Journal of the Geological Society, London, Vol. 165, 2008, pp. 319–332. Printed in Great Britain.

Palaeoecology of the Late Triassic extinction event in the SW UK

LUKE MANDER 1, RICHARD J. TWITCHETT 1 & MICHAEL J. BENTON 2 1School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK (e-mail: [email protected]) 2Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK

Abstract: A high-resolution palaeoecological study of the shelly invertebrate macrofauna across two marine Triassic–Jurassic boundary sections in the UK (St. Audrie’s Bay and Lavernock Point) is presented. Loss of taxonomic richness occurs in the upper Westbury Formation to lower Lilstock Formation (late Rhaetian), but if sample size is taken into account there is little convincing evidence of a catastrophic marine extinction. There is, however, good evidence for significant palaeoecological change in the benthic marine ecosystem at this time. The immediate post-event recovery interval in the upper Lilstock Formation is characterized by assemblages of low abundance, low diversity, high dominance and low evenness. Body-sizes of taxa that survived the event and originated afterwards were low until the later Hettangian. Recovery to higher abundance, higher diversity and higher evenness is recorded in the Psiloceras planorbis Zone. Recovery of the benthic ecosystem in the aftermath of the Late Triassic event was disrupted by marine anoxia and shows additional similarities to the (much slower) recovery that followed the Late Permian event. The pattern of body-size changes recorded in the shelly fossil record closely matches that of the trace fossil record. Shell thickness trends do not support a biocalcification crisis during the Late Triassic biotic event.

The Late Triassic witnessed one of the major extinction episodes and marine animals (Cuny 1995; Hallam 2002). Also, recent of the Phanerozoic, with the loss of 23.4% of marine families analyses have shown that the rates of extinction may have varied and 21.7% of terrestrial families (Benton 1995; McGhee et al. in different environmental settings and were apparently highest 2004). The timing of the Late Triassic event is, however, not well among those taxa with a preference for carbonate substrates understood (Tanner et al. 2004), largely as a result of the ‘lack of (Kiessling et al. 2007). a reliable and widely applicable biostratigraphic framework’ The cause(s) of extinction is also widely debated. Release of (Hesselbo et al. 2007). Scientific interest in the Late Triassic methane from gas hydrate associated with massive volcanism in event continues to increase (Twitchett 2006) and many of the the Central Atlantic Magmatic Province (Marzoli et al. 1999; problems with global correlation should be resolved once a Wignall 2001; Hesselbo et al. 2002, 2004), bolide impact Global Stratotype Section and Point (GSSP) has been selected preceding (Bice et al. 1992; Olsen et al. 2002) and coinciding (Hesselbo et al. 2007). with the Triassic–Jurassic boundary (Simms 2003) or sea-level Regarding the extinction event itself, an additional key issue is changes associated with volcanic plume activity (Hallam & the apparent lack of a marine extinction horizon in many Wignall 1999) have been considered. Release of volcanogenic sections, which has led some researchers to question the severity CO2, as indicated by carbon isotope studies (McRoberts et al. of diversity loss (e.g. Hallam 2002). Partly this may be because 1997; Palfy et al. 2001; Ward et al. 2001; Hesselbo et al. 2002; global compilations of stratigraphic ranges binned at 1–6 Ma Guex et al. 2003; Galli et al. 2005), and consequent climate intervals are unable to resolve the timing and pattern of ancient change is frequently cited as a link between Central Atlantic events in a biologically meaningful way (Bambach 2006). Magmatic Province volcanism and mass extinction. Enhanced Certainly, an extinction horizon has been identified in the higher- atmospheric CO2 may have caused undersaturation of carbonates resolution terrestrial palynological record (Hounslow et al. within the Earth’s oceans, culminating in a biocalcification crisis 2004). A recent global study using data from the Paleobiology at the end of the Rhaetian stage that led to the selective Database did find evidence of a Rhaetian extinction peak, but the extinction of aragonitic molluscs (Hautmann 2004). The role of coarse resolution of the analysis did not allow for a precise extraterrestrial impact in triggering mass extinctions is debated determination of where within the Norian–Hettangian interval (Hallam & Wignall 1997; Twitchett 2006) and is probably of the biotic crisis occurred (Kiessling et al. 2007). Until GSSPs limited significance at the Triassic–Jurassic boundary, not least are ratified for the bases of the Carnian, Norian, Rhaetian and because the largest obvious impact site, the Manicouagan crater, Hettangian stages, and the stratigraphic information within the predates Late Triassic biotic turnover by at least 13 Ma (Hodych Palaeobiology Database is revised where necessary, the resolu- & Dunning 1992). tion of such global studies will always be somewhat inadequate. Recent work has demonstrated that the magnitude of diversity Apart from the stratigraphic problems, the lack of a well- loss and the palaeoecological severity of ancient extinction defined marine extinction horizon could also be because the events may be decoupled (Droser et al. 2000; McGhee et al. distribution of facies in the available rock record masks any true 2004). Events of similar magnitude may have different impacts biological signals (see Smith et al. 2001). Rapid facies changes on the ecological structure of the marine ecosystem: the Late within many Triassic–Jurassic sections worldwide (e.g. Hallam Triassic event is ranked fourth in terms of diversity loss but third & Wignall 2000; Hesselbo et al. 2004) could result in biases in in terms of ecological effect on the global biosphere (McGhee et the apparent extinction and recovery patterns of both terrestrial al. 2004). Understanding the marine palaeoecology of the

319 320 L. MANDER ET AL.

Triassic–Jurassic interval may shed light on not only the timing Geological setting but also the cause of the Late Triassic extinction event. Existing data on the palaeoecology of the marine ecosystem during this The Triassic–Jurassic boundary sections of SW England and period are, however, limited. Palaeoecological studies of the south Wales have been subject to geological analyses for over Triassic–Jurassic marine trace fossil record of the UK, Austria 200 years (Richardson 1905, 1906, 1911) and are of international and Nevada have been undertaken (Twitchett & Barras 2004; significance. The boundary beds studied here were deposited in a Barras & Twitchett 2007), and apparent global changes in series of east–west-trending extensional basins (Fig. 1). The ichnodiversity and the size and depth of bioturbation have been basin depocentre was situated on the NW margin of Tethys and recorded. The most recent global analysis of marine shelly taxa was bounded by outcrops of Palaeozoic basement rocks in the through the Triassic–Jurassic interval concluded that perhaps north and by the London–Brabant landmass in SE England ecological and taxonomic effects were not decoupled, at least (Swift & Martill 1999). Norian depositional environments were amongst the bivalves, but noted that ‘a finer resolution [study] is lacustrine and commonly evaporitic, and were abruptly termi- clearly needed to solve this issue’ (Kiessling et al. 2007). Herein nated by a marine transgression in the early Rhaetian (Hesselbo we present one such study. et al. 2004). Marine conditions were sustained through much of The aims of the present paper are: (1) to characterize the the Rhaetian and, following a period of exposure, continued into palaeoecology of the marine ecosystem during the Triassic– the Hettangian. Jurassic extinction–recovery interval at two marine boundary sections in the UK: St. Audrie’s Bay, SW England and Lavernock Point, south Wales; (2) to elucidate the magnitude, duration, Stratigraphy timing, and potential causes of any palaeoecological changes recorded; (3) to consider the effects of facies bias on the quality The lithostratigraphic scheme used to correlate the studied of the Triassic–Jurassic fossil record. sections follows that of Warrington et al. (1980) (Fig. 2).

