-based rarefaction (Fig. 2b) generates a null model of abundance- and size-independent loss of information due to random species extinction, thus simulating the effect of a “neutral”, abundance- and size-independent extinction crisis on the size–frequency distribution. In both cases, it is worth noting that we performed shell-size-independent random selection of individuals or species. For reasons discussed below (Section 7.1.2), this simple working (null) hypothesis is likely to be satisfactory at a first order of approximation. More complex simulation approaches involving shell-size-dependent random selection of individuals and/or species are currently under development (Escarguel et al., ongoing work). Under this simple working hypothesis of shell-size-independent random sorting, both specimen- and species-based rarefaction simulations clearly show that, whereas the median value of the shell size–frequency distributions remains stable, the [Min–Max] sampled interval strongly


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effect s.l., but rather that large-sized individuals and species may be greatly under-sampled, leading to a spurious right-hand truncation of the size distribution. From this point of view, the minimum, median and maximum shell-size values, obtained from the Early Triassic samples presented and discussed below, rather accurately match the null predictions of our simulation approach (Fig. 3), strongly suggesting that they can be seen as relatively small and species-poor samples randomly selected from a “standard”, non-altered shell size–frequency distribution. Even if such a simple simulation approach does not unambiguously refute the existence of a Lilliput effect in Early Triassic gastropod assemblages, the discovery of abundant, large-sized specimens representing several gastropod taxa in the same areas where a Lilliput effect for gastropods has previously been suggested (e.g., western USA basin) is highly remarkable, given the low Early Triassic gastropod richness (b 100 named species known; Nützel, 2005b). 3. Previous reports of Early Triassic Gulliver occurrences

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1000 2000

Number of sub-sampled species Fig. 2. Results of the rarefaction-based simulations showing the evolution of the minimum (blue), median (black) and maximum (red) sampled adult shell-size with decreasing subsampling of the New-Caledonian modern assemblage (including 77,481 specimens, 1941 species). a) Individual-based rarefaction. b) Species-based rarefaction. Bold curves give the median simulated estimate for each shell-size distribution parameter (Min, Med and Max), and associated thin curves give the related 95% nonparametric confidence intervals, based on: 500, 1000 and 5000 independent random sub-samples for the individual-based rarefaction in the [77,484–10,001], [10,000–1001], and [1000–1] specimen intervals, respectively; and 1000, 5000 and 10,000 independent random sub-samples for the species-based rarefaction in the [1941–501], [500–101], and [100–1] species intervals, respectively.

decreases with sample size. Furthermore, due to the strongly asymmetric (right-skewed) shape of the shell-size distribution (Fig. 1a, c), the Min value only slightly increases as sample size decreases (e.g., exceeding a median estimate of 1 mm for less than ~20 sampled specimens/17 sampled species), whereas the Max value dramatically decreases as sample size decreases (e.g., falling below a median estimate of 10 cm for less than ~350 sampled individuals/46 sampled species, and a median estimate of 5 cm for less than ~50 sampled individuals/11 sampled species). Obviously, this rather counter-intuitive pattern logically results from the strongly right-skewed shape of the parent shell-size distribution: because the largest individuals are (very) rare in the parent assemblage, their probability to be randomly sorted through the rarefaction procedure rapidly becomes so low that they are almost never captured in the rarefied samples below a few hundreds of sampled individuals/tens of sampled species. These simulation results clearly indicate that, when randomly departing from a “standard”, non-altered shell size–frequency distribution, noticeably reduced Max and [Min–Max] range values can be expected without invoking any ad-hoc environmental and/or evolutionary driver(s) when working with relatively small, under-sampled assemblages. Thus, a shift toward small sizes of the whole size distribution (see below, Section 7.5.2) does not necessarily indicate a Lilliput

Fig. 4 displays all known spatio-temporal occurrences of Early Triassic Gullivers. Fraiser and Bottjer (2004, their Fig. 1) and Fraiser and Bottjer (2005, their Figs. 3 and 4) produced the same work for microgastropod beds. The resulting diagrams suggest that Lilliput and Gulliver gastropods may be found in the same regions and time intervals. Gulliver occurrences have not yet been documented from high latitudes, and reports of gastropods from the Boreal domain are rare and specimens have thus far not been illustrated (see Dagys et al., 1979 for Siberia and Mørk et al., 1999 for Spitsbergen). Prior to the study by Brayard et al. (2010), known occurrences of large-sized gastropods were restricted to well-studied Griesbachian assemblages from Oman (Twitchett et al., 2004; Wheeley and Twitchett, 2005) and from brief reports on the Griesbachian–Dienerian interval of northwestern China (Tong and Erwin, 2001) and South Primorye (Kaim, 2009), the Smithian of western Australia (Runnegar, 1969), and the Spathian of South China (Pan, 1982), Qinghai (Zhu, 1995), Serbia (Frech, 1912) and Italy (Neri and Posenato, 1985, Nützel, 2005a,b) (Fig. 4a, d). In addition, Dean (1981, his Pl. 1) illustrated two “representative” specimens (~ 10 and 25 mm in height) from the Smithian Sinbad Formation in the Torrey area (Utah; western USA basin), suggesting the presence of largesized gastropods in this region. Batten and Stokes (1986) mentioned the occurrence of three specimens of Zygopleura, Battenizyga (= Anoptychia) and Coelostylina from the same formation in the San Rafael Swell (Utah) whose respective heights are 18.8 mm, 15.3 mm, and 13.9 mm. Goodspeed (1996) and Goodspeed and Lucas (2007, their Table 1) also reported “large gastropods” from the Sinbad Formation in the San Rafael Swell. Their comment was mainly based on field observations, but some sampled packstones from their collection indeed show several high-spired gastropods ≥ 1.5 cm (S.G. Lucas, personal communication to the first author, 2013). Pietsch et al. (2014) recently illustrated a 17 mm-high specimen of Coelostylina from the same beds. More problematic is the description by Yochelson et al. (1985) of a large specimen of Retispira bittneri (Bellerophon group) with a diameter of 5 cm from the Griesbachian of the Dinwoody Formation in Wyoming. Indeed, as previously discussed by Payne (2005), its stratigraphic position is uncertain, although Yochelson et al. (1985) identified an Early Triassic source for this material (see also Kaim and Nützel, 2011). Brayard et al. (2010) confirmed the occurrence of large-sized Spathian specimens in Serbia and Italy, and they documented abundant Gullivers from the Smithian of west-central Utah. Turculeţ (1987) reported giant specimens of the gastropod Werfenella, the largest one measuring 71 mm in height and 48 mm in width, from the Spathian (“Campilian”) of Romania (Carpathians). Large-sized gastropods have also been recently reported from the Griesbachian of South China (Kaim et al., 2010), the Griesbachian–Dienerian interval of the Salt Range (Kaim et al., 2013) and southeastern Idaho (Hofmann et al., 2013a), and the Smithian of southern Utah (Hofmann et al., 2014; Olivier et al., 2014).

36


2

11

9

10

1.5

4

3

1

6

2

5

8

10 mm

1

Microgastropods


12 13

“Normal-sized” gastropods

2.5

7

0.5

0

-0.5 1

10

100

1000

10,000

80,000

Number of sub-sampled individuals Fig. 3. Median and [Min–Max] range values (thin horizontal and bold vertical lines, respectively) of the shell-size distributions of some of the Early Triassic gastropod samples presented and discussed in this work with respect to the individual-rarefaction-based simulation results (Fig. 2a). Dark-gray area: 50% non-parametric confidence interval of the Min & Max shell-size values; light-gray area: 95% non-parametric confidence interval of the Min & Max shell-size values. 1–4 (black bars): Greenland samples (1: all localities; 2: NAS1; 2: IMMRI-SMA3; 4: WCE1); 5–13 (white bars): Southwestern USA samples (5: all localities; 6: CR; 7: SRS; 8: TO; 9: MM; 10: KAN; 11: VD; 12: BRC; 13: ROC; western USA locality abbreviations are detailed in Fig. 9).

Spath (1930, his Pl. 9) and Spath (1935, his Pl. 22) illustrated a few large specimens of Naticopsis arctica from the Griesbachian “Vishnuites” and “Proptychites” ammonoid beds of Greenland, but he did not include information about their abundance. Extensive fieldwork in Greenland allowed us to resample this assemblage, yielding abundant large Naticopsis specimens from several beds. Notably, as already mentioned and illustrated by Spath (1930, 1935), many Naticopsis preserve their shell and color patterns. In addition, intense fieldwork in Utah, Nevada and Idaho also allowed us to complement here previous reports of Gullivers in the western USA basin.

4. Newly sampled Gulliver collections: the fieldwork contribution In this work we show that the maximum shell size of Early Triassic gastropods was underestimated in previous studies. Extensive field sampling indicates that large sizes were reached in several gastropod genera representing various high-ranked clades. Large-sized specimens increase the known size range for gastropods and limit the significance of the Lilliput effect for this clade. We investigated large areas in Utah (Torrey and San Rafael Swell regions), from which microgastropod shell-beds had been described in the Smithian Sinbad Formation and which were presented as typical of the Early Triassic (Batten and Stokes, 1986; Schubert and Bottjer, 1995; Boyer, 2004; Fraiser and Bottjer, 2004; Fraiser et al., 2005; Pietsch et al., 2014). Gastropod collections from these key-areas and a few other regions (e.g., Werfen Formation, Alps) served to argue for a Lilliput effect among gastropods, subsequently leading to a spatio-temporal extrapolation on a global scale. Because our observations lead to alternative interpretations and conclusions, they not only have direct consequences for the significance, but they even question the existence of the Lilliput effect for gastropods on a regional scale and maybe also on a global scale. Our new collections are stratigraphically well constrained by highresolution ammonoid zonations (zones and horizons; e.g., Brayard et al., 2013; Jenks et al., 2013). Moreover, they integrate several sampling areas of the western USA basin. The Gulliver record from the western USA also covers the entire early/middle Smithian to early Spathian interval. The number of sampled specimens from Greenland is lower,

but it documents several successive assemblages from the earliest Triassic. All studied sections were carefully sampled bed by bed. Small gastropods were present in all sampled assemblages as is usual for both fossil and recent gastropod faunas. In order to document large-sized gastropods and to complement the known size range for this clade effectively, we used a combination of extensive surface collecting and block disintegration whenever possible. Most measured specimens are fragmented, so their original size was larger, especially for high-spired specimens, which are usually broken. In our dataset, each specimen represents a unique individual. Consequently, the terms “specimen” and “individual” can be used interchangeably. Here, “shell size” refers to the maximum measured height along the coiling axis for high-spired conispiral shells and to the maximum measured width for low-spired shells. Many specimens were measured directly in the field. Taxonomic determinations at the genus level follow the descriptions and classification established in Hofmann et al. (2014). Repositories of figured and measured specimens are abbreviated UBGD (Université de Bourgogne, Géologie Dijon — France) and PIMUZ (Paläontologisches Institut und Museum der Universität Zürich — Switzerland), unless otherwise indicated. 5. Griesbachian Gullivers from Greenland 5.1. Geological and biostratigraphical settings East Greenland is a classical area for studying the PT boundary and the Griesbachian–early Dienerian interval given its extensive sedimentary and fossil records (e.g., Nielsen, 1935; Spath, 1935; Trümpy, 1969; Twitchett et al., 2001; Wignall and Twitchett, 2002; Bjerager et al., 2006). All Gulliver gastropods come from the Wordie Creek Formation, which is well-exposed in the northern part of Hold With Hope Peninsula (Fig. 5). During the Early Triassic, this basin, which was part of the Greenland–Norway rift, was located at mid-latitudes in eastern Panthalassa (Fig. 4d). A thorough bio- and lithostratigraphy of this region was published by Bjerager et al. (2006). In the Hold With Hope Peninsula, the Wordie Creek Fm. was deposited on the tectonically sunken part of a westward tilted block, thus recording more than


