A silicified Early Triassic marine assemblage from Svalbard

Understanding how the marine biosphere recovered from the late Permian mass extinction event is a major evolutionary question. The quality of the global fossil record of this interval is, however, somewhat poor due to preservational, collection and sampling biases. Here we report a new earliest Induan (Hindeodus parvus Zone) marine assemblage from the Deltadalen Member of the Vikinghøgda Formation, central Spitsbergen, which fills a critical gap in knowledge. The fully silicified fossils comprise the oldest silicified assemblage known from the Triassic and provide critical new systematic data. For its age, the assemblage is exceptionally diverse with 14 species of bivalves and gastropods, as well as conodonts and ammonoids. Four new bivalve species (Austrotindaria antiqua, A. svalbardensis, Nucinella taylori and N. nakremi) and one new gastropod species (Glabrocingulum parvum) are described, and five families are recorded in the Induan for the first time. Some of the common and globally widespread Early Triassic taxa, such as Unionites, are also present, and their exceptional preservation reveals key morphological characters that are documented for the first time. Taxonomic and ecological revisions based on these new data suggest that shallow-infaunal deposit-feeders were a dominant component of pre-Spathian benthic communities. The gastropods and bivalves all possessed a planktotrophic larval stage, which may have been a particular advantage in the wake of the late Permian mass extinction. http://zoobank.org/urn:lsid:zoobank.org:pub:3EBCAEF3-27C2-4216-9F18-89F195FA534F


Introduction
The aftermath of the late Permian mass extinction represents a key interval in the diversification of marine biota. Even though 78% of marine genera are estimated to have gone extinct during the late Permian event, no novel phyla or classes and only one new mode of life originated during the extinction aftermath (Erwin et al. 1987;Foster & Twitchett 2014). The re-diversification of benthic groups in the wake of the extinction, such as bivalves (Nakazawa & Runnegar 1973;Komatsu et al. 2004;Fraiser & Bottjer 2005;Hautmann 2007;Posenato 2008) and gastropods (Erwin & Pan 1996;N€ utzel 2005;Gr€ undel & N€ utzel 2012), is typically described as occurring in the Middle Triassic, resulting in the traditional view of a 'delayed' post-Permian recovery. The Early Triassic fossil record is, however, notoriously poor, as evidenced by an unusually high number of Lazarus taxa, due to the typical mouldic preservation of shells (Nakazawa & Runnegar 1973;Wheeley & Twitchett 2005) and their small, easily overlooked sizes (Hautmann & N€ utzel 2005), as well as the substantial regional and latitudinal sampling bias towards low (tropical) palaeolatitudes of the Palaeotethys (Foster & Twitchett 2014).
Better preserved benthic fossil assemblages have been reported from the Induan of South China (Kaim et al. 2010;Hautmann et al. 2011Hautmann et al. , 2015 and Primorye, Russia (Shigeta et al. 2009), and from the Olenekian of the western USA (Batten & Stokes 1986;Hautmann & N€ utzel 2005;N€ utzel & Schulbert 2005;Pruss et al. 2015) and Pakistan (Wasmer et al. 2012;Kaim et al. 2013). In addition, a partially silicified fauna is known from the Griesbachian of Oman Wheeley & Twitchett 2005;Oji & Twitchett 2015). Studies of those assemblages have shown that a greater number of bivalve, gastropod and crinoid lineages survived the late Permian mass extinction event and/or were present in the Early Triassic than previously thought. These partially silicified and better preserved assemblages still lack key diagnostic characters of many taxa, however, especially the internal morphology of bivalve shells, making taxonomic assignments often equivocal.
Better preserved early Induan (Griesbachian) fossil assemblages from palaeotropical localities have been critical in demonstrating that taxonomically and ecologically *Corresponding author. Email: w.j.foster@gmx.co.uk diverse benthic ecosystems reappeared locally in some settings by the second conodont zone of the Triassic in the Neotethys . At higher latitudes, however, early Induan shelly benthic assemblages are rarely reported and typically consist of poorly preserved cosmopolitan generalists, such as Claraia, Unionites, Lingularia and Warthia (Spath 1930;Mørk et al. 1999;Zonneveld et al. 2007). In these regions, trace fossils are typically used as a measure of documenting ecosystem collapse and recovery (following Twitchett 2006) and demonstrate that rapid local recovery of the infauna took place within the earliest Induan (Twitchett & Barras 2004;Beatty et al. 2008;Zonneveld et al. 2010). From their analysis of global fossil occurrences, Foster & Twitchett (2014) also concluded that during the Induan the benthic fauna of the extratropical northern palaeolatitudes contained a greater ecological diversity than that of the palaeotropics.
Here, we describe the first silicified fossil assemblage from the Lower Triassic of the northern palaeolatitudes. The assemblage was recovered from the lower Griesbachian of central Spitsbergen, Svalbard, making it the oldest silicified assemblage known from the Mesozoic. It is the first fully silicified assemblage of Early Triassic age to be found, and the exquisite preservation of the internal and external morphology of the bivalve and gastropod shells provides critical new taxonomic data that have major implications for our understanding of the response of marine ecosystems to the late Permian mass extinction event and of the stratigraphical ranges of several molluscan families.

Material and methods
The fossil assemblage described herein was recovered from two carbonate concretions, collected by WJF and RJT in 2013 from 11.9 and 12.6 m above the base of the Deltadalen Member of the Vikinghøgda Formation, Lusitaniadalen, Svalbard (N78 17 0 54.8 00 , E016 43 0 59.3 00 ; Fig. 1). The fossils were extracted in the laboratory by first mechanically disaggregating 3 kg samples to expose larger fossils. Remaining rock fragments were then dissolved using the buffered formic acid technique of Jeppsson & Anehus (1995). To maximize yield, the residue was collected at c. 12 hour intervals, washed thoroughly with tap water to remove any excess solution and to avoid crystal growth, and dried. The buffered solution was renewed every 48 hours. Heavy liquid separation (Mitchell & Heckert 2010) was used to separate the fossils from the remaining residue. The specimens are housed in the Natural History Museum, London (NHMUK).

Geological setting
During the Early Triassic, the Svalbard archipelago was situated at c. 45-50 N in the southern part of the Boreal Ocean ( Fig. 1; Hounslow et al. 2008). In central Spitsbergen, the Vikinghøgda Formation, described by Mørk et al. (1999), records deposition through the latest Permian and entire Early Triassic in a siliciclastic, open-marine, shelf setting. It is divided into three members, of which the Deltadalen Member is the lowest (Mørk et al. 1999). At the study site in Lusitaniadalen, the basal 1.6 m of the Deltadalen Member is composed of bedded, well-bioturbated, fine-to medium-grained, glauconitic sandstones that are very similar to the underlying Kapp Starostin Formation except that they lack diagenetic chert nodules (Mørk et al. 1999;Nabbefeld et al. 2010). These sandstones contain a diverse trace fossil assemblage, indicating a fully functional benthic ecosystem that was living under well-oxygenated conditions (Nabbefeld et al. 2010). In contrast, the body fossil assemblage is limited, comprising mainly the phosphatic-shelled lingulid brachiopods, which may indicate preservational bias.