Blue Anchor Formation

Methods Rydon Member. The Rydon Member is a heterogeneous unit Graphic logs of the boundary sections were produced from detailed field characterized by grey, black and green mudstones with subordi- studies by the lead author (L.M.), and accurate correlation between nate red–brown, dolomitic mudstones and dolomites. The pre- localities was achieved using key stratal surfaces (see Hesselbo et al. sence of gypsum at several horizons indicates a locally evaporitic 2004). During sampling, an attempt was made to control for biases environment. associated with sampling and facies distribution. Bulk samples were collected randomly, to reduce biases related to sample location, approxi- mately every metre from each of the two main lithofacies encountered Williton Member. The Williton Member comprises a thin hetero- (carbonate and fine-grained siliciclastic) and located on the appropriate lith of mudstone and fine-grained sandstone (Hesselbo et al. graphic log. Parity between samples was ensured by analysing 2004). The base of this unit is irregular and erosive, and the 1.6 0.1 kg of each rock sample (see Table 1). Samples from similar uppermost surface of the underlying Rydon Member contains lithofaciesÆ were then subjected to identical analytical techniques: carbo- nate rocks to acetate-peel analyses; siliciclastic rocks to mechanical abundant Diplocraterion burrows, interpreted as a marine firm- breakdown. ground by Mayall (1981). The occurrence of marine fossils The invertebrate macrofauna of each sample was identified as far as indicates that the Williton Member represents a marine transgres- possible and assessed in terms of abundance and mean organism size. For sion, which has been dated as earliest Rhaetian (Mayall 1981; carbonate samples this was achieved by counting only those specimens Hesselbo et al. 2004). preserved on the bedding plane surface, defined here as the maximum area of the upper surface of a 1.5–1.7 kg sample. For siliciclastic 2 samples, this was achieved by breaking down each sample to 15 mm Westbury Formation portions, and counting all fossil specimens present. The palaeoecological indices of Margalef Diversity, dominance and evenness were calculated The Westbury Formation is also of Rhaetian age and comprises for each sample using the statistical program PAST (Hammer et al. dark grey siliciclastic mudstones with subordinate interbedded 2001). Shell thickness was measured for only those specimens preserved calcareous sandstones, bioclastic packstones, wackestones and within the carbonate rock samples. One acetate-peel taken vertical to intraformational conglomerates (Macquaker 1999). It is gener- bedding was produced per carbonate sample. Each was analysed under a ally accepted that the sediments of the Westbury Formation microscope and measurements of shell thickness were made digitally using Image Pro imaging software. were deposited in a storm-dominated, shallow epeiric sea Body-size of individual bivalve fossils was determined as the square (Warrington & Ivimey-Cook 1995). The mudstones were depos- root of the shell height multiplied by the shell length; that is, the ited either in quiet offshore waters or in restricted lagoons geometric body-size of Jablonski (1996), where Height Umbo to subject to fluctuating salinity (see, e.g. Allison & Wright 2005; commissure tip, in a straight line, and Length span of shell¼ at right Barras & Twitchett 2007). Calcareous sandstones represent ¼ angles to height line. The height and length of incomplete shell material barrier-bar deposits and bioclastic packstones represent win- was measured and an estimate of original shell height and width was nowed shallow marine concentrations (Macquaker 1999). The made and recorded separately. Separate records were kept of fossil intraformational conglomerates and ‘bone beds’ have been material that was incomplete because of breakage and specimens that variously interpreted as transgressive lag deposits, condensed were partially obscured by sediment that could not be removed without damage. horizons or storm-generated concentrations (Macquaker 1999). A summary of all palaeoecological data can be found in Table 1. More Coarsening-up and fining-up successions on a stacked para- detailed data are available from the authors on request. Specimens sequence scale within the Westbury Formation suggest that the pertaining to this study are housed in the collections of the Department distribution of facies was controlled by fluctuations in relative of Earth Sciences, University of Bristol. sea level (see Hesselbo et al. 2004). TRIASSIC – JURASSIC PALAEOECOLOGY 321

Table 1. Summary of palaeoecological parameters measured in this study

Sample Lithology Height Mass Abundance Margalef Dominance Evenness Pre-event bivalve Post-event bivalve Shell (m) (kg) Diversity size size thickness

Mean (mm) SE Mean (mm) SE n Mean (mm) SE

St. Audrie’s Bay LGe14 L 28.80 1.51 – – – – – – – – 50 0.11 0.01 LGe13 M 28.22 1.53 21 2.30 0.24 4.20 14 9.7 4 0.3 – – – LGe12 M 27.30 1.65 7 1.54 0.39 2.58 18 – 5 0.3 – – – LGe11 L 26.40 1.57 – – – – – – – – 26 0.11 0.03 LGe10 M 25.40 1.60 0 0.00 – – – – – – – – – LGe8 M 23.72 1.63 7 2.06 0.23 4.45 9 5.4 12 – – – – LG29 L 23.45 1.56 – – – – – – – – 38 0.10 0.01 LGe7 M 22.55 1.52 0 0.00 – – – – – – – – – LG23–24 M 21.53 1.62 30 0.59 0.45 2.23 11 1.8 3 – – – – LGe6 L 20.95 1.67 – – – – – – – – 46 0.14 0.01 LGe5 M 20.55 1.59 13 0.78 0.43 2.32 6 1.0 6 0.8 – – – LG13 M 19.50 1.63 0 0.00 – – – – – – – – – LG10 L 19.30 1.66 2 1.44 0.50 2.00 9 1.3 – – 13 0.10 0.01 LG8 L 18.90 1.64 – – – – – – – – 6 0.10 0.02 LGe4 M 18.60 1.68 9 1.82 0.24 4.26 8 0.9 4 0.3 – – – LG6.1 L 17.65 1.70 – – – – – – – – 50 0.22 0.02 LGe3 M 17.35 1.59 10 1.30 0.52 1.92 11 4.8 6 1.2 – – – LG6 L 17.00 1.64 10 0.00 1.00 1.00 18 2.0 – – 50 0.25 0.03 LGe2 M 16.30 1.60 15 0.38 0.54 1.85 9 1.2 – – – – – LG4 L 15.90 1.52 10 0.43 0.68 1.47 6 1.0 – – 50 0.07 0.00 LG3 M 15.45 1.61 8 0.48 0.63 1.60 10 1.4 3 – – – – LG1 M 15.00 1.53 13 0.39 0.65 1.55 5 0.5 – – – – – LGe1 M 14.35 1.63 0 0.00 – – – – – – – – – LF2 L 13.85 1.58 20 1.00 0.59 1.71 15 1.8 4 0.3 30 0.09 0.01 LFa4 M 13.45 1.53 1 0.00 1.00 1.00 11 – – – – – – LFa2 L 13.30 1.68 8 1.44 0.31 3.20 18 2.0 20 0.6 – – – LFa3 M 12.80 1.63 0 0.00 – – – – – – – – – LFa1 L 12.56 1.61 – – – – – – – – 50 0.11 0.01 LF1 M 12.20 1.51 13 1.56 0.33 3.08 16 1.8 – – – – – WFxA M 11.30 1.63 45 2.36 0.20 5.08 9 0.8 – – – – – WF7 L 10.70 1.59 22 1.94 0.20 5.04 15 1.1 – – 50 0.09 0.01 WF7A M 10.50 1.59 98 1.53 0.43 2.33 11 0.5 – – – – – WF6 L 10.05 1.68 11 1.25 0.36 2.81 17 2.3 – – 50 0.11 0.01 WF6A M 9.45 1.59 20 1.34 0.36 2.78 7 0.2 – – – – – WFe2 M 8.32 1.70 11 1.30 0.30 3.33 14 1.5 – – – – – WF5 L 7.70 1.58 8 – – – 6 0.2 – – 50 0.14 0.01 WF5A M 7.15 1.68 55 1.99 0.29 3.41 6 0.5 – – – – – WFe1 M 6.45 1.50 0 0.00 – – – – – – – – – WF4 L 5.75 1.69 8 0.56 0.72 1.39 4 0.6 – – 50 0.12 0.01 WF4A M 5.30 1.57 38 0.86 0.41 2.46 3 0.1 – – – – – WF3A M 4.25 1.68 13 0.40 0.51 1.95 5 0.3 – – – – – WF2 L 3.80 1.59 23 0.64 0.60 1.68 9 0.6 – – 50 0.13 0.01 WF2A M 3.30 1.55 7 1.03 0.39 2.58 2 0.4 – – – – – WF1a M 2.85 1.51 0 0.00 – – – – – – – – – WF1 L 2.58 1.68 6 0.56 0.72 1.39 15 0.7 – – 50 0.14 0.01 Lavernock Point LG14 L 15.55 1.69 2 1.44 0.50 2.00 49 22.6 – – 50 0.13 0.01 LG12 L 14.65 1.58 3 0.91 0.56 1.80 – – 32 6.5 28 0.11 0.01 LG11 M 14.50 1.69 12 1.56 0.28 3.60 11 2.2 13 3.3 – – – LG10 L 13.85 1.60 – – – – – – – – 50 0.10 0.01 LG9 M 13.37 1.57 20 0.67 0.67 1.50 21 1.6 5 – – – – LG8 L 13.10 1.62 9 0.46 0.80 1.25 13 1.7 – – 50 0.20 0.03 LG7 M 12.55 1.63 24 0.63 0.59 1.69 10 1.5 9 – – – – LG6 L 12.45 1.50 – – – – – – – – 37 0.22 0.03 LG4 L 11.60 1.59 4 – 1.00 1.00 9 1.0 – – 50 0.11 0.01 LG3 M 11.35 1.63 22 0.33 0.91 1.10 6 0.5 – – – – – LG2 L 11.25 1.65 13 0.39 0.53 1.90 17 1.7 – – 7 0.10 0.02 LG1 M 10.90 1.56 3 0.91 0.56 1.80 8 1.9 – – – – – LGB M 10.30 1.60 0 0.00 – – – – – – – – – LGA M 9.55 1.57 0 0.00 – – – – – – – – – LF2 L 8.60 1.69 4 0.72 0.50 2.00 10 1.3 – – 15 0.10 0.01 LF1 M 8.35 1.52 0 0.00 – – – – – – – – – WF10 M 6.80 1.69 109 3.41 0.23 4.43 14 0.8 – – – – – WF9 L 6.05 1.66 0 0.00 – – – – – – 50 0.15 0.01 WF7 L 5.35 1.68 14 0.76 0.48 2.09 18 2.3 – – 50 0.15 0.01 WF6 M 4.95 1.64 15 1.11 0.35 2.85 12 2.0 – – – – – WF5 L 3.90 1.53 5 0.62 0.68 1.47 12 1.9 – – 50 0.12 0.01 WF4 M 3.60 1.57 7 1.54 0.31 3.27 3 0.4 – – – – – WF3 M 2.95 1.67 29 0.59 0.70 1.43 3 0.2 – – – – – WF2 L 2.30 1.52 4 – – – 10 0.8 – – 50 0.17 0.01 WF1 M 1.90 1.60 0 0.00 – – – – – – – – –