a

37

b

d c

Fig. 4. Spatio-temporal distribution of Gulliver gastropods during the Early Triassic. a) Chronostratigraphic subdivisions of the Early Triassic (radiometric ages by Ovtcharova et al. (2006), Galfetti et al. (2007b) and Burgess et al. (2014)) with simplified trends of geochemical (δ13Ccarb; data from Galfetti et al., 2007b) and Tethyan relative temperature fluctuations during this period (data from Romano et al., 2013 [black line] and Sun et al., 2012 [gray line]; w: warmer; c: colder). Gulliver occurrence reports — Oman: Twitchett et al. (2004) and Wheeley and Twitchett (2005); northwestern China: Tong and Erwin (2001); South China: Pan (1982) and Kaim et al. (2010); Romania: Turculeţ (1987); Serbia: Frech (1912), Nützel (2005a,b), Nützel et al. (2010) and Brayard et al. (2010); Italy: Neri and Posenato (1985) and Brayard et al. (2010); South Primorye: Kaim (2009); Salt Range: Kaim et al. (2013); Greenland: Spath (1930, 1935), this work; southwestern USA: Dean (1981), Brayard et al. (2010), Hofmann et al. (2014), Olivier et al. (2014) and this work. R? indicates uncertain stratigraphic occurrence of Retispira bittneri reaching 5 cm in diameter (Yochelson et al., 1985). Question marks indicate uncertainty for stratigraphic position. Qinghai (Zhu, 1995) and West Australia (Runnegar, 1969) illustrated specimens are not located on the figure due to the high uncertainty about their stratigraphic position. b) High-resolution temporal distribution of Gulliver gastropods from southwestern USA. Ammonoid zonation is from Brayard et al. (2013) and Jenks et al. (2013). Western US locality abbreviations are given in Fig. 9. c) High-resolution temporal distribution of Gulliver gastropods from Greenland. Ammonoid and bivalve zonations are from Bjerager et al. (2006) and Meier and Bucher (ongoing work). Locality abbreviations are given in Fig. 5. d) Early Triassic localities with Gulliver gastropods.

650 m of essentially deltaic sediments. At Hold With Hope, the latest Permian is missing and the Triassic unconformably overlies the Permian Ravnefjell Fm. Both formations record a succession of transgressive– regressive cycles, which are particularly well displayed within the Wordie Creek Fm. This indicates a strong tectonic control on the regional Late Permian and Early Triassic sedimentation (Bjerager et al., 2006). In the Wordie Creek Fm., successive gastropod assemblages are documented from beds of late Griesbachian age (Ophiceras commune and Wordieoceras decipiens Zones) and probably of early Dienerian age (Bukkenites rosenkrantzi Zone) (Fig. 4c). The uppermost part of the B. rosenkrantzi Zone is probably early Dienerian in age (Bjerager et al., 2006; Meier and Bucher, ongoing work; Sanson-Barrera et al., submitted). Our field observations are in accordance with the original sedimentological description by Bjerager et al. (2006). Deposits of the O. commune Zone correspond to marine mudstones gradually passing into thin sandstone lobes (bottom-set beds of delta). The W. decipiens Zone is represented by offshore mudstones intercalated with very thin sandstone sheets in its lower part and a prominent unit of marine, channeled density-flow

sandstones in its upper part. The B. rosenkrantzi Zone also consists of offshore mudstones and is overlain by a sandstone-dominated unit representing nearshore deposition. 5.2. Gulliver gastropods The gastropod fauna from Greenland includes taxa of various sizes: Naticopsis, Warthia, Worthenia and high-spired “Loxonema” (or “Polygyrina”) (Spath, 1930, 1935; Kaim and Nützel, 2011). The neritimorph Naticopsis is the dominant large-sized genus in all sampled assemblages from Greenland. The largest Naticopsis specimens are ~ 3 cm wide (Figs. 6–8). Spath (1930, 1935) also illustrated Warthia (Bellerophon) specimens ~ 2.1 cm-wide and high. Spath (1930, 1935) reported the occurrence of a single species of Naticopsis: N. arctica. In accordance with Spath's descriptions, several recently sampled specimens are exceptionally well-preserved, especially from the IMMRI and SMA3 localities (Figs. 4c, 5, 6, 7). Axial sections of specimens show that all original aragonitic shell

38


Table 1 Reported maximum sizes of Early Triassic gastropod genera. Genus

Reported maximum size (mm)

Associated references

“Bellerophon” Coelostylina “Coelostylina sp. A” Omphaloptycha Cylindrobullina Worthenia Angularia Pseudotritonium (Kittliconcha) Polygyrina

17.7 40 85 30 1 22.4 24.6 7.6

Kaim (2009) This work This work This work Batten and Stokes (1986) This work This work Batten and Stokes (1986)

72

Promathilda Neritaria Naticopsis Vernelia Chartronella Battenizyga Warthia Soleniscus Strobeus Natiria Boutillieria Zygopleura Werfenella Abrekopsis Dicellonema Wannerispira Paleonarica Laubopsis Pachyomphalus Jiangxispira Gradellia Trypanostylus Amberleya? Toxoconcha Ananias

2.5 11.1 32 15.2 6.7 15.3 23.8 10.5 38 28.6 7.3 18.8 71 × 48 31.4 16.9 10.03 7.2 20 2.2 1.3 10 14.5 15 17.8 7.2

Undetermined genus SRS Ampezzopleura “Euomphalus” Guizhouspira Scurriopsis Solariconulus Trochotoma “Loxonema” Atorcula Retispira

30.2 1.9 14.6 6.3 4.9 9.4 25.2 5.2 b1 50

This work; Brayard et al. (2010) Batten and Stokes (1986) Hofmann et al. (2014) This work Nützel and Schulbert (2005) This work Batten and Stokes (1986) Kaim et al. (2013) Nützel (2005b) Brayard et al. (2010) Brayard et al. (2010) Batten and Stokes (1986) Batten and Stokes (1986) Turculeţ (1987) This work Kaim and Nützel (2011) Kaim et al. (2010) Kaim and Nützel (2011) This work Batten and Stokes (1986) Pan (1982) Tong and Erwin (2001) Tong and Erwin (2001) Tong and Erwin (2001) Tong and Erwin (2001) Wheeley and Twitchett (2005) This work Nützel and Schulbert (2005) Zhu (1995) Zhu (1995) Zhu (1995) Zhu (1995) Pan (1982) Spath (1935) Kaim et al. (2014b) Yochelson et al. (1985)

material has been dissolved (Fig. 8aa–ab); the specimens are filled with a homogenous sediment and there are no remains of the columella and internal shell walls. It is unclear whether the internal shell material of early whorls was resorbed by the animal (as is typical of Neritidae) or whether it was destroyed by diagenesis. Thus, systematic placement in Neritidae (resorbed) or Naticopsidae/ Neritopsidae (not resorbed) of the material from Greenland remains open, but at this point an assignment to Naticopsis seems reasonable. Only the outermost calcitic shell layer is well-preserved. Although sometimes slightly crushed, most specimens have relatively undamaged shell surfaces that retain various color patterns ranging from axial zigzag stripes to small patches (Figs. 7, 8). These patterns cover the entire shell surface, from the first to the last whorls. Zigzag stripes are spatially heterogeneous with different frequencies, widths and angles. In contrast, patches are more regular. Some rare Naticopsis are ornamented with folds and constrictions at large body sizes. The shell surface is generally smooth as is typical of the diverse Late Paleozoic/Early Mesozoic genus Naticopsis and for many other neritimorph species, making the discrimination of species within Naticopsis difficult (Kaim et al., 2013; Hofmann et al.,

2014). If not resulting from polymorphism (e.g., Krawczyński, 2013), the various color patterns may indicate that different Naticopsis species have coexisted, thus questioning the low gastropod richness previously reported from this area (Spath, 1930, 1935). Nevertheless, this hypothesis needs more substantiation as the number of studied specimens is still low. 6. Smithian–early Spathian Gullivers from western USA 6.1. Geological and biostratigraphical settings During the Early Triassic, the western USA basin was near-equatorial in the eastern edge of Panthalassa (Fig. 4d) and records both continental and marine sedimentation (McKee, 1954; Blakey, 1974, 1977; Collinson et al., 1976). Outcrops of Smithian and Spathian substages (Thaynes and Moenkopi Groups, sensu Lucas et al., 2007) are widely distributed within a large area covering Wyoming, Idaho, Utah and Nevada (e.g., Goodspeed and Lucas, 2007). Deposits of the Thaynes Group consist of alternating limestones and shales reflecting deposition within the relatively shallow western USA basin. These marine sedimentary deposits thin from the northwest to the southeast across Utah, where they interfinger with the more terrestrially dominated sediments of the Moenkopi Group (Lucas et al., 2007). During the Smithian, depositional environments essentially transitioned from a coastal plain with continental deposits to subtidal marine bioclastic limestones (e.g., Olivier et al., 2014, in press). This southward transgressive trend marks a long-term sea level rise that is identified worldwide after the PT boundary (Embry, 1997; Fig. 9). The Smithian sea-level rise reached its maximum extent within the western USA basin during the late Smithian (i.e., Anasibirites kingianus and Xenoceltitidae gen. indet. A beds) and was followed by a rapid sea-level fall around the Smithian/ Spathian boundary (Brayard et al., 2013). A renewed transgression marked the early Spathian, but the shoreline did not reach the maximum southeastward extent of the Smithian sea (Collinson and Hasenmueller, 1978; Carr and Paull, 1983; Paull and Paull, 1993; Olivier et al., 2014). Marine deposits generally contain abundant, although often not well preserved benthic and nekto-pelagic fossils (e.g., Hose and Repenning, 1959; Schubert and Bottjer, 1995; Fraiser and Bottjer, 2004; McGowan et al., 2009; Brayard et al., 2009a, 2010, 2011b, 2013; Stephen et al., 2010; Hofmann et al., 2013a,b, 2014; Hautmann et al., 2013). We sampled Smithian and early Spathian sedimentary successions in the Confusion Range, Mineral Mountains, Cedar City, Kanarraville, Virgin, Black Rock Canyon, Torrey and San Rafael Swell areas in Utah, Rock Canyon area in Arizona, Palomino Ridge in Nevada, and Hot Springs in southeastern Idaho (Fig. 9a). These Lower Triassic exposures represent various environments along a shoreline–offshore profile (Fig. 9b). All studied sections are relatively thick, indicating high sedimentation rates and an increase in accommodation space, but thickness also varies considerably from section to section (Fig. 10). This variation probably arises from the deposition of sediments on a complex paleo-relief formed during the Late Permian–earliest Triassic transition (Collinson et al., 1976; Dean, 1981; Paull and Paull, 1982; Brayard et al., 2013; Olivier et al., 2014). This setting is also suggested by the presence of unconformities, breccias, conglomerates and normal faults underlying the Lower Triassic deposits in Utah and Nevada (e.g., Paull and Paull, 1982; Hofmann et al., 2014; Olivier et al., 2014). Detailed sedimentological and biostratigraphical descriptions of most of these studied sections were recently published in Brayard et al. (2011b, 2013), Jenks et al. (2013), Hofmann et al. (2014), Olivier et al. (2014, in press) and Vennin et al. (2015). The Palomino Ridge (PA) and Confusion Range (CR) sections represent the deepest and quietest environments (lower to upper offshore) with thick shale intervals alternating with limestones in CR (Fig. 10). There is no evidence for active fair-weather-wave reworking. The base of the Mineral Mountains (MM) section corresponds to shallow-water settings containing microbial and metazoan bioconstructions (Brayard et al., 2011b; Vennin et al.,


GRE E

22°

D AN L N

39

20°

Clavering Ø Kap Stosch

IMMRI NAS1

74°

SMA3 WCE1

HOLD WITH HOPE

Gauss Halvø

Fault

25 km

Wordie Creek Fm.