The sandstones record the onset of marine transgression (Mørk et al. 1999;Nabbefeld et al. 2010), with the base of the overlying laminated, silty mudstones marking significant deepening. Several horizons of cemented siltstones and tabular concretions are found in the lower few metres of this mudstone-dominated interval and contain coarser laminae with abundant ichthyoliths and disarticulated lingulid shells. Occasional, 1-5 cm thick, fine-grained, pyritic and glauconitic, graded, cemented sandstones, interpreted as distal tempestites, also occur within the laminated silty mudstones. Biomarker data support the interpretation based on field observations that deposition took place under anoxic and periodically euxinic conditions during transgression and maximum flooding, with evidence of phytoplankton blooms in the surface waters likely driven by nutrient influx (Nabbefeld et al. 2010).
From c. 10 m above the base of the mudstones, the lithologies become noticeably coarser with a greater proportion of interbedded siltstones and very fine sandstones, presumably due to progradation or sea level fall. The coarser, heterolithic beds are bioturbated, initially by millimetre-diameter Planolites and then with the addition of Skolithos and Arenicolites about 1.5 m higher, indicative of environmental amelioration and deposition under more persistently oxygenated conditions. The ichnofabric index remains low (ii2-3), however, and the burrows are small (diameters <5 mm) and do not penetrate deeply, indicating that the environment was probably still not fully optimal for benthic colonization. Earliest Triassic ammonoids, conodonts and a moderate benthic assemblage have already been recorded from the Planolitesdominated interval (Mørk et al. 1999). One of the concretions that yield the silicified assemblage documented in this study derives from that same horizon, 12.6 m above the base of the formation. The second concretion was collected from c. 65 cm lower (Fig. 1).
The age of the Deltadalen Member is well constrained by biostratigraphy (ammonoids and conodonts) and magnetostratigraphy, with deposition recorded from the upper Changhsingian, prior to the late Permian mass extinction event, through most of the Induan (Mørk et al. 1999;Hounslow et al. 2008;Nakrem et al. 2008). Locally, the late Permian extinction event is recorded by the disappearance of prolific bioturbation just below the top of the glauconitic sandstones, 1.6 m above the base of the member (Nabbefeld et al. 2010). From conodont evidence, Nakrem et al. (2008) inferred that the Permian/Triassic boundary occurs between 5 and 11 m above the base of the Deltadalen Member. Claraia cf. wangi is recorded 10.1 m above the base of the member at Lusitaniadalen, indicative of a Griesbachian age. The silicified assemblages described herein are of earliest Griesbachian (earliest Induan) age, and are assigned to the upper part of the Otoceras boreale Zone, which corresponds to the basal Triassic Hindeodus parvus Conodont Zone (Orchard 2007).
Description. Shell outline is subcircular, with its length slightly greater than the width. The dorsal valve is inflated to an apex, which is located about a third of the diameter from the anterior margin. Shell smooth, except for fine concentric growth lines.
Remarks. Extant solitary discinids are sessile, epifaunal invertebrates that attach to hard surfaces with a suckerlike pedicle (Mergl 2010). They have been attaching to shelled invertebrates since the Ordovician (Mergl 2010), and one of the specimens described herein is attached to an ammonoid. Discinids are suspension feeders, and their co-occurrence with lingulids in laminated black shales has led some authors to interpret them as being tolerant of low-oxygen conditions (Savoy 1992;Hallam 1995;Mergl 2010).
Mode of life. Surficial, stationary, attached, suspension feeder (Mergl 2010 Description. Shell is small, mytiliform, inequilateral, prosocline, higher than long, and moderately inflated. Umbo is small, terminal and prosogyrate. Posterior dorsal margin is straight. Posterior margin is slightly convex, forming a rounded posteroventral margin. Anterior margin is long, nearly straight or weakly acute and partly depressed near the umbo. Remarks. The internal characters of the shells cannot be observed because all the specimens are articulated. Externally, the shells resemble Promyalina schamarae from the Griesbachian Lazurnaya Bay Formation, Russia (Shigeta et al. 2009), and are therefore assigned to this species. These specimens differ from P. groenlandica (Newell, 1955), as the beak does not project beyond the dorsal margin, and from P. spathi in having a more convex anterior margin.
Order Solemyoida Dall, 1889Family Nucinellidae Vokes, 1956 Diagnosis. Shell nuculoid, obliquely oval, higher than long, monomyarian with anterior adductor muscle scar only. Hinge with subumbonal taxodont teeth and single elongate lateral tooth on the anterior dorsal margin. Ligament mostly opisthodetic, wholly external or in a sunken resilifer.
Remarks. Although living nucinellids are sometimes classified in the family Manzanellidae, which extends back into the Permian (e.g. Coan & Valentich-Scott 2012), Oliver & Taylor (2012) argued that the Nucinellidae and Manzanellidae should be separated on morphological grounds and their conclusions are followed here. Manzanella, the type genus of Manzanellidae, is dimyarian and subcircular in outline, with its taxodont teeth lying posterior to the beak. In contrast, Nucinella and Huxleyia (i.e. the Nucinellidae) are both monomyarian and elliptical, with their teeth positioned anterior to the beak (Oliver & Taylor 2012).

Diagnosis. Nucinellids with external ligament.
Remarks. Of the two genera described for the Nucinellidae, Huxleyia has a mostly internal ligament set in a sunken resilifer, whereas Nucinella has an external ligament. The specimens described herein have an opisthodetic or amphidetic ligament and so are assigned to Nucinella.
Nucinella taylori sp. nov. (Fig. 3) Diagnosis. A small Nucinella having a nuculoid shape, smooth shell except for growth lines with three subumbonal and two anterior pointed blade-like teeth, with no triangular flat area below the dentition; opisthodetic ligament. Prodissoconch valves: outline nuculoid and suboval. Posterior margin is distinct, long and slightly incurved. Inequilateral, with beaks close to posterior margin, and sculpture consists of irregularly spaced growth laminae. Ventral valve margin has a narrow flat platform. Five anterior and 11 posterior hinge teeth. Amphidetic ligament, lying between the beak and anterior hinge plate.