St. Audrie’s Bay samples prefixed ‘SAB’; Lavernock Point samples prefixed ‘LP’. Lithology: L, limestone; M, mudstone. Pre-event bivalve size includes taxa that originate before the biotic crisis and are lost, and those that originate before the biotic crisis and survive. Post-event bivalve size includes only taxa that originate after the biotic crisis. SE, standard error.

Lilstock Formation ripple marks are ubiquitous and indicate very shallow water and moderately high-energy conditions (Swift 1999). Regional seis- Cotham Member. The Cotham Member comprises pale green– mic activity associated with the opening of the Atlantic, or grey mudstone, fine-grained sandstone and limestone. Sedimen- regional shock caused by extraterrestrial impact, resulted in tary structures such as lenticular bedding, cross-bedding and widespread slumping, microfaulting and dewatering (Swift 1999; 322 L. MANDER ET AL.

Fig. 1. Location of the studied sections in the SW UK.

Simms 2003, 2007). The Lower Cotham Member is a shallowing upward succession and emergence is demonstrated by large desiccation cracks that commonly extend down into the under- lying Westbury Formation. The unstratified, single generation fill of these large cracks suggests that exposure was brief (Hesselbo et al. 2004). The Upper Cotham Member represents a coastal environment and preserves the initial negative carbon-isotope excursion described by Hesselbo et al. (2002) and correlated with similar excursions in Triassic–Jurassic boundary sections in the Pacific and Tethyan regions (Palfy et al. 2001; Ward et al. 2001; Guex et al. 2003). Faunal data support the interpretation that the Cotham Member represents a shoreface equivalent of the Westbury Formation (Hesselbo et al. 2004). The Upper Cotham Member is of latest Rhaetian or earliest Hettangian age (see below) (Orbell 1973; Hesselbo et al. 2002).

Langport Member. The Langport Member is the uppermost unit of the Lilstock Formation and comprises a series of pale, hard, micritic limestones variously interpreted as representing a shelf- lagoon (Hallam 1960; Hounslow et al. 2004), a broad, shallow, relatively quiet seaway (Wignall 2001), or a carbonate ramp formed following the drowning of the underlying Cotham Member (Hesselbo et al. 2004). The Langport Member is of latest Rhaetian or earliest Hettangian age (George et al. 1969; Poole 1979, 1980, 1991; Hallam 1990, 1995).

Blue Lias Formation The rocks of the Blue Lias Formation comprise rhythmic interbeds of laminated organic-rich shale, dark and pale mud- stones, and limestone. The fauna of the lower Blue Lias Formation is marine, but the lowest few metres lack ammonites and have been named the ‘Pre-Planorbis Beds’. Carbonate-rich Fig. 2. Stratigraphy and graphic logs of the sampled localities. lithologies generally reflect well-oxygenated conditions whereas Lithostratigraphic scheme after Warrington et al. (1980). The question organic-rich lithologies generally reflect oxygen depletion (Hal- mark denotes debate over the position of the Triassic–Jurassic boundary. lam 1960; Weedon 1985). Water depths of at least a few tens of Lithology: (1) laminated black shale; (2) dark grey mudstone; (3) grey metres, sufficient to prevent wave-mixing of bottom waters, have mudstone; (4) grey marl; (5) sandstone; (6) limestone; (7) rip-up clasts; been inferred (Wignall 2001). The ‘Pre-Planorbis Beds’ are of (8) shell bed; (9) beef calcite; (10) bioturbation; (11) desiccation cracks; latest Rhaetian or early Hettangian age (Warrington et al. 1994). (12) soft-sediment deformation. LM, Langport Member. TRIASSIC – JURASSIC PALAEOECOLOGY 323