Fig. 5. Greenland map with outcrops of the Wordie Creek Formation within the Hold With Hope Peninsula. Site abbreviations refer to localities of Nielsen (1935).

2015; Fig. 10). The overlying Smithian sediments of MM show a tidal influence followed by deeper offshore environments with intercalated shales and storm-induced limestones that are typical of the Thaynes Group (Hofmann et al., 2014; Vennin et al., 2015). Smithian deposits in the Cedar City (CC) and Kanarraville (KAN) sections record shallow depositional environments with tidal influences (Fig. 10). The Virgin Dam (VD) area as well as the Black Rock Canyon (BRC) and Rock Canyon (ROC) sections correspond to peritidal to shallow subtidal environments in southwesternmost Utah and northernmost Arizona (Blakey, 1977; Lucas et al., 2007; Brayard et al., 2013; Olivier et al., 2014; Fig. 10). Deposits of the Sinbad Formation in the Torrey (TO) and San

All n=67

Rafael Swell (SRS) areas were described in detail by Blakey (1974), Dean (1981), Goodspeed and Lucas (2007), Nützel and Schulbert (2005), Hofmann et al. (2014) and Olivier et al. (in press). They are represented by high-energy environments similar to those of VD, BRC and ROC (Fig. 10). Early Spathian gastropods from Hot Springs (HS) within the Columbites beds were found in concretions deposited below the storm-wave base. In this work, we use the detailed regional biostratigraphic ammonoidbased zonation proposed by Brayard et al. (2013) for the Smithian outcrops of Utah (Fig. 4b), while the early Spathian ammonoid zonation is based on Jenks et al. (2013). Further evidence for ages comes from conodonts (determination by N. Goudemand) and bivalves (Brayard et al., 2013; Hofmann et al., 2014). Sections from the Sinbad Formation of the TO and SRS areas were previously described as contemporaneous (e.g., Blakey, 1974; Fraiser and Bottjer, 2004). Actually, they are slightly diachroneous, the TO sections representing the middle–late Smithian interval and the SRS sections being mainly late Smithian–earliest Spathian in age (Brayard et al., 2013).

NAS1 n=45

6.2. Gulliver gastropods

IMMRI-SMA3 n=7

WCE1 n=15

0

3

6

9

12

15

18

21

24

27

Width (mm) Fig. 6. Raw values documenting the range of sampled Naticopsis specimens from the Wordie Creek Formation.

6.2.1. General observations Dominant large-sized gastropods in the western USA basin are Abrekopsis, Polygyrina, Strobeus, Omphaloptycha, Coelostylina, Laubopsis and a new taxon provisionally identified as “Coelostylina sp. A” (Figs. 11–19). The largest sampled Polygyrina and “Coelostylina sp. A” fragmentary shells reach ~7.2 cm and 8.5 cm in height, respectively, for an estimated original size N 10 cm (only fragments of these large highspired gastropods have been sampled to date). The largest Abrekopsis reaches ~ 3.2 cm in width, and the largest Strobeus, Omphaloptycha, Coelostylina and Laubopsis specimens reach 3.8 cm, ~ 3 cm, 4 cm and ~2 cm in height, respectively (Fig. 20). Low-spired Abrekopsis and highspired Polygyrina, Coelostylina, Strobeus, Omphaloptycha and Laubopsis can reach sizes ≥ 2 cm, and they occur in almost all studied sections. Except for Laubopsis, these genera correspond to the most frequent

40


Fig. 7. Large-sized Naticopsis arctica from the Wordie Creek Formation of Greenland. a–c) PIMUZ 30935, loc. NAS1, Bukkenites rosenkrantzi Zone. d–f) PIMUZ 30936 showing zigzag axial banding, loc. IMMRI, Wordieoceras decipiens Zone. g) PIMUZ 30939 showing zigzag axial banding, loc. IMMRI, Wordieoceras decipiens Zone. h–j) PIMUZ 30940 showing color patches, loc. SMA3, Wordieoceras decipiens Zone. k–l and m–o) PIMUZ 30937 and PIMUZ 30938, respectively, loc. IMMRI, Wordieoceras decipiens Zone. p–q and r–s) PIMUZ 30941 and PIMUZ 30942, respectively, loc. SMA3, Wordieoceras decipiens Zone. t–u, v–w, x–y, z–aa, ab–ac, ad–ae, af–ag, ah–ai, aj–ak) PIMUZ 30946, 30944, 30943, 30949, 30950, 30947, 30948, 30951 and 30945, respectively, loc. WCE1, Ophiceras commune Zone.

components of Early Triassic gastropod assemblages globally (Batten, 1973; Erwin and Hua-zhang, 1996; Nützel, 2005a and references therein). “Coelostylina sp. A” is apparently only present in VD, BRC and ROC, and Polygyrina is apparently absent from these localities. It mainly differs

from Polygyrina by its larger apical angle. Two other large-sized taxa occur in VD, BRC, and ROC: Angularia sp. and Worthenia windowblindensis, which reach ~ 2.5 cm and ~ 2.2 cm, respectively. W. windowblindensis is also present in KAN. Most relatively large


41

Fig. 8. Large-sized Naticopsis arctica from the Wordie Creek Formation of Greenland. a–b, c–d, e–f, g–h, i–j, k–l, m, n–p, q–r, s–t, u–v, w–x, y–z) PIMUZ 30972, 30935, 30973, 30977, 30968, 30975, 30974, 30978, 30971, 30967, 30970, 30976, 30969, respectively, loc. NAS-1, Bukkenites rosenkrantzi Zone. aa–ab) Axial sections of two Naticopsis specimens showing that all original aragonitic shell material has been dissolved.

a

100 km

IDAHO

b

HS

42°N

IDAHO

42°N

WY UTAH

PA

NEVADA

CR

NEVADA SRS

MM

TO 37°N

Studied localities with new Gulliver specimens Previously illustrated Gulliver specimens Localities with previously reported typical microgastropod beds

CC KAN VD BRC ROC

37°N

ARIZONA

basinal facies

ARIZONA

outer shelf facies inner shelf facies 110°W

global trend of the Smithian transgression

110°W

Fig. 9. a) Early Triassic Gulliver gastropod occurrences within the western USA basin. For temporal distributions, see Fig. 4b. b) Schematic extent of the Smithian open marine depositional facies, based on ammonoid data (Brayard et al., 2013) and modified after Carr and Paull (1983), Collinson and Hasenmueller (1978), and Paull and Paull (1993). HS: Hot Springs; PA: Palomino Ridge; CR: Confusion Range; MM: Mineral Mountains; CC: Cedar City; KAN: Kanarraville; VD: Virgin Dam; BRC: Black Rock Canyon; ROC: Rock Canyon; SRS: San Rafael Swell and TO: Torrey area.

42


Mineral Mountains

Cedar City Area

105m 45m

35m

85m

30m

80m

25m

60m

20m

Xe.

20m

10m

I.o.

I.o.

?

A.k.

A.k. 75m

O.

15m

5m

40m

5m

70m

Hidden and faulted

35m

30m

Conglomeratic unit

10m

ult

Fa

65m

ROC3

ROC2

ROC1

5m

Conglomeratic unit 0m

Hidden

Co

0m

40m

0m

?

A. k.

0m

I.b. 45m

15m

A. k.

5m

?

O.

v v

Massive bioconstructed limestone unit

O.

?O.

10m

15m

10m

?O.

?

?

45m

80m

50m

20m

15m

A.k.


A.k.

75m

v v

Rock Canyon

25m

Eumorphotis beds

?

90m

85m

? 30m

Highly bioturbated limestone unit

SPATHAN

95m

? To the Bivalve and Tirolites beds «Green shales»

100m

25m

Black Rock Canyon

I.o.

90m

Virgin Dam


Confusion Range

Kanarraville 30m


40m

To the Virgin Limestone

100m

~200m to the Virgin Limestone

?

To the Virgin Limestone

Eumorphotis beds

v v

25m

DH1-9 35m 60m

?

30m

GS2

Torrey Area

25m

15m

10m

20m

Biostratigraphical correlations

50m

V.u

Lithological correlations

45m

5m

Co

I.b. «Gastropod shales 1»

10m

35m

40m

25m

35m

5m

20m

30m

?

30m

0m

A.k.

15m

RED BEDS 25m

10m

PERMIAN

P.K.

DH1-0A

PERMIAN

0m

Anasibirites kingianus beds

O.

Owenites beds

I.o.

Inyoites oweni horizons

Fle.

Flemingites sp. indet. bed

I.b.

Inyoites beaverensis beds

P.K.

Preflorianites-Kashmirites beds

M.m.

Meekoceras millardense bed

M.o.

Meekoceras olivieri beds

R.e.

Radioceras aff. evolvens beds

V.u

Vercherites undulatus bed

IDAHO

v v v v

CR

NEVADA

SRS MM

Hidden

TO

CC

v v v v

5m

KAN BRC

Co

10m


PERM.? DIEN.?

15m

RED BEDS

PERMIAN

Xenoceltitidae gen. indet. A beds

A.k.

5m

Red unit

20m

Yellow unit

DH1-2

0m

0m

Xe.

O.

15m

5m

A.k.

?

25m

20m

Grey unit Blue unit

GS1 C2


40m

San Rafael Swell

45m

Yellow unit

?15m

10m

Conglomeratic unit

«Gastropod shales 2»

SMITHIAN

50m

THAYNES Grp.

55m

20m

0m

RED BEDS PERMIAN

VD ROC

UTAH

ARIZONA Smithian basinal configuration

Lithologies sandstone limestone

shelly-limestone calcareous shale

sandy-limestone

shale

microbial dep.

conglomerates


gastropods are present as fragmented steinkerns and hence their taxonomic and systematic placement is unclear. In the deepest part of the basin (CR, PA), Gullivers are represented by a large variety of more or less poorly preserved imprints, internal or external molds. These various preservation types coexist in some shale intervals at CR (Figs. 11 and 12). Calcite infillings are observed in specimens from all other sections, as well as a siliceous preservation of the shell in rare individuals from the basal bioconstructions in MM (Brayard et al., 2011b) and TO. Some specimens from KAN, VD and BRC have exceptionally well-preserved shells (Figs. 12t, 16e–r, and 17k–o). In these cases, as in the Griesbachian assemblages with dominant neritimorphs from Greenland, Abrekopsis specimens sometimes show residual color patterns in natural light as weakly perceptible axial dark bands, especially near the aperture (Fig. 16e–r). Additional Triassic neritimorphs with preserved color patterns are from the Salt Range (Fig. 4d; Kaim et al., 2013). These specimens are latest Smithian–early Spathian in age and show a welldeveloped punctuation over the entire shell surface. Unlike other gastropods, preservation of color patterns in neritimorphs is relatively frequent. This phenomenon is explained by a thin outer calcitic layer, typical of this group, that may preserve pigments and color patterns (e.g., Nützel et al., 2007; Kaim et al., 2013; Krawczyński, 2013). All size ranges from microto large-sized gastropods were found in the same beds in all studied areas (Figs. 20 and 21). Fig. 20 displays the raw values that document the range of large-sized specimens. Microgastropods (maximum shell size b 1 cm by definition for fossil gastropods; Fraiser and Bottjer, 2004) do occur but have not always been measured. Even though the largest sample of Gullivers studied herein comes from CR, the gastropod assemblages from VD, BRC and ROC are the most intriguing. These are mostly represented by the largest Gullivers present in the studied material, with only rare occurrences of small specimens (Figs. 18 and 19). These assemblages are difficult to detect in the field and specimens are often visible only after block disintegration. They occur in the first bed overlying the massive microbial constructions for VD (Olivier et al., 2014), and within microbial deposits embedded in conglomerates for BRC and ROC. Within the Inyoites oweni horizons (late middle Smithian) of CR, only countless phosphatic microgastropods were documented (Fig. 21b, c) and they probably correspond to a restricted event in the deepest part of the basin. A similar assemblage of countless microgastropods was found in the correlative I. oweni horizons of PA. However, these microgastropods are associated with large specimens at this site (Fig. 13q–v). Early Spathian gastropods at HS are extremely rare, but they are comparatively well-preserved. From this site, an undescribed and still undetermined vetigastropod with spiral ornament (clearly a distinct species in the studied collections), probably representing Pleurotomarioidea or Turbinidae, reaches 3.3 cm in diameter (Fig. 13m–p). 6.2.2. Gullivers as micro-habitat for epibionts Interestingly, some middle Smithian specimens from the CR, PA and VD areas (Figs. 4, 11 and 14–17) show traces of epibionts. No epibionts were found on neritimorphs, even on the largest specimens with preserved shell material. At VD, epibionts were observed on “Coelostylina sp. A” only. These specimens are represented by poorly-preserved internal molds; ~15% show various epibionts that can coexist on the same specimen. They consist of undetermined tubes, attached bivalves and enigmatic traces (Fig. 22). Their taxonomic determination is often precluded due to poor preservation. Undetermined tubes, which colonized the inner shell surface, both near the aperture and within the inner whorls (Fig. 22a), are characterized by a small diameter (~0.5 mm) and a relatively large length (up to ~ 1 cm). These traces are found all over the shell and it is unlikely that they represent dissolution traces. They are sinuous and sometimes appear to branch, although this pattern may

