[Palaeontology, Vol. 61, Part 3, 2018, pp. 417–439] SMALL CARBONACEOUS FOSSILS (SCFs) FROM THE TERRENEUVIAN (LOWER CAMBRIAN) OF BALTICA by BEN J. SLATER1,3 , THOMAS H. P. HARVEY2 and NICHOLAS J. BUTTERFIELD1 1Department of Earth Sciences, University of Cambridge, Cambridge, UK; [email protected] 2School of Geography, Geology & the Environment, University of Leicester, Leicester, UK 3Current address: Department of Earth Sciences, Palaeobiology, Uppsala University, Uppsala, Sweden Typescript received 28 September 2017; accepted in revised form 8 December 2017 Abstract: We describe a new assemblage of small carbona- Together these data offer a fundamentally enriched view of ceous fossils (SCFs) from diagenetically minimally altered Terreneuvian life in the epicratonic seas of Baltica, from an clays and siltstones of Terreneuvian age from the Lontova episode where records of non-biomineralized life are cur- and Voosi formations of Estonia, Lithuania and Russia. This rently sparse. Even so, the recovered assemblages contain a is the first detailed account of an SCF assemblage from the lower diversity of metazoans than SCF biotas from younger Terreneuvian and includes a number of previously undocu- (Stage 4) Baltic successions that represent broadly equivalent mented Cambrian organisms. Recognizably bilaterian-derived environments, echoing the diversification signal recorded in SCFs include abundant protoconodonts (total-group the coeval shelly and trace-fossil records. Close comparison Chaetognatha), and distinctive cuticular spines of scali- to the biostratigraphical signal from Fortunian small shelly dophoran worms. Alongside these metazoan remains are a fossils supports a late Fortunian age for most of the Lon- range of protistan-grade fossils, including Retiranus balticus tova/Voosi succession, rather than a younger (wholly Stage gen. et sp. nov., a distinctive funnel-shaped or sheet-like 2) range. problematicum characterized by terminal or marginal vesi- cles, and Lontohystrichosphaera grandis gen. et sp. nov., a Key words: Cambrian explosion, small carbonaceous fos- large (100–550 lm) ornamented vesicular microfossil. sils, Terreneuvian, Lontova, acritarchs, Baltica. T HE earliest Cambrian Terreneuvian Series represents one acritarchs from the Fortunian point to relatively low- of the most transformative intervals in Earth history. diversity ecosystems compared to Stage 2, or younger Bracketed by the Ediacaran System below, and Cambrian counterparts. Phosphatic SSF assemblages from this earli- Series 2 above, it witnessed the Cambrian ‘explosion’ of est Cambrian interval are dominated by enigmatic tubular animals, biomineralization, bioturbation and protists, as forms (e.g. Anabarites, Hyolithellus, Hexaconularia) and well as the establishment of an essentially modern marine protoconodonts (Protohertzina), typically referred to the biosphere. Curiously, the most obviously ‘explosive’ part Anabarites trisulcatus–Protohertzina anabarica Zone of the radiations occurs several million years after the c. (Hamdi et al. 1989; Steiner et al. 2004a; Kouchinsky et al. 541 Ma base of the Cambrian, more or less in the middle 2012). Fortunian trace fossils record comparatively low of the Terreneuvian (Budd & Jensen 2000; Maloof et al. degrees of sediment mixing, and are typified by assem- 2010). The Terreneuvian itself consists of two parts: a blages of simple, shallow tiering treptichnid-type habits lower Fortunian Stage (c. 541–529 Ma), and an upper (e.g. Treptichnus, Didymaulichnus), which are joined by ‘Stage 2’ (c. 529–521 Ma), broadly equivalent to the Tom- Rusophycus-type arthropod traces at c. 536 Ma (Jensen motian of Siberia (Kouchinsky et al. 2012). 2003; Mangano & Buatois 2016). The signal from Several lines of palaeontological inquiry confirm that Fortunian plankton is also relatively subdued, character- much of the initial radiation of bilaterian animals does ized by long-ranging acritarch taxa belonging to the indeed lie in the Terreneuvian (Budd 2003, 2013). A pro- Asteridium tornatum–Comasphaeridium velvetum Zone nounced increase in the origination rate of small shelly (Moczydłowska 1991, 1998) and the Asteridium– fossil (SSF) taxa occurs at or near the beginning of Stage Heliosphaeridium–Comasphaeridium assemblage (Yao et al. 2 (Bengtson et al. 1990; Maloof et al. 2010; Kouchinsky 2005). No Burgess Shale-type (BST) Lagerst€atten are et al. 2012). Current records of SSFs, ichnofossils and known from the Terreneuvian, and apart from a handful © 2018 The Authors. doi: 10.1111/pala.12350 417 Palaeontology published by John Wiley & Sons Ltd on behalf of The Palaeontological Association. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 418 PALAEONTOLOGY, VOLUME 61 of sites bearing phosphatized algae and metazoan larvae in Cambrian evolution on Baltica. Here we report a unique South China (Steiner et al. 2004b; Liu et al. 2017), there is SCF accounting of the Lontova and laterally contiguous remarkably little accounting of the non-biomineralizing Voosi Formations of Estonia, Lithuania and western clades that dominate the diversity of most ecosystems. Russia. One alternative source of palaeontological data comes from small carbonaceous fossils (SCFs), which have fun- damentally expanded the view of non-biomineralized GEOLOGICAL SETTING organisms in younger Cambrian strata (Butterfield & Harvey 2012; Harvey et al. 2012; Smith et al. 2015; Slater The Lontova Formation (Figs 1, 2) underlies large areas et al. 2017a). An extension of this SCF record into the of the eastern Baltic States, extending into Russia and the Terreneuvian offers a novel means of tracking diversifica- Gulf of Finland. It is exposed locally in clay quarries tion through the ‘Cambrian Explosion’ interval, and a along the north Estonian coastline and widely encoun- complementary record to the signal from SSFs. Indeed, at tered in drillcore (Mens 2003; Nielsen & Schovsbo 2011). least some SCFs appear to be diagenetically de-minera- In the western Estonian mainland and islands of Hiiumaa lized versions of SSF elements, fossilized by virtue of their and Saaremaa, contemporaneous sedimentary units are periostracum or intercrystalline organic matrix (e.g. But- referred to as the Voosi Formation (Figs 1, 2). terfield & Harvey 2012; Martı Mus 2014; Slater et al. Sediments of the Lontova Formation comprise a rela- 2017a, b); equally, some SSFs could be diagenetically min- tively homogeneous and laterally extensive sequence of eralized remains of originally carbonaceous or lightly poorly lithified illite-smectite rich claystones/siltstones mineralized forms; e.g. phosphatized wiwaxiid sclerites known locally as ‘Cambrian Blue Clay’ (Mens 2003; Rai- (Porter 2000) and hallucigeniid spines (Skovsted & Peel dla et al. 2010; Budd et al. 2011). A combination of shal- 2001). SCFs can therefore extend the ranges of SSF-type low burial depth and a tectonically stable setting has taxa into previously unsampled depositional (or diage- resulted in remarkably little lithification or thermal alter- netic) environments. Importantly, however, a majority of ation over the past half a billion years (Kirsim€ae et al. SCFs record body parts or taxa that are otherwise unrep- 1999; Kirsim€ae & Jørgensen 2000; Winchester-Seeto & resented in the fossil record. McIlroy 2006). Properties of the clay mineralogy and acri- A rich diversity of SCFs has recently been documented tarch colour-alteration in these sediments signify maxi- in early, but not earliest, Cambrian siliciclastic sediments mum burial temperatures not exceeding 50°C, ideal for of the western Baltic Basin (Slater et al. 2017a). Across the preservation of SCFs (Talyzina 1998; Ivanovskaya & this part of the basin Cambrian sediments are draped Geptner 2004). Across its >500 km extent, the Lontova unconformably over a peneplained Proterozoic gneissic Formation rests transgressively above sediments of the basement, and over most of the region the earliest pre- ‘Rovno’ (latest Ediacaran to Fortunian) or ‘Kotlin’ (latest served Cambrian record begins in Stage 4 (i.e. younger Ediacaran) regional Baltic stages, or directly on Protero- than 514 Ma; Nielsen & Schovsbo 2007, 2011; Slater et al. zoic crystalline gneissic basement (Mens et al. 1990; Jen- 2017a). In the eastern Baltic region, however, these epi- sen & Mens 1999). Despite its homogeneity, it is possible cratonic successions continue down into the Terreneu- to recognize four subdivisions of the Lontova Formation, vian, spanning the full range of the early Cambrian based primarily on differences in the relative ratios of (Nielsen & Schovsbo 2011; Meidla 2017). These poorly sediment types. The lowermost S€ami Member consists of lithified clays and silts form the local Baltic ‘Lontovan’ a heterolithic succession of sandstones and silty claystones stage. The Lontovan has been widely regarded as equiva- (Mens & Isakar 1999), and hosts a low-diversity ichnofos- lent to the lowermost part of the ‘Tommotian’ stage of sil assemblage including Treptichnus pedum (Palij et al. the Siberian craton (see Bergstrom€ 1981; Moczydłowska 1983). The S€ami Member is overlain by the green-grey to & Vidal 1988), which forms the upper part (c. 525– red-brown silt-rich claystones of the Mahu Member. The 521 Ma) of Cambrian Global Stage 2 (c. 529–521 Ma) succeeding Kestla Member is predominantly claystone, (see Kouchinsky et al. 2012). This age assignment