Remarks. These specimens are most similar to the extant nucinellid Nucinella serrei in their small size, number of posterior and anterior hinge teeth and opisthodetic ligament. However, they lack a flat triangular area below the teeth dentition and a small circular pit at the end of the lateral tooth, which supports their separation.
The majority of extant Nucinella range from intertidal to 500 m deep (La Perna 2005), but some species have been described from water depths exceeding 3000 m (Oliver & Taylor 2012). A large fossil Nucinella species has been described from a Late Cretaceous cold-seep deposit (Amano et al. 2007), showing that this genus may inhabit a wide range of sulphide-rich environmental settings. Bacterial symbiosis with sulphur-oxidizing bacteria is confirmed for N. owenensis and has been inferred for all species of the Nucinellidae (Oliver & Taylor 2012).
Nucinella taylori sp. nov. supplants N. birkelundi from the Late Jurassic (Clausen & Wignall 1990) and Nucinella? sp. from the Late Triassic (N€ utzel & Kaim 2014) as the oldest known species of Nucinella, and extends the range of the genus to the basal Triassic H. parvus Conodont Zone.
The prodissoconch valves are very similar to adult specimens of Nucinella taylori sp. nov. except that they have more hinge teeth, which appears to reflect their premetamorphosis stage of development. The position of the ligament in the prodissoconch valves also differs from adult specimens of N. taylori sp. nov. in being amphidetic rather than opisthodetic, but this character is known to change after metamorphosis (Bernard 1898). The prodissoconch valves most resemble N. taylori sp. nov. rather than N. nakremi sp. nov., but may represent larval stages for either or both species. Mode of life. Shallow infaunal, fully motile, slow, chemosymbiotic (Oliver & Taylor 2012).
( Fig. 4) Diagnosis. A small Nucinella having a nuculoid shape, smooth shell except for growth lines. Prosogyrate beak, one to three subumbonal teeth. Ligament amphidetic and does not invade the hinge plate.
Derivation of name. Named after Dr Hans Arne Nakrem in recognition of his work on Permian and Triassic fossils from Svalbard.
Description. Shell small, nuculoid and ovate. Posterior dorsal margin distinct, slightly incurved; posterior margin rounded. Inequilateral, prosogyrate, with beaks close to anterior margin. Umbo prominent. Smooth, thin shell with very weak growth lines. Monomyarian: posterior adductor muscle scar absent; anterior adductor large, oval. One to two subumbonal, pointed blade-like teeth plus one anterior tooth. Ligament amphidetic, prominent, external does not invade the hinge plate.
Remarks. These specimens differ from other described nucinellid species in having fewer hinge teeth, a more elliptical shape and a prosogyrate beak. Such differences may occur during the ontogeny of Nucinella (e.g. Bernard 1898) and so are not sufficient for assignment to a separate genus. These specimens are, however, considered to represent a separate species rather than an intermediate ontogenetic stage between the protoconch and adult stage of Nucinella taylori. During ontogeny, the shape, size and position of nucinellid subombonal teeth also vary: in earlier stages of development they are more rounded and later they develop a chevron-blade shape with the older teeth making space below the beak for thinner, newer ones (Bernard 1898;La Perna 2004). In contrast, the subumbonal teeth of specimens assigned to N. nakremi and N. taylori have comparable shapes, and so indicate a similar stage of development. Furthermore, because the position of the ligament in nucinellids is fixed after metamorphosis (Bernard 1898;La Perna 2004), and the only subsequent ontogenetic change is an increase in ligament size with age, the differences in ligament position between N. nakremi and N. taylori cannot be ontogenetic. If the specimens assigned to N. nakremi were included as an intermediate ontogenetic stage of N. taylori, the ontogenetic pattern would not match any known nucinellid (cf. Bernard 1898; La Perna 2004), and, therefore, the differences in the subumbonal teeth and the position of the ligament support their separation. A specimen of Nucinella nakremi sp. nov. includes an example of a transposed hinge on a left valve (Fig. 4D). Instead of the normal left valve arrangement of three hinge teeth and a lateral secondary ridge creating a secondary socket, this specimen has two hinge teeth and a lateral tooth (i.e. the normal right valve arrangement). An alternative interpretation is that this specimen represents an earlier ontogenetic stage, but this is rejected because even though the specimen is slightly smaller, its lateral tooth is more prominent than in a typical left valve arrangement and both the subumbonal and lateral teeth appear to fit with the expected corresponding valve arrangement. In addition, it has been reported that in the early stages of nucinellid development the second lateral tooth is very small and closer to the subumbonal teeth than later in ontogeny (Bernard 1898), which is not the case with this specimen. Transposed hinges have been reported in a number of bivalve families, but this is the first reported occurrence in a species of Nucinellidae.
Remarks. The Neilonellidae are very similar to the Nuculanidae, but the lack of a resilifer in mature adults supports their separation (Coan & Valentich-Scott 2012). The new specimens described herein differ from the Malletiidae in lacking conspicuous gapes, and from the Tindariidae in having a short gap in the dentition below the beaks (Di Geronimo & La Perna 1997). The family is currently known from the Jurassic to the present in all oceans, especially in deep water and soft substrates (Coan & Valentich-Scott 2012).
Diagnosis. Delicate almost smooth shell, with weak concentric sculpture. A short, edentulous gap between the posterior and anterior hinge plates. No pallial sinus and without rostrum.
Remarks. The Neilonellidae comprises three valid genera: Neilonella, Austrotindaria and Pseudoneilonella (La Perna 2007). The convex shape of the posterior margin and the presence of an opisthodetic ligament in our specimens indicate better agreement with Austrotindaria than Neilonella (Di Geronimo & La Perna 1997;La Perna 2007). Austrotindaria differs from Neilonella and Pseudoneilonella by having a delicate, almost smooth shell rather than a sturdy, sculptured shell, and by having a small elongate pit below the posterior and anterior teeth (La Perna 2007). This genus is currently known from the Miocene to Recent (La Perna 2007;Coan & Valentich-Scott 2012), and the specimens identified in his study extend the range of the genus to the beginning of the Triassic. Diagnosis. Shell small, smooth except for concentric growth lines, subtrigonal, inequilateral, prosogyrate; taxodont hinge dentition with more than 50% fewer anterior than posterior ones, obtusely chevron-shaped teeth, hinge plate interrupted below the beak by an edentulous gap, a small rounded triangular pit below the edentulous gap; ligament opisthodetic. Derivation of name. Latin, antiqua (ancient) referring to this species being the oldest known of the genus.