43

correspond to superimposed individuals. Such epibionts probably colonized empty shells before these were filled with sediment. A second category of enigmatic shell inhabitants, characterized by marked paired negative reliefs (Fig. 22d), are solitary, sinuous and sometimes very long (≥ 7 cm). Trace diameter is also rather large (~5 mm). These traces, observed on several specimens, generally follow the whorl coiling near the aperture before abruptly crossing other juvenile whorls. These reliefs were filled by sediments different from the mold matrix; their upper surface was shaped by the shell curvature. After infilling by sediments, this organism bored through the inner side of the gastropod shell before the latter dissolved. Such traces better characterize bioerosion than epibiont coverage. With regard to other VD gastropod epibionts, small (up to ~1.5 cm) epibysally-attached bivalves are frequent (Fig. 22b, c). Theses bivalves colonized the mold after the final dissolution of the shell. Such epizoan bivalves are also frequently observed at CR in the umbilical area of internal molds of the ammonoid Inyoites beaverensis (see Brayard et al., 2013: their Fig. 52). In both cases, they have not been observed in cooccurring ammonoid and gastropod taxa. Whether this selectivity is biological (preferential hosts) and/or represents a taphonomic signal remains to be investigated. The same type of epizoan bivalves has been described from the ammonoid assemblage that occurs in the Dienerian Candelaria Formation of Nevada (Ware et al., 2011). To summarize, three different types of gastropod epibionts were found at VD, and they represent different colonizing phases. The first settlement took place before sediment infill, and it corresponds to a cryptic habitat where worm tubes formed on the inner shell surface. After sediment infill but probably before the end of lithification and shell dissolution, an enigmatic organism bored and burrowed large traces below the inner shell surface. After shell dissolution, a third phase corresponds to the arrival of epibysally-attached bivalves. The Gulliver assemblage from VD thus represents a micro-habitat where successive secondary tierers had enough time to settle and flourish. In the CR and PA areas, epibionts are much rarer (a few specimens for hundreds of gastropods) and were observed only on ≥1 cm Strobeus and Polygyrina individuals. In the CR, epibionts consist of small, sub-circular traces probably corresponding to attached bivalves. One? Strobeus specimen also shows an undetermined sinuous worm tube in its inner whorls (Fig. 22f). In the PA, small bivalves (~1 mm, Fig. 22e; up to 2 coexisting specimens) are found directly attached to preserved shells. Smithian gastropod epibionts are documented here for the first time. Despite considerable efforts, previous studies of the diversity of epi- and endoskeletozoans in diverse Lower Triassic outcrops from the western USA basin did not document any epibiont in the Smithian or colonizing Early Triassic gastropods (Schubert and Bottjer, 1995; Fraiser, 2011). These authors linked this absence to the small size of the studied gastropod specimens, which represented a physically unstable environment. All colonized gastropods reported here are N 1 cm in size; this may explain why epizoans associated with Smithian gastropods have not been previously reported, as only microgastropods were studied and because originally aragonitic shells are not preserved. Observed epibionts are contemporaneously present in the deepest (CR, PA) and shallowest (VD) parts of the basin. They appear more diverse in nearshore environments, but this may result from the larger size reached by the VD gastropods. Shell size is likely a major factor controlling their installation as (i) no epizoan was found to date on specimen b 1 cm, (ii) the largest gastropods in VD are frequently colonized, and (ii) observed epizoans often display relatively large sizes (N5 mm). Epibionts were not observed in BRC and ROC although large-sized gastropods are abundant. This may result from the often poorly preserved, highly recrystallized internal molds.

Fig. 10. Detailed biostratigraphical correlation between the studied Smithian sections illustrating the diachronism of sedimentary deposits (see Brayard et al., 2013, for details). Occurrence of Gulliver gastropods is indicated by the gastropod symbol. Not all studied sections are depicted here, but each illustrated section log corresponds to the best biostratigraphical succession of the region. Co: occurrence of Smithian conodonts.

44


Fig. 11. Large-sized gastropods from the Confusion Range, middle Smithian, Utah (Figs. 9 and 10). a–b) Coelostylina sp., UBGD 278268, loc. C2. c) Polygyrina sp. A, UBGD 278269, loc. C2. d– e) Strobeus batteni, UBGD 277109, loc. GS1. f) Strobeus batteni, UBGD 277110, loc. GS1. g) Polygyrina sp. A, UBGD 277111, loc. DH1-0A. h) Polygyrina sp. A, UBGD 278270, loc. C2. i) Strobeus batteni, UBGD 278271, loc. GS1. j–k) Strobeus batteni, UBGD 278272, loc. C2. l) Strobeus batteni, UBGD 278273, loc. GS1. m) Strobeus batteni, UBGD 278274, loc. GS1. n–o) Strobeus batteni, UBGD 278275, loc. GS1. p) Polygyrina sp. A, UBGD 277113, loc. GS1. q) whorl section of Polygyrina sp. A, UBGD 278276, loc. C2. r) Strobeus batteni, UBGD 278277, loc. GS1. s) Stroebus batteni, UBGD 278278, loc. GS1. t) Polygyrina sp. A, UBGD 277112, loc. GS1. u–v) Polygyrina sp. A, UBGD 277114, loc. GS1. w) Coelostylina sp., UBGD 278279, loc. GS2. x–y) Abrekopsis depressispirus, UBGD 278280, loc. GS1. z) Strobeus batteni, UBGD 278281, loc. GS1. aa) Polygyrina sp. A, UBGD 278282, loc. GS1. ab) Polygyrina sp. A, UBGD 278283, loc. C2. ac–ad) Abrekopsis depressispirus, UBGD 278284, loc. C2. ae) Abrekopsis depressispirus, UBGD 278285, loc. C2. af) Polygyrina sp. A, UBGD 277116, loc. GS1. ag) Polygyrina sp. A, UBGD 278286, loc. GS1. ah) Polygyrina sp. A, UBGD 278287, loc. GS1. ai–aj) Strobeus batteni, UBGD 278288, loc. GS2. ak) Strobeus batteni, UBGD 278289, loc. GS2. al) Strobeus batteni, UBGD 278290, loc. GS2. am–an) Strobeus batteni, UBGD 278291, loc. GS2.


45

Fig. 12. Large-sized gastropods from the Confusion Range (CR), Kanarraville (KAN) and Torrey area (TO), early and middle Smithian, Utah (Figs. 9 and 10). Illustrated KAN specimens are all from loc. Ka2, Owenites koeneni beds. Illustrated TO specimens are all from units B and C (Dean 1981), O. koeneni beds. a, b, c–d) Polygyrina sp. A, UBGD 278292, UBGD 278293 and UBGD 277115, respectively, all from loc. DH1-0A, base of the Preflorianites–Kashmirites beds, CR. e, f, h–i) Strobeus batteni and Abrekopsis depressispirus, UBGD 278294, UBGD 278295and UBGD 278296, respectively, all from loc. DH1–2, Preflorianites-Kashmirites beds, CR. g) Strobeus batteni, UBGD 278297, loc. DH1–9, base of the O. koeneni beds, CR. j–l, m–n) Laubopsis sp. indet., UBGD 278803 and UBGD 278804, loc. GS2. o–p) Polygyrina sp. indet., UBGD 278298, KAN. q) Polygyrina sp. indet., UBGD 278299, KAN. r) Polygyrina sp. indet., UBGD 278300, KAN. s–t) Abrekopsis depressispirus, UBGD 278301, KAN. u) Polygyrina sp. indet., UBGD 278302, KAN. v) Polygyrina sp. indet., UBGD 278303, KAN. w–x, y–aa) Polygyrina sp. A, UBGD 278304 and UBGD 278305, TO. ab, ac) Polygyrina sp. A, UBGD 278306 and UBGD 278307, TO. ad) Coelostylina sp.,UBGD 278308, TO. ae) Polygyrina sp. A, UBGD 278309, TO. af–ag) Strobeus batteni, UBGD 278310, TO. ah) Polygyrina sp. indet., UBGD 278311, TO. ai) Polygyrina sp. indet., UBGD 278312, TO.

46


Fig. 13. Large-sized gastropods from Palomino Ridge (PA), San Rafael Swell (SRS), Hot Springs (HS), middle, late Smithian and early Spathian, Nevada, Utah and Idaho (Figs. 9 and 10). a–j) Undetermined taxon, UBGD 278313 to UBGD 278322, loc. RC43, earliest Spathian, SRS. k–l) packstones showing several high-spired specimens, Anasibirites kingianus beds, SRS (specimens from the lectostratotype of the Sinbad Fm., which is Section 10 in Goodspeed and Lucas, 2007; image courtesy of S.G. Lucas, Albuquerque). m–p) Vetigastropoda indet., J. Jenks private collection, Columbites beds, early Spathian, HS. q–r) Polygyrina sp. A, PIMUZ 30952, loc. PLR11, Inyoites oweni horizons, middle Smithian, PA (microgastropods co-occur and are represented by black calcitic micro-molds and white phosphatic molds). s–t) Polygyrina sp. A, PIMUZ 30954, loc. PLR12, middle Smithian, PA. u–v) Coelostylina sp., PIMUZ 30952, loc. PLR71, middle Smithian, PA.


47

Fig. 14. Large-sized gastropods from Virgin Dam, all from loc. VD4A, Owenites koeneni beds, middle Smithian, Utah (Figs. 9 and 10; see Olivier et al., 2014). a–b, c–d) “Coelostylina sp. A”, UBGD 278323 and UBGD 278324. e–g) “Coelostylina sp. A”, UBGD 275164. h–j) “Coelostylina sp. A”, UBGD 278325. k, l) “Coelostylina sp. A”, UBGD 278843 and UBGD 278844, respectively.

6.2.3. Local lateral variations We carefully explored the Torrey (TO; Fig. 23) and San Rafael Swell (SRS; Fig. 24) areas in Utah, from which microgastropod shell beds were previously reported to be common (Batten and Stokes, 1986; Schubert and Bottjer, 1995; Boyer, 2004; Fraiser and Bottjer, 2004; Fraiser et al., 2005; Nützel and Schulbert, 2005; Pietsch et al., 2014).