Local Triassic–Jurassic boundary definition the Westbury Formation coincides with peak taxonomic richness in both sections. Following this peak in abundance, there is a The ages of the Lilstock Formation and lower Blue Lias dramatic decline in abundance, and the samples from the upper Formation rocks are debated because the base of the Hettangian Cotham Member at both sections yielded no fauna at all. This Stage at present lacks a GSSP (see Hesselbo et al. (2004) for a major decline in abundance occurs at approximately the same recent review of the debate). For the sections studied herein, the level that a significant proportion of taxa disappear (i.e. between alternative positions of the Triassic–Jurassic boundary are: (1) the uppermost Westbury Formation and lowermost Lilstock the base of the Psiloceras planorbis Zone (e.g. Warrington et al. Formation; see above). A period of sustained low abundance is 1994); (2) the base of the Blue Lias Formation (e.g. Hallam then recorded from the middle Langport Member to the middle 1990, 1995); (3) the base of the Langport Member (Poole 1979, Psiloceras planorbis Subzone. 1980, 1991); (4) in the Cotham Member between the Rhaetipollis and Heliosporites miospore zones (Orbell 1973) associated with a negative shift in carbon isotopes (Hesselbo et al. 2002). Diversity, dominance, evenness, size and shell thickness The patterns of palaeoecological change recorded in the para- Results meters assessed herein are broadly similar between the two sections and between the two broad lithofacies that were studied Stratigraphic ranges (Figs 5 and 6). Results and trends were similar when analyses were repeated at familial, generic and specific levels, although The patterns of faunal change through the Triassic–Jurassic only the generic-level results are shown here. interval are similar at both St. Audrie’s Bay and Lavernock Point (Figs 3 and 4). There is a gradual increase in taxonomic richness Margalef Diversity. At St. Audrie’s Bay (Fig. 5), there is a steady through the Westbury Formation, reaching a peak in the upper- rise in diversity through the Westbury Formation, in both most metre. Per sample richness is lower in the overlying lithofacies, that parallels the abundance and taxonomic richness Lilstock and Blue Lias formations. Although taxa disappear data (Fig. 3). However, the peak Margalef Diversity in the throughout the succession, there is an apparent peak in the uppermost Westbury Formation is not very much greater than number of disappearances in the upper metre of the Westbury peaks reached in the overlying beds. Diversity fluctuates within Formation and the lower metre of the Lilstock Formation, the succeeding Blue Lias Formation. At Lavernock (Fig. 6), there although at Lavernock this is exaggerated somewhat by the is little change in diversity recorded in samples of carbonate singleton occurrences in the uppermost Westbury Formation lithofacies. In the siliciclastic samples, there is a trend of sample (Fig. 4). At St. Audrie’s Bay, a total of 20 species in 15 increasing diversity through the Westbury Formation, followed genera were recorded in the upper Westbury Formation (above by dramatic decline, which parallels the abundance and richness 7.7 m). Of these taxa, nine species and five genera disappear by data (Fig. 4). In both sections, maximum diversity is recorded at the top of the Westbury Formation, with a further three species the top of the Westbury Formation and samples with the lowest and three genera disappearing in the lower Cotham Member. At diversity derive from the upper Cotham member to lower ‘Pre- Lavernock, 20 species in 17 genera were recorded in the upper Planorbis Beds’. half of the Westbury Formation. Of these, 15 species and 14 genera disappear by the top of the Westbury Formation, although the pattern is distorted as 10 of these species (nine genera) were Dominance and evenness. Dominance and evenness fluctuate recorded only from a single bed. If the two datasets are throughout the Westbury Formation and low dominance (high combined, 12 out of 23 species (52%) and eight out of 18 genera evenness) values are recorded at the top of this unit (Figs 5 and (44%) disappeared in the uppermost Westbury Formation. This is 6). There then follows a sharp, dramatic, and short-lived change: the only horizon where a significant proportion of the marine assemblages from the middle Langport Member to the middle macrofauna disappears. ‘Pre-Planorbis Beds’ are characterized by high dominance (low Patterns of disappearances in local studies may reflect facies evenness). From the middle ‘Pre-Planorbis Beds’ assemblages change, and/or local range contraction or emigration rather than return to relatively low dominance (high evenness) with smaller- true biological extinction. To account for this problem, the ranges scale fluctuations through the upper ‘Pre-Planorbis Beds’ and of taxa known to have undergone global or regional extinction into the Alsatites liasicus Zone. have been extended using the literature (e.g. Hallam 1960; Waters At St. Audrie’s Bay the bivalve Oxytoma is responsible for & Lawrence 1987; Swift & Martill 1999; Hodges 2000) and the high dominance in the upper Langport Member, and the bivalves Paleobiology Database. These range extensions highlight a cluster Liostrea and Modiolus are dominant in the lower Lias Group. At of global and regional last appearance data in the uppermost Lavernock Point Liostrea is responsible for high dominance in Westbury Formation and Lilstock Formation (Figs 3 and 4). the lower Lias Group.

Abundance Size. Body-size among bivalves fluctuates throughout most of the Westbury Formation. A sharp reduction affecting surviving and Abundance trends between the two sections are similar (Figs 3 newly originating taxa occurs in the middle Langport Member. and 4). Within the carbonate lithofacies there is little difference At St. Audrie’s Bay, suppressed body-size persists until the A. in abundances through the section. Within the siliciclastic liasicus Zone, where an increase is recorded by holdover taxa samples, fossil abundance in Westbury Formation samples ranges (Fig. 5). At Lavernock Point, large body-size is recorded by up to 109 specimens per sample, whereas maximum abundance newly originating taxa in a single limestone sample from the P. in samples from the overlying Lilstock and Blue Lias formations planorbis Zone at 14.65 m (Fig. 6). A brief body-size increase is just 30 specimens per sample. If the single high-abundance among holdover taxa occurs in limestone samples from 17 m at sample at Lavernock is removed, however, there is little differ- St. Audrie’s Bay, and from 11.25 m at Lavernock Point (see Figs ence in abundance through the section. Peak abundance within 5 and 6, respectively). 324 L. MANDER ET AL. TRIASSIC – JURASSIC PALAEOECOLOGY 325

Fig. 4. Occurrences (d) and ranges (continuous lines) of taxa recovered from samples taken from the section at Lavernock Point. Dashed lines indicate range extensions for taxa known to have undergone global extinction (+) and regional extinction (s). Total abundance of invertebrate macrofossils per sample is shown on the right, with filled black circles joined by dashed lines representing samples from mudstone lithofacies, and filled grey circles joined by dashed lines representing samples from limestone lithofacies. Lithology as for Figure 2.

Fig. 3. Occurrences (d) and ranges (continuous lines) of taxa recovered from samples taken from the section in St. Audrie’s Bay. Dashed lines indicate range extensions for taxa known to have undergone global extinction (+) and regional extinction (s). Total abundance of invertebrate macrofossils per sample is shown on the right, with filled black circles joined by dashed lines representing samples from mudstone lithofacies, and filled grey circles joined by dashed lines representing samples from limestone lithofacies. Lithology as for Figure 2. 326 L. MANDER ET AL.

Fig. 5. Selected palaeoecological parameters for samples collected from the St. Audrie’s Bay locality, based on counts of invertebrate macrofossil genera. Black symbols and lines indicate samples from mudstone lithofacies. Grey symbols and lines indicate samples from limestone lithofacies. Bivalve size axis shows holdover taxa (black symbols and lines) against new arrivals (grey symbols and lines), with no distinction between limestone and mudstone lithofacies. Error bars represent the standard error of the mean for a given sample. Where error bars are not shown, the standard error was less than, or equal to, the width of the symbol used. Lithology as for Figure 2. TRIASSIC – JURASSIC PALAEOECOLOGY 327

Fig. 6. Selected palaeoecological parameters for samples collected from the Lavernock Point locality, based on counts of invertebrate macrofossil genera. Black symbols and lines indicate samples from mudstone lithofacies. Grey symbols and lines indicate samples from limestone lithofacies. Bivalve size axis shows holdover taxa (black symbols and lines) against new arrivals (grey symbols and lines), with no distinction between limestone and mudstone lithofacies. Error bars represent the standard error of the mean for a given sample. Where error bars are not shown, the standard error was less than, or equal to, the width of the symbol used. Lithology as for Figure 2.