Description. Equivalve, inequilateral shell with a subtrigonal outline and a beak positioned approximately 30% along the length of the dorsal margin from the posterior; H/L ratio 0.6-1. Conspicuously tumid. Umbo prosogyrate, prominent, moderately broad, rounded and projects above the hinge margin. Slightly rounded and gently sloping anterodorsal margin. Posterodorsal margin almost straight to slightly rounded, gently sloping, with a slight angled junction with the posterior margin. Ventral margin deeply rounded. Escutcheon short, relatively broad, elliptical; lunule narrow. Shell smooth with fine concentric growth lines; entire inner margin smooth. Small ligament, external, opisthodetic, with a well-defined margin and wellrounded triangular pit seated beneath the edentulous gap.
Hinge plate with taxodont teeth in two series, sometimes separated by a narrow, plain area, without resilifer, that is narrow below the beak, broadening towards the anterior and posterior ends. Robust teeth, moderately long and blunt, with more than 50% fewer anterior than posterior ones. Smooth ventral margin. No pallial line, sinus or muscle scars are present. As the size of the shell increases the edentulous gap becomes proportionally smaller and more central, and moves externally.
Remarks. The external morphology of these specimens is identical to most Early Triassic specimens previously described as 'Unionites' fassaensis, which is one of the most widespread bivalve species from the Lower Triassic and is also a problematic dustbin taxon that includes a range of different morphologies. These specimens differ from the original description and figures of 'Myacites' fassaensis Wissmann, 1841 in having a less elongated posterior margin and a more prosogyrate beak. Due to poor preservation, however, little is known about the internal morphology of Early Triassic specimens assigned to U. fassaensis, which has created some uncertainty . Internally, these new specimens lack the following characters that Geyer et al. (2005) determined were diagnostic of Unionites: a nymph extending nearly half of the posterior margin; an impressed adductor muscle scar; an overlap of the anterior hinge; and a deeply impressed lunule and posterior keel. Thus, these specimens cannot be assigned to Unionites. The presence of taxodont hinge dentition, an opisthodetic ligament and lack of ornamentation are, however, characteristic of the genus Austrotindaria. The more prosogyrate beak means that these specimens have a similar external morphology to Middle Triassic Unionites specimens (e.g. Geyer et al. 2005), but apart from possessing a faint posterior keel they lack the diagnostic criteria of Unionites. Furthermore, the Unionites specimens described by Geyer et al. (2005) and from this study (see below) all have a strong posterior keel. Thus, these specimens are not assigned to Unionites. These specimens, and all previous specimens assigned to Unionites fassaensis that possess a prosogyrate beak and lack a well-defined posterior keel, are therefore assigned to Austrotindaria antiqua sp. nov.
Mode of life. Shallow infaunal, fully motile, slow, miner (Stanley 1968 Derivation of name. Named after the Svalbard archipelago. Description. Shell outline subtrigonal, equivalve, inequilateral with beak positioned approximately 60% of the distance along the dorsal margin from the posterior; Height/Length (H/L) ratio 0.6-1; conspicuously tumid. Orthogyrate umbo, prominent, moderately broad, rounded and projected above the hinge margin. Anterodorsal margin slightly rounded and gently sloping. The posterodorsal margin is almost straight to slightly rounded and gently sloping. Ventral margin is smooth and deeply rounded. Escutcheon is short, relatively broad, elliptical; lunule narrow. No sculpture except for fine concentric growth lines; entire inner margin is smooth. Prodissoconch is smooth, broadly subovate, with H/L ratio of 0.6-0.9. Ligament small, external with well-defined margin, opisthodetic, with a well-rounded triangular pit seated beneath the edentulous gap. Hinge plate with taxodont teeth in two series; separated by narrow, plain area, without resilifer, narrow below the beak, broadening towards the anterior and posterior ends. Teeth are robust, moderately long and blunt, with more posterior than anterior teeth, separated by an edentulous gap. As the size of the shell increases the edentulous gap becomes proportionally smaller, more central and moves externally. No pallial line, sinus or muscle scars present.
Remarks. These specimens differ from Austrotindaria antiqua sp. nov. in having a more equal number of anterior to posterior teeth and an orthogyrate beak. The direction of the beak also separates these specimens from unequivocal species of Unionites. In the specimens described here and other Austrotindaria species the beak is orthogyrate to posteriorly opisthogyrate, whereas in Unionites it is prosogyrate. Based on this key character, most specimens previously assigned to Unionites fassaensis, and similar Early Triassic specimens with an orthogyrate beak and no internal morphological detail preserved, are herein assigned to Austrotindaria svalbardensis sp. nov. These specimens have a very similar shape, size and ornamentation to the type species of Austrotindaria (A. wrighti Fleming); however, they differ in possessing a small, rounded, triangular pit below the edentulous gap, and differ from other Austrotindaria species, such as A. mawheraensis, in lacking a weak posterior rostrum. For these reasons, they are therefore considered a separate species.
Mode of life. Shallow infaunal, fully motile, slow, miner (Stasek 1961;Stanley 1968 Description. Outline subovate to elongate, equivalve, inflated below the umbo; inequilateral with beak lying approximately 62% along the dorsal margin length from posterior; H/L ratio 0.4-0.7. Posterior margin elongated and almost straight, anterior margin narrowly rounded. Escutcheon and lunule indistinct. Umbo orthogyrate, prominent, moderately broad, rounded, projecting above the hinge margin. Ornamented externally with fine concentric growth lines; entire inner margin smooth. Remarks. The internal characters and hinge in these specimens were not observed and have not been reported for Unionites canalensis. Externally, the shell is virtually identical to those that are typically assigned to U. canalensis (e.g. Hofmann et al. 2015), with a medially placed umbo that is a diagnostic feature of U. canalensis (Catullo, 1846) and an orthogyrate beak. In contrast, all other species of Unionites have a more anteriorly located umbo, and the beak in Unionites is prosogyrate (Geyer et al. 2005). Thus, these Early Triassic specimens clearly do not belong to the genus Unionites. The external features of these specimens are most similar to species of Neilonellidae, such as Austrotindaria benthicola (Dell, 1956). Austrotindaria is the only genus of the Neilonellidae that is reported from the Early Triassic, and these specimens are, therefore, tentatively assigned to it. Without observation of the internal characters, an unequivocal generic assignment cannot be made. The posterior margin is more elongate than in Austrotindaria svalbardensis sp. nov., and these specimens are therefore considered to represent a separate species.
Description. Shell is equilateral, elliptical, H/L ratio of 0.7, and moderately inflated. Umbo is broad, orthogyrate, with beak positioned centrally. Shell smooth except for concentric growth lines. Hinge plate has three anterior and three posterior teeth in two series separated by a large plain area with a groove. Ligament is predominantly external, amphidetic and weak.