In the Torrey area (Fig. 23), we investigated sections previously described by Dean (1981) that were expected to contain large-sized gastropods according to the two specimens illustrated by this author, but that have been neglected in previous works. Other paleontological studies of the Early Triassic marine outcrops within this area are sparse: Schubert and Bottjer (1995) sampled three sections (without providing

48


Fig. 15. Large-sized gastropods from Virgin Dam, all from loc. VD4A, Owenites koeneni beds, middle Smithian, Utah (Figs. 9 and 10; see Olivier et al., 2014). a–t) “Coelostylina sp. A”, UBGD 278326 to UBGD 278336; n: UBGD 278845; o; UBGD 278846; p: UBGD 278847.

detailed information about the locations), Fraiser and Bottjer (2004) sampled two sections, and Nützel and Schulbert (2005) a single section. We thus extended our sampling to new sites that these authors did not include in their reports of typical microgastropod beds (Fig. 23a). According to lithological descriptions of Dean (1981), the units that we sampled in the new localities were the same units that were studied

by Fraiser and Bottjer (2004) (units A, B and D of Dean) and additional beds (units C and E of Dean) at various localities within the Torrey area (Dean's sections and eight supplementary sites). According to the figures of the studied facies, Nützel and Schulbert (2005) likely sampled Dean's unit A. Our spatially expanded sampling of this area indicates that large-sized gastropods are frequently present and that micro-


49

Fig. 16. Large-sized gastropods from Virgin Dam, all from loc. VD4A, Owenites koeneni beds, middle Smithian, Utah (Figs. 9 and 10; see Olivier et al., 2014). Arrows indicate color pattern remains. a–d) “Coelostylina sp. A”, UBGD 278337 to UBGD 278339. e–g, h–i, j–k, l–o) Abrekopsis depressispirus, UBGD 278340, UBGD 278341, UBGD 278848 and UBGD 278342, respectively; o: close-up view of the dark band remains at the aperture. p–p) Abrekopsis depressispirus, UBGD 278343.

and Gulliver gastropods do occur in the same units and beds at several localities (Figs. 20 and 23b, c). Clearly, the shell-size range of gastropods within the TO area has been underestimated. The largest sampled gastropod fragment measures ~ 4.5 cm in height, which, considering its incompleteness, suggests that the original shell

length was likely 7 cm or more (Fig. 12w–x), whereas the largest complete specimen observed in the field reaches ~ 5 cm (Fig. 23c). Well-sorted microgastropod concentrations often occur, especially in unit A, confirming the observation by Dean (1981) and Fraiser and Bottjer (2004). The gastropod fauna studied by Nützel and Schulbert

50


Fig. 17. Large-sized gastropods from Virgin Dam (VD) and Black Rock Canyon (BRC), middle Smithian, Utah (Figs. 9 and 10). Illustrated VD specimens (a–j) are all from loc. VD4A, Owenites koeneni beds. Illustrated BRC specimens (k–w) are all from the beds underlying the Anasibirites kingianus beds, at the base of the section. a–b) “Coelostylina sp. A”, UBGD 278849, VD. c–d) Angularia sp., UBGD 278850, VD. e–f) Worthenia windowblindensis, UBGD 278851, VD. g–k) “Coelostylina sp. A”, UBGD 278852 to UBGD 278856. l–n) Angularia sp., UBGD 278857. o) Angularia sp., UBGD 278858. p–t) “Coelostylina sp. A”, UBGD 278859 to 278862. u) Block with several specimens of “Coelostylina sp. A” (UBGD 278863 to 278865) and one specimen of Angularia sp. (“A”; UBGD 278866). v–w) Blocks with specimens of “Coelostylina sp. A” (UBGD 278867 to 278871).

(2005) was derived from a single, graded shell bed, typical of tempestites, including size sorting by transport. Based on our field observations, some of the mentioned coquinas laterally correspond to horizons with Gullivers. Some large specimens observed in the different

TO units are illustrated in Fig. 23c. Large-sized specimens appear to be more abundant in units B, C and D in the western and southern parts of the area. Large bivalves are also abundant in unit D. Overall, Gullivers seem to be absent only at the base of unit A. Although microgastropod


51

Fig. 18. Field illustrations of large-sized gastropods from various localities in Utah. MM: specimens from loc. MIA1, Vercherites undulatus beds, early Smithian, Mineral Mountains (see Brayard et al. 2011b). CC: specimens from TW12, early?–middle Smithian, Cedar City area. SRS: specimens from loc. RC43, earliest Spathian, San Rafael Swell (see Fig. 10). BRC: specimens from the Anasibirites kingianus beds, late Smithian, Black Rock Canyon.

beds are present, they are not exclusive of units A–D since Gullivers occur contemporaneously with them in nearly the entire sedimentary sequence. Both size classes display a heterogeneous distribution within the same bed, site and also within the entire TO area. This may be explained by the complex interplay of local environments, ranging from dominant tidal flats to subtidal and more open-marine settings (Dean, 1981; Hofmann et al., 2014; see environmental reconstructions in Olivier et al., in press). Qualitatively, Gullivers seem to be more abundant within slightly more open-marine environments (sites #1–3 and 7 in Fig. 23a), whereas microgastropods appear often concentrated within the lower microbial deposits of unit A (see also Olivier et al., in

press for a likely explanation based on autocyclic processes). Nevertheless, in most cases they coexist. This spatially-expanded sampling protocol therefore suggests that, due to spatial heterogeneity, a widespread sampling effort has a significant impact on estimated diversity and shell-size range, as much as, if not more than sampling intensity per se (number of samples and specimens). Concerning the large SRS area (Fig. 24), we concentrated our sampling effort to the top of the Sinbad Formation, from which field observations of large-sized gastropods were succinctly reported by Goodspeed (1996) and Goodspeed and Lucas (2007). Diversified small-sized mollusk assemblages (mainly gastropods, bivalves and

52


Fig. 19. Field illustrations of large-sized gastropods from various localities in Utah and Arizona. BRC: specimens from the basal microbial beds, middle? Smithian, Black Rock Canyon. ROC: specimens from loc. ROC1 and 3, middle and late Smithian, Rock Canyon.

scaphopods) are characteristic of the A. kingianus beds (late Smithian) and of underlying levels (roughly correlative of Dean's (1981) unit D in TO), as previously discussed by Batten and Stokes (1986), Boyer (2004), Fraiser and Bottjer (2004), Hautmann and Nützel (2005), Nützel and Schulbert (2005), Goodspeed and Lucas (2007), Hofmann et al. (2014), and Pietsch et al. (2014). The environments are often described as being influenced by storms, which may have resulted in substantial size-sorting. However, transport seems to have been limited, resulting in poor sedimentary sorting and the presence of numerous articulated bivalves and unbroken ammonoid conchs (Brayard et al., 2013; Hofmann et al., 2014). These storm-induced deposits are recognized over the entire SRS area, although lateral variations may be pronounced (Goodspeed and Lucas, 2007). Batten and Stokes (1986) reported 16 genera, but only three high-spired specimens with a size N13 mm: Zygopleura, Battenizyga (Anoptychia) and Coelostylina. The material of Nützel and Schulbert (2005); same occurrence as Batten and Stokes (1986) yielded several specimens larger than 1 cm,

e.g., “Zygopleura”: 18.5 mm in height and 3.7 mm in width, and Vernelia venestravella: 15.2 mm in height and 10 mm in width. Nützel (2005b) added the occurrence of a fragmented specimen of Strobeus or Soleniscus with a size N10.5 mm from the same locality. Field observations and collections by Goodspeed (1996) (nine reported genera) and Goodspeed and Lucas (2007) indicate that correlative packstones contain accumulations of high-spired specimens N15 mm in some places, as illustrated in Fig. 13k and l. These specimens were found in the lectostratotype of the Sinbad Formation (section 10 in Goodspeed and Lucas, 2007). Pietsch et al. (2014) also reported a Coelostylina specimen of 17 mm from this site. Taken together, these observations thus indicate that the abundance of large specimens may vary greatly laterally and that Gulliver gastropods co-occur with abundant minute representatives. Overlying beds are much less fossiliferous and correspond to tidal and peritidal deposits. We note that an abundant and apparently monospecific gastropod assemblage with large-sized specimens is present in the topmost


53

All

n=2513

HS

IDAHO

PA CR n=1619

NEVADA

SRS n=67

TO

n=349

MM n=9

CC KAN n=245

Smithian basinal configuration Neritimorph

VD n=93

Strobeus

Polygyrina, Coelostylina or “Coelostylina sp. A”

BRC n=46

ROC n=71 0

8

16

24

32

40

48

56

64

72

80

88

96

Size (mm) Fig. 20. Raw values documenting the range of sampled gastropod specimens within the western USA basin. Horizontal dotted lines indicate prolonged size ranges by field and thin-section observations. Dashed area shows the microgastropod range 0–1 cm. HS, PA, KAN, VD, BRC and ROC: from blocks reduced to fragments by hammer (KAN: 2 replicates of ~600 cm2). CR, CC: surface collecting. SRS, TO, MM: surface collecting and from blocks reduced to fragments by hammer.

beds of the Sinbad Formation at some places in northernmost SRS. The largest individual reaches ~3 cm (Figs. 13, 18, 20). This assemblage may be earliest Spathian in age (Fig. 4b; see Brayard et al., 2013). It reinforces the idea that environmental lateral variations strongly determine the heterogeneous spatial distributions of microgastropods and Gullivers, which in turn calls for spatially-widespread sampling strategies to recover better-constrained estimates of diversity and shell-size range. 7. Discussion 7.1. Sampling and preservation effects Because the mode of preservation and type of sampling influence observed richness and shell-size range, it is worth distinguishing their potential role in controlling the observed size distribution among Early Triassic gastropods. 7.1.1. Sampling In the case of Early Triassic gastropods, four sampling issues regarding the full assessment of body size are most relevant sources of bias: (1) the lack of repeated sampling of fossil sites and insufficient regional coverage; (2) large specimens are usually rare in living assemblages, which results in a paradoxical sampling bias against large-bodied specimens; (3) an incomplete survey of the scientific literature; and (4) an insufficient taxonomic work. (1) Spatial clustering and local rarity are among the main factors determining the efficiency of sampling in both neontological and paleontological studies. Therefore, repeated sampling and sufficient geographic coverage are necessary to increase the

probability of recovering rare taxa including rare large specimens due to the patchiness of their occurrence. Sampling intensity also enhances statistical confidence of relative abundances (Hayek and Buzas, 1997; Bennington and Rutherford, 1999; Bennington, 2003, Zuschin et al., 2005, 2006, McGowan et al., 2009). However, Early Triassic gastropod faunas have rarely been collected to a sufficiently intense degree. Many of the sampled outcrops correspond to shallow-marine depositional environments that are characterized by strong lateral variation and patchiness at local scale. Thus, sufficient spatial coverage and high sampling intensity are mandatory. (2) Most of the large Early Triassic gastropods reported here are poorly preserved fragments of internal molds. Therefore, their taxonomic value is very limited, which might explain why these specimens have not been recognized previously, although they come from the very same outcrops and horizons from which microgastropods were first reported. Moreover, large and incomplete internal molds of gastropods can be difficult to detect in the field, especially when the apical part is not visible — the characteristic helicoidal shape can be hard to spot when only the last whorls are exposed (see, e.g., Olivier et al., 2014, their Fig. 8). The present report shows that large gastropods have been overlooked in the field until recently, although they are usually collected preferentially due to their attractiveness to fossil collectors and are easier to detect by surface collection (e.g., Kidwell and Bosence, 1991; Cooper et al., 2006). In addition, the evaluation of gastropod richness or density based on observation of thin sections has obviously favored the reports of diminutive specimens. (3) Research on the Lilliput effect in Early Triassic gastropods also suffered from an insufficient assessment of the literature which

54


Fig. 21. a) Field sample illustrating the co-occurrence of Gulliver and microgastropods (black calcitic micro-molds) within the same bed, GS2, Confusion Range, middle Smithian (Fig. 10). b–c) Close-up views of late middle Smithian microgastropod accumulation from the Inyoites oweni horizons, CR. Diameter of Inyoites oweni in c is ~3 cm. d–e) Field illustrations of cooccurring large- and small-sized specimens within a “typical” microgastropod bed, Kanarraville, middle Smithian (Fig. 10).

reported large specimens. To some degree, this literature is not easily available and known only to specialists, but it still must be considered. (4) When compared to ammonoids or bivalves, Early Triassic

gastropods were rarely collected with a taxonomic goal and monographed (with Batten and Stokes, 1986, being one noticeable exception). This likely results from their poor preservation and may explain at least partially the paucity of documentation of

Fig. 22. Smithian gastropod epibionts from the Sinbad (Timpoweap) Formation in the Virgin Dam (VD: a–d) area and from the Thaynes Group at Palomino Ridge (PA: e) and the Confusion Range (CR: f). a) Undetermined worm tubes which colonized the inner shell surface (VD). b–c, e) Epibysally attached bivalves. d) Close-up of an enigmatic burrowed large trace found on several Gullivers from VD. f) Undetermined worm tube in inner whorls (CR).


large-sized specimens prior to our work. Since gastropods are not used for biostratigraphy, intense stratified collection is therefore less common than in ammonoids.