Shell thickness. Shell thickness remains constant from the base abrupt faunal extirpation apparently occurs near the top of the of the Westbury Formation and through the Lilstock Formation Westbury Formation, although at Lavernock this is accentuated (Figs 5 and 6). A brief but dramatic increase occurs in the by a large number of taxa that appear in just one, preceding middle ‘Pre-Planorbis Beds’ at both localities. Above this point, sample. This loss of taxonomic richness is accompanied by a shell thickness measurements return to the values recorded in the reduction in abundance and fossil taxa are rarer in the overlying Westbury and Lilstock Formations and remain constant through strata. Does this represent a catastrophic extinction event? to the A. liasicus Zone. The Margalef Diversity index takes into account fluctuations in sample size (numbers of individuals). At St. Audrie’s Bay, although the Margalef Diversity does decline from the uppermost Discussion Westbury Formation through the lower Lilstock Formation, the rate of change appears no more catastrophic than the other fluctuations Evidence for a catastrophic marine extinction? in diversity recorded through the succession (Fig. 5). Thus, once The stratigraphic range charts from St. Audrie’s Bay and fluctuating numbers of specimens are taken into account the local Lavernock Point (Figs 3 and 4) show the shape of the Late diversity loss appears less dramatic. Indeed, several taxa range into Triassic crisis at two sections in SW UK. A significant and fairly the lower Cotham Member, which appears to be more closely 328 L. MANDER ET AL. related in terms of its faunal composition to the Westbury extinction events, such as the Late Permian event (e.g. Twitchett Formation than to the upper Cotham Member and overlying et al. 2001). Langport Member. This lends support to the assertion of Hesselbo et al. (2004) that the Cotham Member represents a shoreface Patterns of recovery and comparative palaeoecology equivalent of the Westbury Formation. The change of facies between the upper Westbury Formation and lower Cotham Mem- The upper Langport Member and overlying Lias Group sedimen- ber may account for some of the apparent extinctions at that level. tary rocks provide a record of the recovery interval following the At Lavernock, however, the change in diversity from the upper Late Triassic extinction crisis. Fossil assemblages from the upper Westbury Formation to upper Cotham Member does appear Langport Member and ‘Pre-Planorbis Beds’ are characterized by catastrophic in the context of other fluctuations within the high dominance (low evenness), low abundance and low diversity section, although this is not apparent in the carbonate samples (Figs 3–7). Body-sizes in surviving taxa and those that originate (Fig. 6). Even when the anomalously high abundances recorded afterwards are reduced from the middle Langport Member. With in the uppermost Westbury Formation sample are taken into the exception of low abundance, this suite of attributes is typical account, the diversity decline appears to be a real phenomenon. of marine assemblages in the aftermath of extinction events The rocks of the uppermost Westbury Formation and lower- according to Urbanek (1993), who coined the term ‘Post- most Lilstock Formation appear to record an extinction event Extinction Syndrome’ to describe assemblages of low diversity, (biotic crisis) in the shelly marine taxa. Facies change across the small body-size and high abundance that are found in the formational boundary may play a role, although the very low aftermath of extinction events in the Silurian. This ‘syndrome’ levels of biodiversity in the uppermost Cotham Member and may be applicable to assemblages after most Phanerozoic events overlying beds appear real. at all scales. For example, almost all shallow marine assemblages in the immediate aftermath of the Late Permian event are characterized by high dominance, low diversity and small-sized Timing of the biotic crisis? animals (e.g. Schubert & Bottjer 1995; Wheeley & Twitchett 2005). The assemblages from the Triassic–Jurassic successions The disappearance of shelly macroinvertebrate taxa and the documented herein differ only in also having low abundance. ecological changes described herein occurred in the stratigraphic Size decrease is also recorded in bivalves in the aftermath of the interval spanning the upper Westbury Formation and lower end-Cretaceous event (Aberhan et al. 2007). Lilstock Formation. Regardless of which of the previously Recently, Twitchett (2006) proposed a model to describe the proposed definitions of the Triassic–Jurassic boundary is ac- ecological recovery of the benthic marine ecosystem in the wake cepted, this biotic event is dated as latest Rhaetian in age of a major extinction event, using parameters such as dominance (Hesselbo et al. 2004; see discussion above). It apparently began and size, as well as trace fossils and tiering, based on recovery just prior to the onset of the negative carbon isotope excursion after the end-Permian event. Recovery Stage 1 (sensu Twitchett and the palynological changes (see Hesselbo et al. 2004), 2006) is characterized by assemblages of low diversity, high although this needs to be confirmed by more detailed study and dominance (low evenness), low levels of epifaunal and infaunal statistical analysis of taxon ranges. These data support the recent tiering, and small size. This description is applicable to the study of Kiessling et al. (2007), which also demonstrated a assemblages of the Langport Member, especially at St. Audrie’s significant Rhaetian peak in global-level extinction rates. Bay (Fig. 5), and most of the ‘Pre-Planorbis Beds’ at both locations (Figs 5 and 6). Tiering is a major component of the recovery model of A post-extinction ‘Dead Zone’? Twitchett (2006), and an increase in infaunal tiering, in particu- The beds of the upper Cotham Member and lower Langport lar, is used to mark the onset of Recovery Stage 2. The first clear Member, immediately following the interval of major taxon loss, indicator of increased infaunal tiering in the St. Audrie’s Bay are virtually devoid of fossil specimens. Below these beds (the section is the appearance of Arenicolites in the upper ‘Pre- lower Cotham Member and Westbury Formation) fossil assem- Planorbis Beds’ (Barras & Twitchett 2007). This is accompanied blages are characterized by low dominance, high evenness and by an increase in the diversity and evenness of the shelly fluctuating but commonly high abundance and diversity, and assemblages, although size (at least among the holdover taxa) large body-size. Above these beds (from the upper Langport shows little change (Fig. 5). Member to the lower Blue Lias Formation), assemblages are The reappearance of higher tier epifaunal organisms marks the characterized by high dominance (low evenness), low abundance, onset of Recovery Stage 3. No evidence of these higher tier taxa low diversity and small body-size (Figs 3–6). These ecological was found in this study. This may reflect sampling failure, as differences are consistent with the presence of a real extinction crinoids, for example, have been recovered from the Blue Lias event. The depauperate rocks may therefore represent a ‘Dead Formation at St. Audrie’s Bay in previous studies (Warrington & Zone’ (sensu Harries & Kauffman 1990), typical of the immedi- Ivimey-Cook 1990). Recovery Stage 4 marks the return to ate aftermath of most extinction events (e.g. Looy et al. 2001). ‘normal’ background conditions (Twitchett 2006) and from trace The rocks of this depauperate ‘Dead Zone’ are interpreted as fossil evidence apparently occurs within the angulata Zone, an early transgressive systems tract above the sequence boundary higher in the successions and unsampled in this present study in the Lilstock Formation (12.5 m at St. Audrie’s Bay; 7.95 m at (Twitchett & Barras 2004). Lavernock Point). The base of this transgression coincides with the extinction event recorded in the terrestrial palynoflora and Body-size trends the initial onset of a negative carbon isotope excursion recorded in the sedimentary organic matter (Hesselbo et al. 2004). The Urbanek (1993) coined the phrase ‘the Lilliput effect’ to describe coincidence of ecological upheaval, loss of marine taxa, a the temporary appearance of a subnormal (dwarfed or stunted) negative carbon isotope excursion, palynofloral change and phenotype in surviving taxa in the immediate aftermath of an transgression is a common local signature of many global extinction event. The body-size trends recorded in this study TRIASSIC – JURASSIC PALAEOECOLOGY 329

Fig. 7. Representative photographs of samples collected from St. Audrie’s Bay and Lavernock Point illustrating size reduction of Early Jurassic bivalve assemblages. Scale bar represents 4 mm. (a) Late Triassic Westbury Formation, St. Audrie’s Bay; Protocardia rhaetica and Chlamys valoniensis; Sample SABWF7A; height 10.50 m; mean size 11 mm; standard error 0.46. (b) Late Triassic Westbury Formation, Lavernock Point; Chlamys valoniensis and Protocardia rhaetica; Sample LPWF10; height 6.80 m; mean size 14 mm; standard error 0.85. (c) Early Jurassic Lias Group, Lavernock Point; Liostrea; Sample LPLG3; height 11.35 m; mean size 6 mm; standard error 0.55. (d) Early Jurassic Lias Group, St. Audrie’s Bay; Modiolus hillanus and Liostrea hisingeri; Sample SABLG1; height 15.00 m; mean size 5 mm; standard error 0.45.