Remarks. Malletiidae are very similar to Nuculanidae, but the lack of a resilifer in mature adults supports their separation (Coan & Valentich-Scott 2012). These specimens do not belong to the Neilonellidae because they possess a small conspicuous gape between the valves. The Malletiidae is a long-ranging family known from the Ordovician to the Recent, and three genera (Malletia, Palaeoneilo and Taimyrodon) belonging to the family have previously been reported from the Lower Triassic (e.g. He et al. 2007;Wasmer et al. 2012). Externally, these specimens resemble the larval shells of Paleoneilo? fortistriata figured by Wasmer et al. (2012), but they have far fewer hinge teeth. There are also equal numbers of anterior to posterior hinge teeth in these specimens, whereas in P. fortistriata there are many more posterior than anterior teeth (Wasmer et al. 2012).
Mode of life. Shallow infaunal, fully motile, slow, miner (Stanley 1968 Description. Outline elongate-elliptical. Shell equivalved, inflated below the umbo, and inequilateral with beak positioned approximately 85% along the length of the dorsal margin from the posterior. Lower part of anterior dorsal margin projects slightly beyond plane of commissure. Posterodorsal margin is almost straight to slightly round and gently sloping. Lunule is long and narrow, with deeply impressed escutcheon. Umbo is prosogyrate and rises above the hinge margin. Ornamented with fine concentric growth lines. The entire inner margin is smooth. Prodissoconch is smooth and orbicular. A small subumbonal groove limits the shell projection posteriorly. Short lateral tooth, left valve possesses a weak secondary ridge creating a shallow socket. Hinge of left valve has an anterior platform that bears a depression for the corresponding anterior hinge margin of the right valve; posterior to this is a small, tuberculiform subumbonal tooth fitting above posterior lateral tooth of right valve. Ligament is fixed to a nymph, which extends about half the length of the posterior dorsal margin. Isomyarian muscle scars with a deeply impressed anterior adductor muscle scar. Remarks. Insufficient knowledge of the internal morphology of Early Triassic bivalves in general, and Unionites in particular, has created uncertainty regarding their systematic position. Based on their external morphology alone these specimens would be assigned to Triaphorus aff. multiformis (Kumagae & Nakazawa 2009), but they possess the following characters that are diagnostic of Unionites: the anterior hinge margin of the right valve overlaps that of the left; a nymph that extends nearly half the length of the posterior hinge margin; an impressed anterior adductor muscle scar; a deeply impressed lunule; and a long and narrow escutcheon. Thus, these specimens are assigned to Unionites. Diagnosis. The diagnosis follows Newell & Boyd (1975). Orthogyrous to moderately prosogyrous shell with slightly incurved beak. Posterior ridge angular to subangular in transverse profile. Myophorian hinge; teeth and sockets smooth or bearing transverse striations.
Remarks. These specimens have most of the required diagnostic criteria for assignment to Neoschizodus: an orthogyrate to moderately prosogyrate shell with an incurved beak and a posterior ridge angular to subangular in transverse. The nymph, teeth and umbonal platform were not present in these specimens so it is not known whether these specimens possess the myophorian hinge that is also diagnostic of this genus. These specimens do, Figure 8. Unionites aff. subrectus (Bittner, 1901 Description. Shell is trigonally subovate, equivalve, inflated below the umbo, inequilateral, slightly higher than long. Umbo is small and orthogyrate, with an elevated, subangular to rounded, umbonal ridge. Indistinct posterior ridge. Anterodorsal margin recurvate, passing to widely arched ventral margin, posterodorsal margin straight, entire inner margin smooth. Smooth sculpture except for faint concentric growth lines. The larval shells have a similar external morphology to the adult shells, with a short and subumbonal hinge and short, narrow nymphs running down the anterior and posterior margins. The prodissoconch is smooth with fine concentric growth lines.

Remarks. Neoschizodus laevigatus is a cosmopolitan
Early and Middle Triassic species with high variability in its morphological characters. The shell shape and indistinct posterior ridge mean that the present specimens agree with the characters of N. laevigatus, and similar specimens identified from the Early Triassic (e.g. Kumagae & Nakazawa 2009;Hautmann et al. 2011).
One of the adult specimens preserves a prodissoconch (Fig. 9I) and its morphology is identical to that of the other prodissoconchs that were found as isolated specimens. The hinge plate of the prodissoconchs was not observed to be myophorian -instead the hinge dentition is typical of a taxodont -but it is not known how a myophorian hinge plate develops through ontogeny (Newell & Boyd 1975). Hautmann & N€ utzel (2005) suggested that, in bivalves, the presence of a small prodissoconch I and a relatively large prodissoconch II indicates a planktotrophic larval stage. Therefore, a planktotrophic larval stage is interpreted for these specimens.
Description. Shell globular, almost as long as wide. Slit short and broad at base of U-shaped sinus. Weakly depressed selenizone. In well-preserved specimens growth lines can be observed. Aperture is arched around earlier whorls, and curved inwards by the selenizone.
Remarks. The shell of Warthia is usually entirely involute and overgrows the umbilicus. In some of our smaller specimens, the umbilicus is still visible, but in larger specimens it has become overgrown during subsequent Figure 10. Warthia zakharovi Kaim, 2009 Waterhouse 1963) and, therefore, we consider our specimens to belong to Warthia. The question of whether bellerophontids had a planktonic larval stage is unresolved (N€ utzel & Mapes 2001). The protoconch in bellerophontids may be very small, less than one whorl, and is succeeded by the teleoconch (Fr yda 1999), and in our specimens the initial whorl is overgrown almost immediately. The small size (<0.05 mm), bilateral symmetry and lack of ornamentation in the initial whorls of Warthia zakharovi recorded in this study (Fig. 10H) are comparable to the embryonic shells of Bellerophon from the late Silurian that Fr yda (1999) interpreted as indicating planktotrophy. A planktotrophic larval stage is, therefore, inferred for Warthia zakharovi.
Order Vetigastropoda Salvini- Plawen, 1980 Family Eotomariidae Wenz, 1938Subfamily Eotomariinae Wenz, 1938Genus Glabrocingulum Thomas, 1940 Type species. Glabrocingulum beggi Thomas, 1940;Carboniferous, Scotland. Diagnosis. Low-to moderately high-spired and turbiniform shell shape. The upper whorl surface forming an angle of <45 with the selenizone located on the upper edge of whorl face. Sutures sharply defined. Upper whorl face with both spiral and collabral ornament; most strongly developed near the suture, weakest near the selenizone. Anomphalus to widely phaneromphalus, with or without funicle.
