The earliest Laurasian unionoids? Freshwater bivalves from the Middle Triassic of Devon, southern UK

Abstract

Bivalve moulds and putative resting traces are recorded from a mudstone unit of lacustrine origin close to the top of the mainly alluvial Otter Sandstone Formation (Anisian, Middle Triassic) near Sidmouth, south-east Devon, southern UK. The bivalves resemble members of the Triassic – Recent Order Unionoida, to which they are tentatively attributed. Previous records of Early and Middle Triassic freshwater bivalves are largely Gondwanan. Our record provides evidence for a significant mid-Triassic radiation into Laurasia, as freshwater ecosystems recovered after the end-Permian mass extinction, and through the ensuing Early Triassic environmental crises.

Introduction

In Triassic times (approximately 252 – 201 million years ago, Ogg et al., 2014), our planet was dominated by the Pangea supercontinent, formed through the late Palaeozoic amalgamation of Laurasia (Asia, Europe and North America) and Gondwana (Africa, South America, India, Madagascar, Australia and Antarctica). Pangea was surrounded by the Panthalassa Ocean, and indented from the east by the low latitude oceanic Tethys gulf (Stampfli and Borel, 2002).

The Triassic world lacked polar ice caps, and following the end-Permian mass extinction (EPME) and ensuing Early Triassic greenhouse crises (Retallack et al., 2011; Sun et al., 2012), the Middle Triassic was characterised by recovery of ecosystems (Benton and Newell, 2014). Middle Triassic Pangea was strongly influenced by monsoonal circulation, and humid phases (Preto et al., 2010). Equatorial Pangea became occupied by a broad arid belt, flanked towards the poles by warm-temperate climatic zones. Terrestrial ecosystems involved re-establishment of coniferous forests (Looy et al., 1999) and initial radiations of important animal groups such as bird-line archosaurs (Avemetatarsalia; Nesbitt et al., 2017) and true flies (Diptera; Shcherbakov et al., 1995).

The early Middle Triassic (Anisian) of the southern UK is dominated by arenaceous continental lithofacies, developed locally as red-beds. Deposition occurred predominantly in alluvial systems within linked rift basins, at a palaeolatitude of approximately 20 °N within the eastern part of Pangea (Benton et al., 2002; Hounslow and Ruffell, 2006; Newell, 2017a). Remnant Armorican uplands lay to the south (McKie and Williams, 2009). Climates were warm to hot and traditionally considered as subtropical in aspect and relatively dry (Ziegler et al., 1993). Mixed freshwater – terrestrial biotas and ichnofaunas are locally present (Benton et al., 1994, 2002; Coram and Radley, 2015; Coram et al., 2019), peaking in abundance and diversity late in the Anisian, perhaps due to increasing humidity linked to Tethyan (Muschelkalk) transgression, farther east (Newell, 2017a).

Extant rivers and freshwater lakes on all continents except Antarctica host bivalves of the unionoid clade (Order Unionoida, Graf and Cummings, 2006, 2007; Graf, 2013), partly reflecting the mid-Mesozoic break-up of Pangea (Bogan and Roe, 2008). The Unionoida is a diverse non-marine molluscan group which arose during the Triassic from late Palaeozoic marine ancestors (Newell and Boyd, 1975; Graf and Cummings, 2006; Skawina and Dzik, 2011), following the end-Permian mass extinction, which apparently extirpated freshwater bivalves (Yates et al., 2012). Extant unionoids are predominantly shallow infaunal to epi-benthic taxa in alluvial and lacustrine environments (Saarinen and Taskinen, 2003; Schwalb and Pusch, 2007; Knoll et al., 2017) and characterised by a larval phase involving parasitic attachment, generally to fish (McIvor and Aldridge, 2007; Bogan and Roe, 2008). Comparable life modes are commonly attributed to fossil unionoids through analogy of shell morphology and association with alluvial and lacustrine lithofacies (Skawina, 2013), occasional preservation of larval shells (Aldridge and Horne, 1998), preservation of shells in inferred life position and their association with bivalve ichnofossils (Radley and Barker, 1998; Radley et al., 1998).

Records of Early and Middle Triassic unionoids remain sparse (Skawina and Dzik, 2011; Yates et al., 2012), despite the global abundance of Triassic alluvial successions (Benton and Newell, 2014). Small (possibly ‘dwarfed’) bivalves resembling unionoids are recorded from vertebrate coprolites, preserved within Early Triassic (Olenekian) fluvio-lacustrine lithofacies of the Burgersdorp Formation in the Karoo Basin, South Africa (Yates et al., 2012). Among the oldest confirmed unionoids (sensu Skawina and Dzik, 2011) are Tihkia karrooensis (Cox) from Middle Triassic (Anisian) beds in Tanzania and Zambia (Cox, 1932; Dixey, 1937; Van Damme et al., 2015), with several possible small unionoid taxa also known from the Anisian Wianamatta Shales of Australia (Etheridge, 1888; Skawina and Dzik, 2011). These records raise the possibility that the Gondwanan region of Pangea hosted the early evolution and radiation of the clade, shortly after the EPME (Yates et al., 2012). Skawina and Niedźwiedzki (2012) recorded freshwater bivalves showing possible unionoid characters from the Anisian of the Holy Cross Mountains (Poland). These await detailed documentation, but nevertheless indicate the presence of unionoids or unionoid-like freshwater bivalves in Laurasian Pangea by the Middle Triassic. The Carnian Pluvial Episode (CPE; e.g. see Dal Corso et al., 2018) possibly marks a stronger radiation of freshwater bivalves into Laurasian Pangea; unionoids being well documented from Carnian and younger Late Triassic freshwater lithofacies in Europe and North America (e.g. see Dubiel et al., 1991; Skawina and Dzik, 2011; Bogan and Weaver, 2012; Rinehart and Lucas, 2013; Skawina, 2013).

Here we report bivalve moulds, tentatively attributed to the Unionoida, from the Anisian of the southern UK.