(Figs 5–7) document the ‘Lilliput effect’ in the shelly marine been implicated as one driver of biotic turnover across the fauna in the recovery interval of the Late Triassic biotic crisis in Triassic–Jurassic boundary (Hautmann 2004; Galli et al. 2005). SW Britain, and affected both surviving taxa and new arrivals. Such a biocalcification crisis might be expected to be expressed There appears to be a slight lag between the event, the change to by a reduction in shell thickness in surviving organisms as they high dominance assemblages and the appearance of small size. struggled to calcify in suboptimal conditions. Data collected With the exception of a brief peak in post-event body-size, from St. Audrie’s Bay and Lavernock Point, however, show no caused by an influx of Liostrea, body-sizes of both survivors and evidence of reduced shell thicknesses anywhere within the stud- newly originating shelly taxa remain low for most the studied ied interval. The only significant change in shell thickness is a interval. Body-size among the soft-bodied benthos remained low temporary twofold increase in shell thickness in the ‘Pre- until the angulata Zone in the upper Hettangian (Twitchett & Planorbis Beds’ accompanying a bloom in Liostrea (Figs 5 and Barras 2004; Barras & Twitchett 2007). 6; Hodges 2000). These data do not support the biocalcification crisis hypothesis. A recent analysis of a global database of benthic marine taxa through the Triassic–Jurassic interval also Possible controls on biotic turnover and palaeoecological found no evidence in support of the biocalcification hypothesis, change as no selectivity with respect to skeletal mineralogy was found It has been suggested that the Late Triassic biotic crisis was (Kiessling et al. 2007, p. 219). coincident with a rise in the flux of volcanigenic carbon dioxide Oceanic anoxia may have played a role during and after many to the atmosphere, which led to temporary acidification of the ancient extinction events (e.g. Bambach 2006; Twitchett 2006, surface ocean and the undersaturation of seawater with respect to and references therein). Evidence for oxygen-depleted conditions aragonite and calcite (Hautmann 2004). This inferred change in in the shales of the lower ‘Pre-Planorbis Beds’, followed by a ocean chemistry may have led to a biocalcification crisis that has gradual improvement in oxygenation through the Blue Lias 330 L. MANDER ET AL.

Formation, has been documented by Wignall (2001). There is a sampling of facies or environments across the Triassic–Jurassic well-known link between small body-size and low oxygen boundary, the palaeoecological changes recorded within the Blue concentrations (e.g. Rhoads & Morse 1971) and the patterns of Lias Formation (the post-extinction recovery) are less affected, as body-size change in the Blue Lias Formation described herein similar facies were sampled throughout. Finally, although the and in previously published studies of the shelly macrofauna sampling strategy was adequate to address the questions of (Hallam 1975) and the trace fossil record (Twitchett & Barras Triassic–Jurassic palaeoecological change on the metre scale, it 2004; Barras & Twitchett 2007) may be a response to the gradual may not have been of sufficient intensity to locate all fossil taxa return to normal oxygen-rich conditions. Certainly the onset of and reveal their full stratigraphic ranges. An even higher small body-size in the lower Blue Lias corresponds well to the resolution study would be needed to pick up changes that might appearance of oxygen-restricted facies (see Wignall 2001). How- be present at, for example, the decimetre scale. As was the case ever, our study found no evidence that anoxia had a direct role in with a recent analysis of European bivalves (Hallam 2002), we the extinction event, as there is no sudden switch to anoxic consider our study to be a robust, first-order representation of conditions at the level of the ecological collapse. real biological trends as recorded by the commoner taxa. A similar situation is recorded during the Permian–Triassic extinction–recovery interval: there is good evidence that marine anoxia is one factor that controls the rate of ecological recovery Conclusions and summary (Twitchett et al. 2004), although the evidence that anoxia had a This study represents the first detailed palaeoecological study of direct role to play in the extinctions is equivocal (e.g. Kozur the shelly fossil assemblages through the Triassic–Jurassic 2007). The post-extinction recovery stages of Twitchett (2006), interval of the UK. The following key conclusions are drawn. described above, may partly reflect the influence of anoxic (1) A real biotic crisis in the marine ecosystem occurred in the bottom waters on the benthic fauna; for example, low levels of short stratigraphic interval spanning the uppermost Westbury epifaunal and infaunal tiering (Recovery Stage 1) are typically Formation and lower Lilstock Formation (i.e. latest Rhaetian). associated with the most oxygen-restricted biofacies (e.g. Savrda This supports the recent data of Kiessling et al. (2007) that show & Bottjer 1991). a significant bioevent in the Rhaetian. Another parameter that has been inferred in many extinction (2) Fossil assemblages in the upper Westbury Formation are episodes is primary productivity collapse. Because food supply is characterized by high abundance, high diversity and low dom- a key control on body-size, a temporary loss of productivity inance (high evenness), which is typical of pre-event, normal could provide an additional explanation for the low abundance marine communities. and small body-size observed in the ‘Pre-Planorbis Beds’ and (3) Fossil assemblages of the upper Lilstock Formation and lower–middle P. planorbis Subzone (see Twitchett 2001; Aber- lower Blue Lias Formation (‘Pre-Planorbis Beds’) are character- han et al. 2007). Evidence for productivity collapse is, however, ized by low diversity, low abundance, high dominance (low limited. The negative excursion in carbon isotopes (Hesselbo et evenness) and small body-size. This suite of characters is typical al. 2004) may have been partly caused by productivity collapse, of assemblages in the initial stages of recovery following an but other plausible interpretations of the carbon-isotope record extinction event. The preceding upper Cotham Member and exist. Post-crisis size changes have also been interpreted in terms lower Langport Member beds are almost devoid of benthic shelly of changes in life strategy (Hallam 1975), whereby selective macroinvertebrates, and may be interpreted as representing the extinction of large K-selected organisms during the crisis would immediate post-extinction ‘Dead Zone’. lead to an abundance of smaller r-selected organisms in the (4) The upper Langport Member and lower Blue Lias Forma- immediate aftermath (Twitchett 2006). tion record a post-event recovery interval in the benthic fauna. There are similarities in the patterns of palaeoecological recovery Bias in the Triassic–Jurassic fossil record in the earliest Jurassic and the Early Triassic, although the absolute timing was different in the two events. Integration of The biological interpretations made thus far must be balanced by data presented herein with previously published data on the trace consideration of potential facies and palaeoenvironmental biases. fossil record suggests that final recovery was reached by the It appears that lithofacies bias is relatively unimportant in the angulata Zone of the Hettangian. Bottom water oxygenation sections studied. Trends in diversity, dominance and evenness seems to be a plausible explanation for the observed changes in follow the same patterns in limestones as they do in siliciclastic body-size, and possibly tiering, through the early recovery rocks (Figs 5 and 6). This indicates that interpretations based on interval, although changes in other factors such as food supply datasets derived exclusively from siliciclastic rocks (for example) and productivity cannot be ruled out. will be as robust as those that contain both limestones and (5) Body-size of bivalves apparently returned to pre-event siliciclastic rocks. Further, because each sample was collected levels within the Hettangian, although most communities had randomly and standardized to a fixed weight, assuming rates of already returned to low dominance–high evenness by this level. deposition were constant or at least similar, comparisons between Other published studies have shown that body-size continued to samples are valid and record real biological signals. increase through the Hettangian and beyond. A more serious bias concerns palaeoenvironments. The same (6) Trends in shell thickness do not support a biocalcification lithofacies sampled on either side of the apparent extinction crisis during the Late Triassic biotic turnover. With the exception event could have been deposited in differing environments, and of a brief, temporary increase in the middle–upper ‘Pre-Planor- there is no guarantee that the same range of palaeoenvironments bis Beds’ (associated with a bloom of Liostrea) no changes in was sampled on either side of the event (see Twitchett 2006). shell thickness are recorded. There are no laminated black shales within the Westbury (7) Although the disappearance of taxa in the upper Westbury Formation, for example, and thus there is no way of assessing the Formation to lower Lilstock Formation transition coincided with palaeoecology of the pre-crisis communities preserved in the dramatic palaeoecological changes, the significant post-event environment represented by this facies. Despite the caveats on palaeoecological changes occurred with little change in taxo- our biological interpretations of extinction caused by non-even nomic richness. This supports the arguments of McGhee et al. TRIASSIC – JURASSIC PALAEOECOLOGY 331