Remarks. These specimens resemble the Permian genera Wannerispira, Ananias and Glabrocingulum, and the Triassic genus Kamupena. They differ from Wannerispira by possessing a selenizone in the upper third of the whorl, and only having two rather than three strong spiral ribs; from Ananias by being low-rather than high-spired and having a less conspicuous and thinner concave band below the selenizone; and from Kamupena by lacking a strong umbilical callus plug. These specimens also differ from other neilsoniines by having spiral ribs and no axial ornamentation, and by being less elongated. Another genus with a comparable whorl profile is Rhaphistomella, which has been considered a synonym of Glabrocingulum (Batten 1989;Erwin & Pan 1996), but it differs from these specimens by the absence of a prominent medial concave band and a more strongly nodulose keel under the suture.
These specimens are therefore assigned to the genus Glabrocingulum.
Wannerispira is the only other unequivocal eotomariid genus to have been reported from the Early Triassic (Kaim et al. 2010;Hautmann et al. 2015), and belongs to the Subfamily Neilsoniinae. Since Glabrocingulum, in contrast, is dextral, low-rather than high-spired, and with a moderately deep slit developing into a selenizone with rounded margins, it belongs within the Subfamily Eotomariinae. These specimens represent the first Early Triassic record of the Subfamily Eotomariinae and are the first Early Triassic record of the genus Glabrocingulum, which is rarely recorded after the Permian period.
Description. Shell is dextral, turbiniform, low-spired, with simple sutures. The upper whorl surface is slightly concave and gently sloping, and bears the selenizone between sharply protruding edges, with the lower of the edges on the shell periphery. Selenizone is concave and moderately deep. Narrow, concave band immediately below the lower rib. Whorl profile below the lower rib is gently convex. Basal angulation is relatively sharply defined, but convex; base with a rounded circum-umbilical shoulder; small umbilical chink. Aperture is a rounded trapezoid; inner lip is reflexed; peristome interrupted by a slit in the outer lip. Shell ornamented with closely, irregular spaced fine spiral lirae. Growth lines visible with small knobs at the intersection of spiral ribs, otherwise no axial ornamentation observed. On the top of the keel, near the suture, the growth lines form small nodules. Protoconch: openly coiled; first two whorls smooth; third whorl possesses »15 evenly spaced, rounded, spiral threads; peristome uninterrupted (Fig. 11F).
Remarks. These specimens resemble Glabrocingulum texanum Batten, 1989 with the selenizone being located in the upper third of the whorl and lacking axial ornamentation. They differ from G. texanum in having a broader selenizone in relation to whorl height; a more concave selenizone with sharper edges; a weakly developed funicle or none at all; and in being moderately low-spired. The uncoiling that has been described for some G. texanum specimens from the Permian of the south-western US (Batten 1989) was not observed. These specimens are also considerably smaller (max. size: H D 3.6 mm, W D 4.6 mm) than the type material of G. texanum (max. size: H D 8.7 mm, W D 9.9 mm), which may be a consequence of environmental stress in the immediate aftermath of an extinction event and an expression of the Lilliput effect in this genus (cf. Twitchett 2007). Due to their excellent preservation, these specimens reveal the morphology of the larval stages, which show a similar ontogenetic development to other species of Glabrocingulum (e.g. Pan & Shen 2008).
The Vetigastropoda have a diverse range of living habits including being described in association with woodfall communities (Kiel et al. 2008). The specimens in this study do occur in association with wood; however, no direct relationship was observed. Eotomariidae recorded from Zechstein reefs are described as motile algal grazers that were probably confined to a hard substrate (Hollingworth & Pettigrew 1988). The lack of evidence of a hard substrate in their depositional environment, however, suggests that these specimens probably had a similar life habit to deep-sea vetigastropods that typically consume sediment (Hickman 1988).
Mode of life. Surficial, fully motile, slow, surface deposit feeder.
Description. Teleoconch not observed. Protoconch is conical, elongate and composed of five whorls. The initial whorl is smooth and has a diameter of 0.08 mm. Collabral ornamentation is initiated on the second whorl, continues to the base of the protoconch, and consists of narrow costellae that intersect at or just below the mid-whorl. Costellae are sigmoidal: on the upper part of the whorls they are slightly curved and oriented at 40 to the shell axis, and on the lower part of the whorls they are oriented at 200 to the shell axis. Growth lines are visible as faint collabral ribs between, and perpendicular to, the costellae. Aperture is circular, with a small columellar fold. Four whorls are present in these specimens.
Remarks. Hoare & Sturgeon (1978) showed that protoconchs of species of Pseudozygopleuridae are very similar, but can be readily differentiated from those of the Zygopleuridae. Diagnostic characters of pseudozygopleurid protoconchs that are present in these specimens include an elongate, conical shape with 3-5 whorls; smooth initial whorl with collabral ornamentation from the second whorl; and narrow opisthocline to sigmoidal transverse costellae that are equally spaced, extend up and below from the suture and curve uniformly to the midwhorl (Hoare & Sturgeon 1978). In contrast, protoconchs of the Zygopleuridae have smooth whorls with fine riblets at the sutures (N€ utzel & Mapes 2001;Kaim 2004) or straight ribs in the Ampezzopleurinae (N€ utzel 1998, 2005). Protoconchs of the Ladinulidae are similar to those of the Pseudozygopleuridae but can be differentiated due to their vertical costellae (Bandel 2006). These specimens are, therefore, representatives of the Pseudozygopleuridae. No teleoconch is apparently present in these specimens as the sculpture of the prodissoconch is uninterrupted. Pseudozygopleuridae is primarily a Palaeozoic family. Apart from the Early Triassic specimens described herein, the only other Mesozoic pseudozygopleurid is Plocezyga from the Jurassic of Poland (Kaim 2004). The specimens attributed to Plocezyga by Kaim (2004), however, lack a protoconch with the diagnostic sigmoidal pseudozygopleurid ornamentation described by Hoare & Sturgeon (1978, 1980. Instead they have a reticulate pattern, shouldered whorls and transverse costae on the protoconch, indicating that they belong to a different genus and family . N€ utzel (1998) used the diameter, ornamentation and whorl number of the protoconch to separate planktotrophic from non-planktotrophic pseudozygopleurid species. The specimens recorded in this study have small protoconch I diameters (0.12-0.14 mm), and the same ornamentation and number of whorls as pseudozgopleurid larval shells interpreted as planktotrophic by Mapes & N€ utzel (2009), suggesting that they too had a planktotrophic larval lifestyle. A teleoconch was not developed in any of the specimens, which may indicate that the adults were unable to live in this environment.
Diagnosis. The generic diagnosis follows Gr€ undel & N€ utzel (2012). The shell is fusiform with a distinctly elevated spire. The teleoconch whorls have a subsutural ramp. The transition from the ramp to the outer whorl face is either rounded or angular, sometimes demarcated abapically by a spiral concavity. Whorls are smooth in most species, although faint spiral furrows occur on the base in a few of them. Growth lines are prosocyrt on the outer whorl face but strongly curving in an abapertural direction and opisthocyrt at the ramp. The aperture is relatively low for the group, teardrop-shaped and lacks a columnellar fold. The protoconch is trans-to medioaxial.