7.1.2. Preservation The issue of preservation is a recurring theme in the ongoing debate about post-crisis recovery, whether for assessing the Lazarus effect and taphonomic megabiases on a global scale (e.g., Erwin and Hua-zhang, 1996; Kauffman and Harries, 1996; Wignall and Benton, 1999; Fara, 2001; Twitchett, 2001; Fraiser and Bottjer, 2005), or for evaluating the fidelity of fossil assemblages on a regional or local scale (Fraiser and Bottjer, 2004). Assessing the quality of the fossil record is difficult because preservation processes integrate both biological traits (e.g., generation time, body size, shell thickness and mineralogy, population density) and paleoenvironmental/taphonomic features (e.g., time-averaging, sedimentation rates, hydrodynamics) that may act at various spatiotemporal scales (Valentine, 1989; Kidwell and Bosence, 1991; Kidwell and Flessa, 1996; Stoner and Ray, 1996; Cherns and Wright, 2000; Wright et al., 2003; Tomašových, 2004, 2006; Fraiser and Bottjer, 2005). In addition, some of these associated parameters may have opposite effects on preservation potential. For example, short-lived mollusks would be expected to be overrepresented in fossil assemblages due to their high population turnover (Levington, 1970; Kidwell and Flessa, 1996; Vermeij and Herbert, 2004), but these taxa are usually smallbodied and should, therefore, be more prone to dissolution (e.g., Cummins et al., 1986; Kidwell and Flessa, 1996). Conversely, large mollusks will tend to be taphonomically robust, but their generation time is usually slow and their population density low (e.g., Powell and Cummins, 1985; Powell and Stanton, 1985; Valentine and Jablonski, 1986; Stoner and Ray, 1996). Overall, these opposite effects probably compensate for each other on a large scale, at least in the generation of thanatocoenoses (Kidwell, 2001). This is the reason why the simulations presented above (Section 2) were achieved under the working hypothesis of a shellsize independent random selection of individuals or species. Fraiser and Bottjer (2005) acknowledged that shell preservation can depend on facies-controlled factors and that the comparison of bodysize ranges and extent of epibiont cover will help to detect selective shell dissolution. For the Early Triassic gastropods, we found no significant preservation contrast between large and small specimens in terms of facies or diagenetic controls. Molds and recrystallized fossils occur in all of the size classes we investigated, as well as rare cases of shell silicification. A lack of size-controlled differential preservation is expected because, as far as we know, there is no evidence of a correlation between shell size and mineralogy (aragonite predominates in the shells of most marine gastropod clades). In addition, data from VD suggest that the frequency of epizoans and bioeroders positively correlates with shell size and occurs at various taphonomic stages. Furthermore, we found no relationship between shell size and shape, corroborating the absence of selective transportation or preservation processes based on these shell parameters. Organic-rich microstructures have also been suspected as being a factor lessening the probability of shell preservation (e.g., Kidwell and Flessa, 1996). However, evidence for this relationship is lacking, and there is no apparent correlation between shell size and richness in organic compounds among gastropod shells. The actual composition of amino acids seems to be a better predictor of diagenesis than the simple richness of organic molecules (e.g., Jackson and Bischoff, 1971; Marin and Gautret, 1994). Overall, sampling biases and stochastic effects linked to the low known Early Triassic gastropod diversity are probably the main reasons for the rarity of reports of large Early Triassic gastropods. The role of preservation is secondary and is much more difficult to qualify and quantify.

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7.2. Distribution within the western USA basin: local and regional controls The common occurrence of Gullivers in the western USA basin strikingly contrasts with the Early Triassic microgastropod concentrations that were first described from this basin. We found that small-sized and Gulliver gastropods have a patchy distribution at all spatial scales (site or basin), and they can occur within the same beds (Figs. 23, 24). Sampling of a few shell beds only in a restricted number of sections is therefore insufficient to characterize the local/regional fauna in terms of diversity and size–frequency distribution. Sampling intensity (i.e., the numbers of sampled beds and specimens) is important, but its geographical (i.e., number of sampled environments) and temporal resolution (i.e., detailed biostratigraphy and correlation) is of prime importance. Focusing on a single outcrop, even if the number of counted specimens is high, leads to an underestimation of the neighboring diversity components. Sufficient spatial sampling coverage, based on highresolution correlations, is often missing in Early Triassic studies (see He et al., 2015 for an exception) so that the observed patterns may be biased to various degrees. Recently, Marenco et al. (2013) described successive microgastropod beds from an Early Triassic shallow-marine environment in Montana. By comparison with large specimens from the deeper environments of the Confusion Range section (Brayard et al., 2010), they postulated that gastropods were subject to differential environmental constraints on shell size along a bathymetric gradient, leading to a higher frequency of large specimens in deeper settings. They also stated “extreme abundances of smaller-than-1 mm gastropods per kg of rock”. In our opinion, they may have actually sampled an assemblage of larval shells and early juvenile shells similar to that reported from the Early Triassic of Vietnam by Kaim et al. (2014a,b). Late Paleozoic/Early Mesozoic caenogastropod larvae have typical shell sizes of 0.5–1 mm. Such thanatocoenoses are also known from the Late Paleozoic and may occur in oxygen depleted (or otherwise limited) settings where metamorphosis of planktic larvae and maturation are impossible (Nützel and Mapes, 2001; Mapes and Nützel, 2009; Nützel, 2014). The widespread occurrence of Gullivers within a large array of environments of the western USA basin demonstrates that microgastropods and large gastropods coexisted in all sampled environments, even in shallow nearshore settings. Moreover, in three shallow-water environments (VD, BRC and ROC), Gullivers are dominant, contradicting the hypothesis of Marenco et al. (2013) that large gastropods were restricted to deeper offshore environments. However, more information about the assemblage studied by Marenco et al. (2013) is needed because this work has been published as an abstract, greatly limiting the amount of information available for this fauna. For instance, a detailed biostratigraphic framework for the Marenco et al. (2013) data would enable direct and robust comparison of both settings. Relative abundances of the Smithian Gulliver taxa identified in the deeper parts of the basin (mainly Polygyrina, Strobeus, Abrekopsis, Laubopsis, Coelostylina and Omphaloptycha) tend to be more equable than those in shallow settings in which mainly Polygyrina, Abrekopsis, Strobeus, “Coelostylina sp. A”, Angularia and Worthenia are found. Slightly more abundant Gulliver specimens can be found locally in deeper settings, e.g., in the Torrey area. Observed maximum sizes of “deep” and “shallow” gastropods at the specimen- or species-levels are nonetheless similar, with, e.g., Abrekopsis specimens and high-spired taxa reaching in both environments ~3 cm and N7 cm, respectively (Fig. 20). The regional spatial distribution of body sizes between juveniles and adults, or among species or guilds, is common in modern gastropods. It mainly results from habitat suitability, feeding preferences, biotic interactions and dispersal ability. This is often documented in tidal environments (e.g., Chapman, 1994, 2000; Ray and Stoner, 1995; Stoner, 2003; Haubois et al., 2004). During the Smithian–Spathian interval, the western USA basin was mainly a relatively shallow epicontinental sea with large bay systems dominated by tidal deposits. In this context, it is not surprising that gastropods show large lateral variation in richness,

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abundance and body size. Microgastropods may have accumulated for reasons independent of stressful conditions. Small-sized specimens may have aggregated according to current strength and food availability, and sheltered in the available pool habitats, whereas more mobile larger individuals may have moved more easily to other habitats (see paleoenvironmental reconstructions in Olivier et al., 2014, in press, for VD and TO, respectively). Such tidal shelters may also explain the common preservation and dominance of microgastropods in some shallow environments (e.g., microbial deposits in TO), without excluding contemporaneous, dense clusters of large specimens at some sites (e.g., in the VD area). The abundant microbial mats, as well as the variable sources of nutrients from the continent, may have provided profuse resources for grazing taxa such as many gastropods (e.g., Browne et al., 2000), thus explaining their high abundance in peritidal settings of the western USA basin. 7.3. Early Triassic spatio-temporal distributions Brayard et al. (2010, 2011a) and a survey of the literature showed that several large gastropod taxa were already present soon after the mass extinction (e.g., during the Griesbachian Ophiceras Zone in South China; Kaim et al., 2010), at different localities during the Griesbachian–Dienerian interval (Fig. 1). Naticopsis is the most widespread taxon yielding large specimens early in the Griesbachian up into the Dienerian; it often reached ~3 cm in width at different latitudes and in various environments. Many other taxa also have a size ≥1 cm (Bellerophon, Amberleya?, Gradellia, Toxoconcha, Trypanostylus, Wannerispira) or even N 2 cm (Coelostylina, Warthia, Neritaria), sometimes co-occurring (e.g., Tong and Erwin, 2001). Assemblages from Greenland, South China, South Primorye or Oman (see Fig. 4) show size distributions that do not differ from those of the Late Triassic or the present-day (Nützel, 2005b; Nützel et al. 2010), thus calling into question the existence of a Lilliput effect on gastropods during the earliest Triassic. The Dienerian is the most poorly-documented time interval in the Early Triassic, and this poor record complicates the analysis of body-size trends for taxa persisting from the Griesbachian to the Smithian. The Smithian is the best documented substage in terms of environmental settings and taxonomic identifications. Several Gulliver taxa have been reported for this time interval (Polygyrina, Strobeus, Abrekopsis, Laubopsis, Coelostylina, Omphaloptycha, “Coelostylina sp. A”, Angularia, Worthenia) along with some other forms with sizes N1 cm (Battenizyga, Soleniscus?, Zygopleura). Several Smithian gastropods had body-sizes larger than the largest known Late Permian as well as many Anisian (early Middle Triassic) taxa (Payne, 2005). This reinforces Brayard et al.'s (2010) view that the Lilliput effect is not applicable at the clade level at that time. As a first-order approximation, based on published measurements or illustrated specimens, Early Triassic genera comprising large gastropods were as diversified as microgastropods (Table 1), contradicting previous observations by Fraiser and Bottjer (2004). Occurrences of Spathian gastropods nearly always comprise several genera yielding large individuals (e.g., Natiria, Werfenella, Vetigastropoda indet.; Nützel, 2005b; Brayard et al., 2010; this work), suggesting that large body sizes have a wide phylogenetic distribution also in this time interval. 7.4. Gulliver gastropods vs. global Early Triassic environmental changes Rediversification after the end-Permian mass extinction was explosive for some nekto-pelagic groups such as ammonoids (Brayard et al., 2009b) and conodonts (Orchard, 2007). However, it was clearly not a