(2004) that ecological changes during extinction events may be carbon-cycle perturbation: a link to initiation of massive volcanism? Geology, decoupled from diversity trends. 30, 251–254. Hesselbo, S.P., Robinson, S.A. & Surlyk, F. 2004. Sea-level change and facies This paper is based on L.M.’s MSc in Palaeobiology thesis, and he thanks development across potential Triassic/Jurassic boundary horizons, SW Britain. Journal of the Geological Society, London, 161, 365–379. the Bob Savage Memorial Fund for support. M. Hautmann is thanked Hesselbo, S.P., McRoberts, C.A. & Palfy, J. 2007. Triassic/Jurassic boundary for scientific discussions and his assistance with Triassic bivalve events: problems, progress, possibilities. Palaeogeography, Palaeoclimatology, taxonomy. R. Schouten is thanked for his assistance with the limestone Palaeoecology, 244, 1–10. samples and acetate peels. We are grateful to M. Aberhan, J. Marshall Hodges, P. 2000. The early Jurassic Bivalvia from the Hettangian and lower and an anonymous reviewer for their constructive and insightful com- Sinemurian of south-west Britain: part 1. Palaeontographical Society Mono- ments. graph, 154. Hodych, J.P. & Dunning, G.R. 1992. Did the Manicouagan impact trigger the end-of-Triassic mass extinction? Geology, 20, 51–54. References Hounslow, M.W., Posen, P.E. & Warrington, G. 2004. Magnetostratigraphy and biostratigraphy of the Upper Triassic and lowermost Jurassic succession, Aberhan, M., Weidemeyer, S., Kiessling, W., Scasso, R.A. & Medina, F.A. St. Audrie’s Bay, UK. Palaeogeography, Palaeoclimatography, Palaeoecology, 2007. Faunal evidence for reduced productivity and uncoordinated recovery 213, 331–358. in Southern Hemisphere Cretaceous–Paleogene boundary sections. Geology, Jablonski, D. 1996. Body size and macroevolution. In: Jablonski, D., Erwin, 35, 227–230. D.H. & Lipps, J.H. (eds) Evolutionary Paleobiology. University of Chicago Allison, P.A. & Wright, P.V. 2005. Switching off the carbonate factory: Press, Chicago, IL, 256–289. A-tidality, stratification and brackish wedges in epeiric seas. Sedimentary Kiessling, W., Aberhan, M., Brenneis, B. & Wagner, P.J. 2007. Extinction Geology, 179, 175–184. trajectories of benthic organisms across the Triassic/Jurassic boundary. Bambach, R.K. 2006. Phanerozoic biodiversity: mass extinctions. Annual Review Palaeogeography, Palaeoclimatology, Palaeoecology, 244, 201–222. of Earth and Planetary Sciences, 34, 127–155. Kozur, H.W. 2007. Biostratigraphy and event stratigraphy in Iran around the Barras, C.G. & Twitchett, R.J. 2007. Response of the marine infauna to Permian–Triassic boundary (PTB): implications for the causes of the PTB Triassic/Jurassic environmental change: ichnological data from southern biotic crisis. Global and Planetary Change, 55, 155–176. England. Palaeogeography, Palaeoclimatology, Palaeoecology, 244, 223–241. Looy, C.V., Twitchett, R.J., Dilcher, D.L., Konijnenburg-Van Cittert, Benton, M.J. 1995. Diversification and extinction in the history of life. Science, J.H.A. & Visscher, H. 2001. Life in the end-Permian dead zone. Proceedings 268, 52–58. of the National Academy of Sciences of the USA, 98, 7879–7883. Bice, D.M., Newton, C.R., McCauley, S., Reiners, P.W. & McRoberts, C.A. Macquaker, J.H.S. 1999. Aspects of the sedimentology of the Westbury 1992. Shocked quartz at the Triassic/Jurassic boundary in Italy. Science, 255, Formation. In: Swift, A. & Martill, D.M. (eds) Fossils of the Rhaetian 443–446. Penarth Group. The Palaeontological Association, London, 39–48. Cuny, G. 1995. French vertebrate faunas and the Triassic/Jurassic boundary. Marzoli, A., Renne, P.R., Piccirillo, E.M., Ernesto, A., Bellieni, G. & De Palaeogeography, Palaeoclimatology, Palaeoecology, 119, 343–358. Min, A. 1999. Extensive 200-million-year-old continental flood basalts of the Droser, M.L., Bottjer, D.J., Sheehan, P.M. & McGhee, G.M. 2000. Decoupling Central Atlantic magmatic province. Science, 284, 616–618. of taxonomic and ecologic severity of Phanerozoic marine mass extinctions. Mayall, M.J. 1981. The Late Triassic Blue Anchor Formation and the initial Geology, 28, 675–678. Rhaetian marine transgression in South-West Britain. Geological Magazine, Galli, M.T., Jadoul, F., Bernasconi, S.M. & Weissert, H. 2005. Anomalies in 118, 377–384. global carbon cycling and extinction at the Triassic/Jurassic boundary: McGhee, G.R., Sheehan, P.M., Bottjer, D.J. & Droser, M.L. 2004. Ecological evidence from a marine C-isotope record. Palaeogeography, Palaeoclimato- rankings of Phanerozoic biodiversity crises: ecological and taxonomic graphy, Palaeoecology, 216, 203–214. severities are decoupled. Palaeogeography, Palaeoclimatography, Palaeoecol- George, T.N., Harland, W.B. & Ager, D.V. et al. 1969. Recommendations on ogy, 211, 289–297. stratigraphical usage. Proceedings of the Geological Society, London, 1656, McRoberts, C.A., Furrer, H. & Jones, D.S. 1997. Palaeoenvironmental 139–166. interpretation of a Triassic/Jurassic boundary section from Western Austria Guex, J., Bartolini, A., Atudorei, V. & Taylor, D. 2003. Two negative ä13C based on palaeoecological data and geochemical data. Palaeogeography, excursions near the Triassic/Jurassic boundary in the New York Canyon area Palaeoclimatography, Palaeoecology, 136, 79–95. (Gabbs Valley range, Nevada). Bulletin de Ge´ologie, Nume´ro Spe´cial, 360, Olsen, P.E., Kent, D.V. & Sues, H.D. et al. 2002. Ascent of dinosaurs linked to 1–4. an iridium anomaly at the Triassic/Jurassic boundary. Science, 296, 1305– Hallam, A. 1960. A sedimentary and faunal analysis of the Blue Lias of Dorset 1307. and Glamorgan. Philosophical Transactions of the Royal Society, London, Orbell, G. 1973. Palynology of the British Rhaeto-Liassic. Bulletin of the Series B, 243, 1–44. Geological Survey of Great Britain, 44, 1–44. Hallam, A. 1975. Evolutionary size increase and longevity in Jurassic bivalves Palfy, J., Demeny, A., Haas, J., Hetenyi, M., Orchard, M.J. & Veto, I. 2001. and ammonites. Nature, 258, 493–496. Carbon isotope anomaly and other geochemical changes at the Triassic/ Hallam, A. 1990. Correlation of the Triassic/Jurassic boundary in England and Jurassic boundary from a marine section in Hungary. Geology, 29, 1047– Austria. Journal of the Geological Society, London, 147, 421–424. 1050. Hallam, A. 1995. Oxygen-restricted facies of the basal Jurassic of North West Poole, E.G. 1979. The Triassic/Jurassic boundary in Great Britain. Geological Europe. Historical Biology, 10, 247–257. Magazine, 116, 303–311. Hallam, A. 2002. How catastrophic was the end-Triassic mass extinction? Lethaia, Poole, E.G. 1980. The Triassic/Jurassic boundary in Great Britain: correspondence. 35, 147–157. Geological Magazine, 117, 619–620. Hallam, A. & Wignall, P.B. 1997. Mass Extinctions and their Aftermath. Oxford Poole, E.G. 1991. Further discussion on correlation of the Triassic/Jurassic University Press, Oxford. boundary in England and Austria. Journal of the Geological Society, London, Hallam, A. & Wignall, P.B. 1999. Mass extinction and sea level-change. Earth- 148, 619–620. Science Reviews, 48, 217–258. Rhoads, D.C. & Morse, J.W. 1971. Evolutionary and ecologic significance of Hallam, A. & Wignall, P.B. 2000. Facies changes across the Triassic/Jurassic oxygen-deficient marine basins. Lethaia, 4, 413–428. boundary in Nevada, USA. Journal of the Geological Society, London, 157, Richardson, L. 1905. The Rhaetic and contiguous deposits of Glamorganshire. 49–54. Quarterly Journal of the Geological Society of London, 61, 385–424. Hammer, Ø., Harper, D.A.T. & Ryan, P.D. 2001. PAST: Paleontological Statistics Richardson, L. 1906. On the Rhaetic and contiguous deposits of Devon and Software Packages for Education and Data Analysis. Palaeontologia Electro- Dorset. Proceedings of the Geologists’ Association, 14, 401–409. nica, 4(1), Article 4. Richardson, L. 1911. The Rhaetic and contiguous deposits of West, Mid, and part Harries, P.J. & Kauffman, E.G. 1990. Patterns of survival and recovery following of East Somerset. Quarterly Journal of the Geological Society of London, 67, the Cenomanian–Turonian (Late Cretaceous) mass extinction in the Western 1–74. Interior Basin, United States. In: Kauffman, E.G. & Walliser, O.H. (eds) Savrda, C.E. & Bottjer, D.J. 1991. Oxygen-related biofacies in marine strata: an Extinction Events in Earth History. Lecture Notes in Earth History, 30, 277– overview and update. In: Tyson, R.V. & Pearson, T.H. (eds) Modern and 298. Ancient Continental Shelf Anoxia. Geological Society, London, Special Hautmann, M. 2004. Effect of end-Triassic CO2 maximum on carbonate Publications, 58, 201–220. sedimentation and marine mass extinction. Facies, 50, 257–261. Schubert, J.K. & Bottjer, D.J. 1995. Aftermath of the Permian–Triassic mass Hesselbo, S.P., Robinson, S.A., Surlyk, F. & Piasecki, S. 2002. Terrestrial and extinction event: palaeoecology of Lower Triassic carbonates in the Western marine extinction at the Triassic/Jurassic boundary synchronized with major USA. Palaeogeography, Palaeoclimatography, Palaeoecology, 116, 1–39. 332 L. MANDER ET AL.