Material. Specimen lost by WJF after photography.
Description. The shell is high-spired, slender and fusiform. Teleoconch whorls have a subsutural ramp. The ramp from the outer whorl face is rounded with a rib on the shell periphery. Whorls are smooth, except for growth lines which are prosocyrt on the outer whorl face curving in an apertural direction and become opisthocyrt towards the ramp. The surface of the shell shows a coloured spiral band around the subsutural ramp. The aperture is an elongated teardrop shape. Protoconch is heterostrophic, sinistral, nearly discoidal with lightly elevated spire 30 offset from the shell axis; protoconch has 1-2 round whorls.
Remarks. Seven species are included in Sinuarbullina, and S. convexa ( D 'Cylindrobullina' convexa) is the only accepted species from the Lower Triassic (Gr€ undel & N€ utzel 2012). These specimens are more slender than S. convexa, described from the Sinbad Limestone of the western USA by Batten & Stokes (1986), and better resemble Jiangxispira yangouensis from the Induan Dayie Formation, China. The shell morphology is similar to that of Meekospira, which has been interpreted as a slow-moving shell dragger (Hughes 1986), but could have also been a burrower (Hollingworth & Pettigrew 1988). Interpreting the feeding strategy of fossil gastropods is difficult because information on the organ system, including the ctenidium, is not usually preserved. The ancestral ecology of high-spired gastropods is presumably as algal grazers on hard substrates (Declerck 1995). Given the absence of hard substrates in this study, the specimens described herein were probably detritus feeders or possibly micro-carnivorous on sedentary prey, like many modern shelled opisthobranchs (e.g. Lobo da Cunha et al. 2009).

Implications for Permian-Triassic extinction and diversification
The fully silicified, diverse, earliest Triassic assemblage described herein includes gastropod and bivalve taxa that have not previously been recognized from the Early Triassic. Some occurrences extend stratigraphical ranges back to the basal Triassic (Fig. 14), with implications for timing and rates of diversification, whereas others show unequivocally that some taxa that were previously thought to have become extinct during the late Permian mass extinction event actually survived.
Three of the four gastropod taxa represent families that originated in the Palaeozoic, of which two (Warthia and Wannerispira) have previously been recognized from the earliest Griesbachian and have been interpreted as 'dead clades walking' (sensu Jablonski 2002) by Kaim et al. (2010) and . The family Pseudozygopleuridae is recorded in the Triassic for the first time (Fig. 14), confirming that it too survived the late Permian extinction, and could also be interpreted in this way. As noted above, the Jurassic specimens assigned to the Pseudozygopleuridae by Kaim (2004) do not have the Figure 13. Sinuarbullina yangouensis (Pan et al., 2003). A, B, lateral views; C, D, views of the protoconch. Scale bars D 1 mm, except D. Note: specimen lost by WJF after photography. expected suite of diagnostic characters and it may be that they represent a different family. If so, then the specimens described herein from Svalbard would represent the youngest occurrence of the Pseudozygopleuridae.
The other gastropod described in this study, Sinuarbullina yangouensis, represents one of the oldest occurrences of the architectibranchs (sensu Gr€ undel & N€ utzel 2012), a group that diversified in the late Triassic and Jurassic. Although architectibranchs have been described from the Carboniferous, the Carboniferous specimens do not have a heterostrophic protoconch, which means that they are not true architectibranchs (Gr€ undel & N€ utzel 2012). Thus, while the oldest unequivocal architectibranchs occur in the first conodont zone of the Triassic (this study; Pan et al. 2003), as hypothesized for other benthic invertebrate groups, such as the articulate crinoids (Baumiller et al. 2010;Oji & Twitchett 2015), their origin may have been Palaeozoic.
Of the five bivalve genera identified in this study, Promyalina, Unionites and Neoschizodus have previously been recorded in the earliest Induan (e.g. Hautmann et al. 2015). The occurrences of the protobranchs Nucinella and Austrotindaria represent significant range extensions back to the Induan and are the oldest occurrences of the Nucinellidae and Nuculanida, respectively. Recent phylogenetic analyses show that the protobranchs rapidly diversified in the late Triassic and Jurassic (Bieler et al. 2014), but these new occurrences demonstrate that the Nucinellidae and Neilonellidae appeared c. 50 million years before that, in the basal Triassic, which better supports the view of Sharma et al. (2013) that the late Permian mass extinction event triggered protobranch diversification. Future phylogenetic analyses will need to take into account our new findings in order to better calibrate molluscan evolutionary trees.
The excellent preservation of the fossil assemblage described in this study has been critical in recognizing that most specimens previously assigned to Unionites fassaensis and U. canalensis (Palaeoheterodonta) from Lower Triassic rocks worldwide are likely to have been misidentified and better resemble the protobranch Austrotindaria. Other specimens from this study are, however, unequivocally identified as Unionites, confirming its presence in Svalbard during the earliest Triassic. The palaeoheterodonts Unionites and Neoschizodus represent two lineages that were part of a major Early Triassic diversification of the Palaeoheterodonta (Newell & Boyd 1975;Ros et al. 2011;Sharma et al. 2013;Bieler et al. 2014).
The re-assignment of most Early Triassic 'Unionites' specimens to Austrotindaria has important implications for understanding how benthic marine ecosystems functioned in the wake of the late Permian mass extinction event, as pre-Spathian shelly benthic assemblages are typically described as being dominated by 'Unionites' (Fraiser & Bottjer 2007;Hofmann et al. 2013aHofmann et al. , 2014Hofmann et al. , 2015Foster et al. 2015). Our taxonomic re-assignment has implications for the functional interpretation of those specimens previously described as 'Unionites' because the motility and feeding of Unionites and Austrotindaria are significantly different -that is, the former is a facultatively motile suspension feeder and the latter is a motile deposit feeder. Other non-protobranch taxa with a similar morphology, such as Tellina, also have a deposit-feeding mode of life, so this ecological reinterpretation will still be valid even if the tentative generic reassignment from 'Unionites' canalensis to Austrotindaria? canalensis is subsequently revised. This new functional interpretation means that the palaeoecology of Early Triassic benthic shelly assemblages is in better agreement with the ichnofaunal record than previously thought (cf. Twitchett 2006), and that in many post-extinction, pre-Spathian benthic communities the dominant functional group was infaunal, motile, deposit-feeders.