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continuous process. Indeed, it was interrupted at least once during a brief, marked extinction event at the end of the Smithian (e.g., Tozer, 1982; Dagys, 1988; Brayard et al., 2006; Orchard, 2007; Brosse et al., 2013). This episode is concomitant with a drastic ecological turnover of floras during which a middle Smithian major spore peak was followed by an early recovery of gymnosperms during the late Smithian (Hermann et al., 2011). The Smithian–Spathian boundary corresponds to an abrupt, global change from hygrophytic to xerophytic plant associations (Galfetti et al., 2007c; Hermann et al., 2011). The global carbon isotope record is characterized by a marked negative peak during the middle Smithian, followed by an abrupt positive shift in the late Smithian (e.g., Payne et al., 2004; Galfetti et al., 2007b; Horacek et al., 2007; Clarkson et al., 2013; Grasby et al., 2013; Fig. 4). In the Tethys, the oxygen isotope record from biogenic phosphate tends to track the carbon isotope record and indicates a temperature drop of ~8 °C near the Smithian–Spathian boundary (Romano et al., 2013; Fig. 4). The end-Smithian event therefore had a deep impact on the rediversification of nekto-pelagic organisms. The most likely explanations call upon the combined roles of the carbon cycle (e.g., Galfetti et al., 2007b, 2008; Goudemand, 2014), sea-level change (e.g., Embry, 1997; Olivier et al., 2014), and climate change (Sun et al., 2012; Romano et al., 2013). Based on marine faunas from the western USA basin, it appears that benthic communities were taxonomically and ecologically rather stable throughout the Smithian and were not affected by significant turnovers at this time (Hofmann et al., 2014). Despite a reduced taxonomic richness and a few disappearances of gastropod genera (e.g., among the Bellerophontoidea and Strobeus; Kaim and Nützel, 2011; Kaim et al., 2013), there is currently no evidence for a major extinction event of benthic faunas at the Smithian–Spathian boundary. Within the western USA basin, this boundary is marked by important bivalve blooms (Brayard et al., 2013; Hofmann et al., 2014). The Lilliput effect in the marine realm is commonly attributed to the outcome of deleterious environmental conditions such as anoxia, salinity fluctuations, acidification and high water temperatures, which may have limited the growth of organisms, or accelerated their development, or modified their population growth and generation time (e.g., Twitchett, 2007). These abiotic parameters fluctuated widely during the Early Triassic, especially during the Smithian (Fig. 4a). For instance, the middle and early late Smithian may have witnessed the highest temperature reached in the Tethys during the Early Triassic (Sun et al., 2012; Romano et al., 2013). Numerous hypotheses state that such a warming should be beneficial for dwarf forms but unfavorable for large taxa (e.g., Sibly and Atkinson, 1994; Daufresne et al., 2009; Smith et al., 2009; Sheridan and Bickford, 2011; Ohlberger, 2013). However, the largest living gastropods are known from the tropics and the largest Permian gastropods are also known from the tropical shallow-water carbonates (Nützel and Nakazawa, 2012). Smithian Gulliver specimens and species occur both in deep and shallow environments within the western USA basin, especially at a time when global temperature and carbon cycle perturbations were most severe (e.g., during the middle and late Smithian; Fig. 4a). In contrast to recent statements by Sun et al. (2012), Song et al. (2014) and Pietsch et al. (2014), the middle to end-Smithian event did not prevent the occurrence of Gullivers, although it corresponds to a global oceanographic and climatic event. It may be argued that this observation is valid only for the epicontinental western USA basin (e.g., Fraiser et al., 2011), which was acting as a potential ecological refugium for large taxa. However, although gastropod data are very scarce, large taxa (e.g., Strobeus and Naticopsis) are also known from correlative beds from the Salt Range, Pakistan (Nützel, 2005b; Kaim et al., 2013; Fig. 4), in an entirely different tectonic and environmental context. Thus, if we rule out a

Fig. 23. Spatio-temporal distribution of Gullivers within the Torrey area. a) Explored Gulliver localities (this work) and previously described microgastropod sites (Fraiser and Bottjer, 2004; Nützel and Schulbert, 2005). b) Temporal distribution of sampled large-sized gastropods compared with distribution of microgastropods beds, highlighting their contemporaneous occurrences. Depicted section #1 is for illustration purpose, but succession and ages of lithological units are identical within all areas. c) Field illustrations of large-sized gastropods from various localities and units.

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Fig. 24. Distribution of large gastropods within the San Rafael Swell area with previously detailed microgastropod sites (Batten and Stokes, 1986; Fraiser and Bottjer, 2004; Nützel and Schulbert, 2005; Pietsch et al., 2014). Extensive studies of Goodspeed (1996) and Goodspeed and Lucas (2007) refer to “large gastropods”, but without specific sampling sites.

regional refugium effect, gastropod shell size was likely not influenced by the globally fluctuating environments during the Smithian. Studies of some present-day organisms indicate synergistic effects of at least pH, hypoxia and temperature on gastropod shell-size and morphological plasticity (e.g., Melatunan et al., 2013), but this link does not always correspond to data from natural populations (e.g., Angilletta and Dunham, 2003; Vanden Byllaardt and Cyr, 2011). The upper limits of tolerable temperatures are high in modern gastropods, especially in intertidal taxa (e.g., Song et al., 2014). This suggests that elevated temperatures during the middle–late Smithian interval had no or little impact on the diversity and distribution of these benthic organisms in all depositional environments of the epicontinental sea of the western USA basin. This agrees with our observation of abundant Gullivers at that time. Gullivers thus demonstrably occurred during the warm period of the middle–late Smithian. Although represented by different families and morphologies, they are also present in the potentially colder environments of the Spathian (e.g., Vetigastropoda indet., Werfenella, and Natiria). Unfortunately, no survivors of the Smithian/Spathian boundary have been recorded in any basin at this point, thus preventing an analysis of size trends before, during and after the end-Smithian event (such an analysis has been performed for conodonts; Chen et al., 2013). Neritimorphs (e.g., Naticopsis, Abrekopsis, and Neritaria) probably represent the longest ranging group during the Early Triassic, and thus may provide some indication of the within-clade body-size evolution. The maximum size of neritimorph gastropods (~3 cm in width) apparently did not vary during the late Griesbachian (three successive zones), nor during the Smithian. This suggests that the influence of global environmental fluctuations on their maximum size was probably limited. 7.5. Is there an Early Triassic Lilliput effect? 7.5.1. Lilliput effect sensu stricto A Lilliput effect sensu stricto, i.e., size reduction in surviving species (Urbanek, 1993; Harries and Knorr, 2009), is not applicable to

gastropods at the Permian/Triassic boundary because there is not a single known nominate gastropod species crossing the boundary. Triassic species must obviously be derived from Permian ancestors, but there is currently no evidence to document any phylogenetical link at the species level. The Lilliput effect has been generally proposed for marine invertebrates at supra-specific taxonomic levels and at a restricted geographical scale for the Early Triassic (e.g., Luo et al., 2008; Metcalfe et al., 2011). This approach (i.e., sensu lato; see Section 7.5.2) results from: (i) the low number of boundary-crossers at low taxonomic levels available for statistical treatment, (ii) the low number of species with a temporal range long enough to identify statistically significant trends, (iii) uncertainties in taxonomic assignments when working with small, poorly-preserved specimens, and (iv) the lack of a good fossil record spanning the PT boundary and/or representing a long-enough interval of the Early Triassic. It is also commonly difficult to find out whether small individuals are fully grown adults, all the more when only poorly preserved internal molds are available (see Metcalfe et al., 2011, for an exception). The studied Smithian gastropod faunas from the western USA comprise large and small individuals with highly heterogeneous spatial distributions. In modern contexts, spatial distributions are controlled by several factors such as habitat/feeding preferences, biotic interactions (e.g., predation), reproductive strategies (e.g., fecundity) or migration of adults (Ray and Stoner, 1995; Takada, 1996; Ray-Culp et al., 1999; Stoner, 2003; Haubois et al., 2004). These fluctuating parameters also become much more complex when applied to environmental gradients (e.g., latitude or depth; McClain et al., 2005, 2009), trophic groups (Roy, 2002) and different clades (e.g., Beck, 2000; Chapman, 2000). To our knowledge, no previous study of the Lilliput effect during the Early Triassic has repeated the same analytical procedures over two or more distant basins to test whether the proposed size-reduction is (i) not a local environmental phenomenon and/or (ii) results of an environmental (e.g., latitudinal or depth) gradient (see Roy, 2002; McClain et al., 2005). This lack of spatially-replicated observation is


rather understandable due for instance to the prohibitively timeconsuming analyses required. For gastropods, it is also very difficult to match larval or juvenile shells to later growth stages. For instance, it is possible that some of the microgastropods reported from the western USA basin are actually early stages of Gullivers (e.g., Battenizyga eotriassica could be a juvenile of “Polygyrina”). Other genera, such as the Early Triassic to Jurassic Sinuarbullina (Cylindrobullina) are generally small, and even Late Triassic and Jurassic species never exceed a size of 10 mm. Moreover, the preservation of gastropods is commonly so poor that species identity of pre- and post-extinction material is commonly impossible to recognize. These problems and the lack of highlyresolved temporal framework and correlation as well as taphonomic and sampling biases make it nearly impossible to detect the Lilliput effect sensu stricto in gastropods at the Permian/Triassic boundary. For Early Triassic gastropods, the Lilliput effect s.s. has not yet been demonstrated in spite of the wealth of papers dealing with this phenomenon, simply because not a single species is known both from the Changhsingian and the Griesbachian (Nützel, 2005b; Twitchett, 2007; Brayard et al., 2011a). For bivalves, two examples of boundarycrossing species have recently been described by Wasmer et al. (2012): Leptochondria curtocardinalis and Permophorus costatus. For the latter, Permian (Zechstein) size data are available from Logan (1967), showing no significantly larger body size in comparison with the Early Triassic data from Wasmer et al. (2012) (shell length of Permian specimens: n = 6, Min = 6.7 mm, Max = 25.9 mm, Median = 14 mm; shell length of Triassic specimens: n = 4, Min = 6 mm, Max = 22 mm, Median = 15.5 mm). 7.5.2. Lilliput effect sensu lato To circumvent these issues, many authors consider a Lilliput effect sensu lato, i.e., they consider taxa above the species level and newly originating taxa during the Early Triassic, or compare global sizedistribution of pre- and post-extinction faunas (Harries and Knorr, 2009). The consideration of supra-specific taxonomic levels implies a hypothesized phyletic lineage or an entire clade (e.g., Payne, 2005; Peng et al., 2007). On that ground, and although representatives of some Early Triassic clades do have small body sizes (e.g., heterodont bivalves, Hautmann and Nützel, 2005; chondrichtyans, Mutter and Neuman, 2009; foraminifers, Song et al., 2011; Rego et al., 2012), the Lilliput effect has been inappropriately generalized as a global-scale decrease in size characterizing all clade members, or more rarely all clades over a large part of or the entire Early Triassic (e.g., Twitchett, 2001, p. 341; Twitchett, 2007, p. 143; Fraiser and Bottjer, 2004, p. 272; Peng et al., 2007, p. 124; Metcalfe et al., 2013: their Fig. 12). The lack of thorough taxonomic identification and of a robust phylogenetic framework, combined with low-resolution stratigraphic scales and paleoenvironmental reconstructions, may impede the study of actual size trends (Harries and Knorr, 2009; Huttenlocker, 2014). These shortcomings make it difficult to answer the following questions: – Did the mass extinction select against large-sized species (“faunal stunting” of Harries and Knorr, 2009)? – Did the presumably harsh environments in the aftermath of the PT crisis directly select in favor of small-sized species (e.g., due to life history traits such as rapid growth, short longevity and generation time, early sexual maturity, high fecundity and semelparity, all of which are usually associated with relatively small body sizes and classically related to stressed, instable and/or unpredictable environments; e.g., Pianka, 1970; Reznick et al., 2002; Twitchett, 2007)? – Are body-size distributions observed during the Early Triassic a stochastic assemblage-scale result from the random removal of species during the extinction crisis and/or random selection of specimens during the sampling procedure, as suggested by the simulations presented above (see Section 2)? – Are species originating in the Early Triassic just small by nature —

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whatever the evolutionary mechanism driving the body-size decrease during the speciation process?