Simms, M.J. 2003. Uniquely extensive seismite from the latest Triassic of the Urbanek, A. 1993. Biotic crises in the history of Upper Silurian graptoloids: a United Kingdom: evidence for bolide impact? Geology, 31, 557–560. palaeobiological model. Historical Biology, 7, 29–50. Simms, M.J. 2007. Uniquely extensive soft sediment deformation in the Rhaetian of Ward, P.D., Haggart, J.W., Carter, E.S., Wilbur, D., Tipper, H.W. & Evans, the UK: evidence for earthquake or impact? Palaeogeography, Palaeoclima- T. 2001. Sudden productivity collapse associated with the Triassic/Jurassic tology, Palaeoecology, 244, 407–423. boundary mass extinction. Science, 292, 1148–1151. Smith, A.B., Gale, A.S. & Monks, N.E.A. 2001. Sea-level change and rock record Warrington, G. & Ivimey-Cook, H.C. 1990. Biostratigraphy of the Late Triassic bias in the Cretaceous: a problem for extinction and biodiversity studies. and Early Jurassic: a review of type sections in Southern Britain. Cahiers Paleobiology, 27, 241–253. Universitaires Catholiques de Lyon, Sciences, 3, 207–213. Swift, A. 1999. Stratigraphy (including biostratigraphy). In: Swift, A. & Warrington, G. & Ivimey-Cook, H.C. 1995. The Late Triassic and Early Jurassic Martill, D.M. (eds) Fossils of the Rhaetian Penarth Group. The Palaeonto- of coastal sections in West Somerset and South and Mid-Glamorgan. In: logical Association, London, 15–30. Taylor, P.D. (ed.) Field Geology of the British Jurassic. Geological Society, Swift, A. & Martill, D.M. (eds) 1999. Fossils of the Rhaetian Penarth Group. London, 9–30. The Palaeontological Association, London. Warrington, G., Audley, M.G. & Elliott, R.E. 1980. A Correlation of Tanner, L.H., Lucas, S.G. & Chapman, M.G. 2004. Assessing the record and Triassic Rocks in the British Isles. Geological Society, London, Special causes of Late Triassic extinctions. Earth-Science Reviews, 65, 103–139. Report, 13. Twitchett, R.J. 2001. Incompleteness of the Permian–Triassic fossil record: a Warrington, G., Cope, J.C.W. & Ivimey-Cook, H.C. 1994. St. Audrie’s Bay, consequence of productivity decline? Geological Journal, 36, 341–353. Somerset, England: a candidate Global Stratotype Section and Point for the Twitchett, R.J. 2006. The palaeoclimatology, palaeoecology and palaeoenviron- base of the Jurassic System. Geological Magazine, 131, 191–200. mental analysis of mass extinction events. Palaeogeography, Palaeoclimatol- Waters, R.A. & Lawrence, D.J.D. 1987. Geology of the South Wales Coalfield, ogy, Palaeoecology, 232, 190–213. Part III, the Country around Cardiff, 3rd. Memoirs of the British Geological Twitchett, R.J. & Barras, C.G. 2004. Trace fossils in the aftermath of mass Survey, Sheet 263 (England and Wales), 69–76. extinction events. In: McIlroy, D. (ed.) The Application of Ichnology to Weedon, G.P. 1985. Hemipelagic shelf sedimentation and climatic cycles: the Palaeoenvironmental and Stratigraphic Analysis. Geological Society, London, basal Jurassic (Blue Lias) of south Britain. Earth and Planetary Science Special Publications, 228, 397–418. Letters, 76, 321–335. Twitchett, R.J., Looy, C.V., Morante, R., Visscher, H. & Wignall, P.B. Wheeley, J.R. & Twitchett, R.J. 2005. Palaeoecological significance of a new 2001. Rapid and synchronous collapse of marine and terrestrial ecosystems Griesbachian (Early Triassic) gastropod assemblage from Oman. Lethaia, 38, during the end-Permian biotic crisis. Geology, 29, 351–354. 37–45. Twitchett, R.J., Krystyn, L., Baud, A., Wheeley, J.R. & Richoz, S. 2004. Wignall, P.B. 2001. Sedimentology of the Triassic/Jurassic boundary beds in Rapid marine recovery after the end-Permian mass extinction in the absence Pinhay Bay (Devon, SW England). Proceedings of the Geologists’ Associa- of marine anoxia. Geology, 32, 805–808. tion, 112, 349–360.

Received 22 February 2007; revised typescript accepted 28 February 2007. Scientific editing by John Marshall