The Middle and, especially, the Late Triassic are frequently described as being key intervals in the radiation of extant marine invertebrate groups from both fossil and molecular evidence (e.g. Nakazawa & Runnegar 1973;N€ utzel 2005;Hautmann 2007;Posenato 2008;Gr€ undel & N€ utzel 2012;Rouse et al. 2013;Bieler et al. 2014;Hausmann & N€ utzel 2014). The poor quality of the Early Triassic fossil record is, however, widely recognized (Fraiser & Bottjer 2005) and it is demonstrably biased towards certain regions, palaeolatitudes and depositional settings (Foster & Twitchett 2014). As shown by other studies (e.g. Hautmann & N€ utzel 2005;Oji & Twitchett 2015), taxa may be overlooked because they are small or poorly preserved, or inhabited particular depositional settings, and there is significant hidden diversity in the Early Triassic. The present study reinforces that view as the assemblage documented herein is the first fully silicified Early Triassic fauna, comes from a remote location, comprises mainly small-sized fossils, and demonstrates that Early Triassic diversity was higher than previously recognized.
The high-fidelity silicification, which has preserved shells that were originally aragonitic in exquisite detail, is a key factor. A significant post-Permian reduction in the number of silicified assemblages has been attributed to a decline and movement offshore of siliceous sponges (Schubert et al. 1997), driven by changes in climate, ocean circulation and productivity (Kidder & Erwin 2001). Although partially silicified assemblages are known from the Early Triassic (e.g. , which Fraiser & Bottjer (2005) argued are adequate for palaeoecological analyses, the present study has demonstrated the necessity of early, complete and highfidelity silicification for detailed taxonomic and ecological analysis. Thus, it appears that there is significant hidden biodiversity in the Early Triassic and that the diversification of many extant marine groups probably began earlier than is presently recognized, with implications for both the timing and rate of evolution. The most diverse silicified assemblages of the earliest Induan are known from offshore settings ; this study), consistent with the hypothesis of Schubert et al. (1997) that siliceous sponges moved offshore after the late Permian extinction event, and so a search strategy focussing on those depositional settings is likely to yield critical new information, especially if regions that are currently underrepresented are prioritized.

The importance of planktotrophy
The exceptionally preserved prodissoconchs and protoconchs in this study provide valuable insights into the early ontogeny of these taxa. Valentine & Jablonski (1983, 1986 suggested that during the late Permian extinction event there was selection against benthic invertebrates with planktotrophic larval stages. More recently, however, planktotrophic larval shell development has been inferred for many Early Triassic gastropods (N€ utzel & Erwin 2002;Pan et al. 2003;N€ utzel & Schulbert 2005) and this selectivity has been questioned (N€ utzel 2014). All gastropod taxa recorded in the present study (i.e. Warthia, Glabrocingulum, Sinuarbullina and the Pseudozygopleuridae) are inferred to have had planktotrophic larvae (N€ utzel & Mapes 2001;this study), and in addition, the bivalves Nucinella, Austrotindaria, Unionites and Neoschizodus all possess a small prodissoconch I and a relatively large prodissoconch II which imply planktotrophic larval development (cf. Hautmann & N€ utzel 2005). Thus, we infer that the possession of planktotrophic larvae was a particular advantage for benthic molluscs inhabiting shelf settings of the Boreal Ocean in the earliest Triassic. This may simply be a consequence of sampling this particular depositional setting, as benthic taxa with plankotrophic larvae were common in similar mid-outer shelf settings during the Palaeozoic (e.g. Fr yda 2001; N€ utzel & Mapes 2001;Bandel et al. 2002). Alternatively, it may be a consequence of the extinction event and associated environmental changes.
One possible advantage of a planktotrophic larval development is protection from benthic predation (N€ utzel & Fr yda 2003). Although predators are rarely identified in Early Triassic benthic assemblages (e.g. Schubert & Bottjer 1995;Hautmann et al. 2011;Hofmann et al. 2013aHofmann et al. , b, 2014Hofmann et al. , 2015Foster & Twitchett 2014;Foster et al. 2015), it is possible that some nektobenthic conodonts, ammonoids or fish may have fulfilled this role or that the predators were largely non-mineralized. Fish with durophagous dentitions, such as Bobasatrania, are locally common in the lowest Triassic of the Boreal Ocean (e.g. East Greenland; Stensi€ o 1932), and trace fossil evidence of vagile crustaceans has been recorded from the Induan of north-west Canada Zonneveld et al. 2010) and central Spitsbergen (WJF & RJT pers. obs.).
An alternative advantage is that planktotrophic larvae enable more effective dispersal and a wide geographical distribution. The cosmopolitan opportunists that thrived in the wake of the late Permian mass extinction (e.g. Lingularia and Claraia) all have an inferred planktotrophic larval stage (Hammond & Poiner 1984;Yang et al. 2001). Possession of planktotrophic larvae would have been particularly useful for Early Triassic benthic invertebrates given the spatial and temporal fluctuations in benthic oxygen concentrations that have been inferred for the earliest Triassic seafloor (Wignall & Twitchett 1996;Thomas et al. 2004;Nabbefeld et al. 2010). Effective dispersal and wide distribution would have afforded greater protection from extinction and enabled rapid colonization of vacated seafloor once harsh environmental conditions had ameliorated. The late Permian mass extinction did not select against animals with planktotrophic larval development (N€ utzel 2014;Posenato et al. 2014), and possession of that character may have been a key factor in the survival and radiation of certain groups.
The occurrence of well-preserved planktotrophic larval shells in this newly described benthic assemblage has additional palaeoenvironmental implications. First, it implies that primary production in surface waters was adequate to sustain planktic food webs. Second, the exquisite preservation of the earliest formed larval stages and the complete absence of any signs of shell dissolution or repair (cf. Garilli et al. 2015) demonstrates that neither the surface waters nor benthic habitat in this particular region were affected by ocean acidification at this time, and the pH was not low enough to have curtailed biomineralization.

Conclusions
The fossil assemblage from the Lusitaniadalen section, Svalbard, is the first fully silicified fauna to have been described from the Early Triassic and provides new critical systematic data. The fauna includes four new bivalve species: Nucinella taylori, N. nakremi, Austrotindaria antiqua and A. svalbardensis and a gastropod species: Glabrocingulum parvum. The silicified fauna from Svalbard demonstrates that the aftermath of the late Permian mass extinction was a key interval for the diversification of the Architectibranchs, Protobranchia and Palaeoheredonta. The assignment of Early Triassic specimens previously identified as Unionites to Austrotindaria has important palaeoecological implications demonstrating that infaunal deposit-feeders dominated benthic assemblages prior to the Spathian. The gastropod and bivalve taxa recorded in the present study are inferred to have had planktotrophic larvae, and it is likely that this was an important adaptation for bivalves and gastropods in midouter shelf settings of the Boreal Ocean during the earliest Triassic.