This has important implications because the last two hypotheses represent simple alternative explanations for an apparent Lilliput effect. In the literature, this phenomenon is almost invariably related to deleterious, stressful environmental conditions thought to prevail in the aftermath of an extinction crisis, although the nature of these conditions as well as proximate causes and processes driving the proposed bodysize reduction remain elusive (Twitchett, 2001, 2006; Harries and Knorr, 2009; Wade and Twitchett, 2009).

7.5.3. The paradox of the western USA basin and the risk of across-scale extrapolation The definitions, analyses and underlying explanations of the Lilliput effect during and after the PT mass extinction are highly variable (Harries and Knorr, 2009; Huang et al., 2010). Local observations of Smithian gastropods from the western USA basin initiated a large discussion about the Lilliput effect s.l. in the wake of the endPermian mass extinction (e.g., Schubert and Bottjer, 1995; Fraiser and Bottjer, 2004), including extrapolation to other geographical and temporal Early Triassic contexts. Paradoxically, we found the largest known Early Triassic gastropods in the western USA basin. We also showed that microgastropod accumulations supposedly typical of Early Triassic assemblages actually correspond to assemblages of coexisting small- and large-sized specimens. This demonstrates that the Early Triassic fossil record is still insufficiently known, especially for poorly-preserved fossils such as gastropods, even in what may be considered as a “well-sampled and wellstudied” basin. Occurrences of Gulliver gastropods are actually frequent in the western USA basin in the very same beds where “typical” Early Triassic microgastropod assemblages were first described, thus invalidating the worldwide extrapolation of typical microgastropod assemblages in the Early Triassic. In a similar manner, some recent contributions have also questioned or downplayed the Lilliput effect previously suggested for Early Triassic ophiuroids (Twitchett et al., 2005), lingulid brachiopods (Rodland and Bottjer, 2001) or fishes (Mutter and Neuman, 2009). Indeed, Chen and McNamara (2006) and Zatoń et al. (2008) documented Paleozoic and Middle Triassic ophiuroid assemblages with much smaller specimens than those of the Early Triassic. In addition, Romano et al. (in press) and Scheyer et al. (2014) showed that fishes and marine tetrapods did not experience a global size reduction during the Early Triassic. Similarly, the exhaustive study of the benthic fauna from the Spathian Virgin Formation in southern Utah by McGowan et al. (2009) did not find any support for a significant reduction in body size when compared to later Triassic faunas. Overall, our own observations together with those of the abovementioned recent studies show that dense, well-dated and geographically dispersed field data at the basin-scale level are a prerequisite before extrapolating local observations to a more general model. In any case, although some Early Triassic taxa are particularly small (e.g., Hautmann and Nützel, 2005; Mutter and Neuman, 2009), a global extrapolation seems to be unwarranted at present. Concerning gastropods, a Lilliput effect neither s.s. nor s.l. can be demonstrated based on the current state of knowledge. Furthermore, the Greenland samples suggest that a global-scale Lilliput effect (either s.s. or s.l.) for gastropods cannot be demonstrated for the earliest Triassic (≲0.3 Myr after the PT boundary). Indeed, the gastropod body size distributions sampled from Griesbachian Greenland assemblages show essentially the same range as those from the Smithian of the western USA basin (compare Figs. 6 and 20). In both cases, the body size ranges are compatible with the expectation of individuals or species randomly drawn from a “standard”, non-altered shell size–frequency distribution (Fig. 3).

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TRIASSIC

Fig. 25. Maximum estimated width and height of known large gastropods from the Permian to Middle Triassic. Trend modified after Payne (2005; light gray conical shapes) by including data discussed in this work (dark gray conical shapes). Corrected data for the (Middle Permian) from Nützel and Nakazawa (2012). Assel.: Asselian; Sak.: Sakmarian; Wuchia.: Wuchiapingian; Changhsin.: Changhsingian.

8. Conclusions We have documented abundant large-sized gastropods from different levels and environments in the Griesbachian–earliest Dienerian of Greenland and the Smithian–early Spathian of the western USA. When coupled with a literature survey, it clearly appears that the maximum size of numerous Early Triassic gastropods has been repeatedly underestimated for all Early Triassic substages. Our new data show that previous reports on gastropod maximum sizes (Griesbachian specimens were b1.5 cm, Smithian specimens were b2 cm and that specimens N2 cm only occur in the Spathian; e.g., Fraiser and Bottjer, 2004; Payne, 2005) must be updated (Fig. 25). The newly reported large-sized specimens from the Griesbachian outcrops of Greenland highlight that Gulliver gastropods were present rapidly after the PT mass extinction, therefore questioning the existence of a Lilliput effect on gastropods at the clade-level during the earliest Triassic. We demonstrate that the western USA basin records a high number of gastropod taxa and specimens characterized by a large body size, well above the 1 cm cut-off value separating microgastropods and large gastropods (Fraiser and Bottjer, 2004; Figs. 1, 3, 6 and 20). The question is no longer: “where and when, if any, do Gulliver gastropods occur during the Early Triassic?” but rather: “where and when, if any, do Gulliver gastropods not co-occur alongside micro-gastropods during the Early Triassic?”. The Lilliput effect in gastropods was first proposed mainly based on studies of “typical microgastropod beds” from this basin (e.g., Fraiser and Bottjer, 2004) and subsequently often considered as a global Early Triassic phenomenon (e.g., Pietsch et al., 2014). As small and large size classes occur contemporaneously within the same area and environments, the paradigmatic character of microgastropod beds in the western USA basin has to be rejected. Heterogeneous distributions of large and small specimens within the western USA basin are likely related to normal lateral variations as observed in present-day contexts. The observed distribution of large and small taxa does not seem to be constrained by a bathymetric gradient or any stressful condition.

The fossil record of the western USA basin covers most of the time interval in which severe oceanographic and climatic perturbations occurred, i.e., during the Smithian–early Spathian interval (e.g., Payne et al., 2004; Galfetti et al., 2007b; Hermann et al., 2011; Kaim et al., 2013; Romano et al., 2013). Contrary to ecological predictions and statements by Sun et al. (2012), Song et al. (2014) and Pietsch et al. (2014), these potential deleterious conditions apparently did not select against large-sized gastropods. On the contrary, the largest individuals and taxa occur during the warmest periods, which strongly suggests that, whether at local community or at global levels, the structuring and evolution of gastropod body-size distributions during the Early Triassic were disconnected from major climate fluctuations. Potential drivers may have to be searched within the still poorly known inter-species interactions in Early Triassic ecological networks. These biotic interactions result from the complex interplay of competition, predation, mutualism and parasitism, and also involve the richness, abundance and evenness of the various components of each trophic layer (e.g., Goudemand, 2014). From this point of view, our observations on gastropod assemblages from the western USA basin call for an in-depth appraisal of food resources which have sustained the proliferation of large-sized specimens and species at several places in this basin. 9. Outlook: research challenges This study exemplifies that the knowledge of the Early Triassic fossil record is still incomplete for a major group of benthic organisms – gastropods – and that new field data are required before general conclusions about macroevolutionary patterns for that time can be made. It also highlights that a comprehensive spatio-temporal survey of a basin, i.e., at a meaningful (paleo)ecological level, is more appropriate than an extrapolation of local lines of evidence for building global macroecological/macroevolutionary models. Accordingly, the first challenge is the correct taxonomic identification of accurately-sampled specimens of all size-classes, despite an often poor state of preservation. The temporal and geographic scales of such


taxonomical studies must also be carefully considered: spatially extended field data calibrated by high-resolution temporal zonations are routinely required. Furthermore, there are still methodological challenges to overcome and new standards to adopt collectively, notably for sampling protocol and for evaluating size distributions. This calls for a common agreement on a unified or at least an explicit sampling methodology. Parallel to this mandatory field and systematic effort, the effect of sample-size on several diversity and disparity (e.g., body size) metrics remains to be further investigated. While realistic direct experimental approaches of the various biotic and abiotic parameters driving the construction of a taphocoenosis from a parent biocoenosis may prove difficult, all the more in the context of deep-time local to regional marine assemblages, a numerical simulation-based approach building on the one presented here is expected to provide useful information. For instance, the behavior (unbiasedness, accuracy, robustness, etc.) of various metrics of the body-size range (e.g., Min–Max range, inter-quartile range, standard-deviation, Coefficient of Variation) may be investigated in order to better constrain comparisons between Early Triassic and other fossil or extant assemblages. In this manner, it may become possible to describe the diversity and disparity of gastropod assemblages in the aftermath of the end-Permian crisis more realistically. Acknowledgments This work is a contribution to the CNRS-INSU Interrvie 2011, 2012 and 2013 programs, and to the ANR project AFTER (ANR-13-JS06-0001-01). A.N. thanks the Deutsche Forschungsgemeinschaft (Project NU 96/6-1). Most fossil localities in the western US mentioned in this report are located on US public land under the stewardship of the Bureau of Land Management (BLM) of the US Department of the Interior; their management and permits to access to these lands are much appreciated. The sections located in the Capitol Reef National Park were studied and collected under permits #CARE-2013-SCI-0005 and CARE-2014-SCI0011, thanks to Sandy Borthwick for her help with the permitting process. N. Goudemand (Zurich) is thanked for help on the field. P. Bouchet and P. Lozouet (MNHN, Paris) kindly provided us their New Caledonian dataset upon which the rarefaction-based simulations presented in this work are based, and J. Thomas (Dijon) kindly shared his data on Eocene gastropods from Grignon (France). E. Steinmetz (Dijon) is thanked for her help with gastropod measurements. We thank F. Marin (Dijon) for the fruitful discussion on mollusk shell mineralogy. S.G. Lucas (Albuquerque) is acknowledged for his help on gastropod specimens from the San Rafael Swell. A. Strasser, S.G. Lucas and P.J. Harries provided constructive reviews, which helped us to improve the paper. References Angilletta, M.J.J., Dunham, A.E., 2003. The temperature-size rule in ectotherms: simple evolutionary explanations may not be general. Am. Nat. 162, 332–342. Batten, R.L., 1973. The vicissitudes of the gastropods during the interval of Guadalupian– Ladinian time. In: Logan, A., Hills, L.V. (Eds.), Permian Triassic Systems and Their Mutual Boundary. Can. Soc. Petrol. Geol., pp. 596–607. Batten, R.L., Stokes, W.L., 1986. Early Triassic gastropods from the Sinbad Member of the Moenkopi Formation, San Rafael Swell, Utah. Am. Mus. Novit. 2864, 1–33. Beatty, T.W., Zonneveld, J.-P., Henderson, C.M., 2008. Anomalously diverse Early Triassic ichnofossil assemblages in northwest Pangea: a case for a shallow-marine habitable zone. Geology 36, 771–774. Beck, M.W., 2000. Separating the elements of habitat structure: independent effects of habitat complexity and structural components on rocky intertidal gastropods. J. Exp. Mar. Biol. Ecol. 249, 29–49. Bennington, J.B., 2003. Transcending patchiness in the comparative analysis of paleocommunities: a test case from the Upper Cretaceous of New Jersey. Palaios 18, 22–33. Bennington, J.B., Rutherford, S.D., 1999. Precision and reliability in paleocommunity comparisons based on cluster-confidence intervals: how to get more statistical bang for your sampling buck. Palaios 14, 506–515. Bjerager, M., Seidler, L., Stemmerik, L., Surlyk, F., 2006. Ammonoid stratigraphy and sedimentary evolution across the Permian-Triassic boundary in East Greenland. Geol. Mag. 143, 635–656.

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