[Palaeontology, Vol. 63, Part 5, 2020, pp. 733–752]

COCHLEATINA: AN ENIGMATIC SURVIVOR AMONG SMALL CARBONACEOUS (SCFS) by BEN J. SLATER1 ,THOMASH.P.HARVEY2, ANDREY BEKKER3 and NICHOLAS J. BUTTERFIELD4 1Department of Sciences, Palaeobiology, Uppsala University, Villav€agen 16, Uppsala 752 36, Sweden; [email protected], [email protected] 2School of Geography, & the Environment, University of Leicester, University Rd, Leicester LE1 7RH, UK 3Department of Earth & Planetary Sciences, UC Riverside, 900 University Av., Riverside, CA 92521, USA 4Department of Earth Sciences, University of Cambridge, Downing St, Cambridge CB2 3EQ, UK

Typescript received 26 November 2019; accepted in revised form 6 March 2020

Abstract: Conspicuously few body- taxa are known descriptions for Cochleatina and C. canilovica, and critically to span the Ediacaran–Cambrian boundary, a pattern usually evaluate previous biological interpretations, drawing compar- taken to signal either a terminal mass extinction, isons with metazoan, algal and protistan analogues. We or taphonomic failure. We draw attention to the emerging reject hypotheses supporting Cochleatina as a metazoan record of small carbonaceous fossils (SCFs), which exhibit mouthpart, and suggest new grounds for viewing Cochleatina continuous preservation spanning this critical interval. Here as a potential multicomponent predator that trapped we focus on the enigmatic SCF Cochleatina, a morphologi- among microbial mats. Most occurrences are from , cally complex coil-shaped problematicum that ranges across but we synthesize sporadic reports of Cochleatina from other the Ediacaran–Cambrian divide, and is potentially among palaeocontinents, pointing to its global distribution during the oldest fossil occurrences of metazoans. We report new the latest ~10 myr of the Ediacaran and majority of the ear- material of Cochleatina canilovica from the Ediacaran of liest Cambrian . As a rare example of an Estonia and Ukraine, which offers new characters for assess- ‘Ediacaran survivor’, Cochleatina highlights the broader sig- ing its palaeobiology. Significantly, new specimens include nificance of SCFs as a novel means of tracking evolutionary sets of three-alike triplets of Cochleatina adhering to organic patterns through the Proterozoic– transition. sheets, suggesting a clustering habit, or grouping of elements within an individual during ; an important step in con- Key words: Ediacaran–Cambrian survivor, oldest - straining the morphology and ecology of this Ediacaran– zoan, Proterozoic mass extinction, small carbonaceous fos- Cambrian problematicum. We present revised systematic sils, fossil problematica.

T HE Ediacaran–Cambrian boundary, approximately 541– 1991; Butterfield 1997; Nowak et al. 2015). Identification 539 Ma (Linnemann et al. 2019), is widely recognized as of such ecological or evolutionary perturbations is heavily a juncture of exceptional ecological and evolutionary reliant on taphonomic continuity; in other words, the fac- importance (Conway Morris 2000; Butterfield 2007; Budd tors governing fossil preservation should not substantially & Jensen 2017). At around this point, the fossil record is change through the time interval of interest. If they do, permanently transformed by the appearance and radiation then the traceability of lineages/taxa can be seriously of diverse biomineralizing and agglutinating forms compromised. The coincident opening and closure of sev- (Kouchinsky et al. 2012). This switching-on of the ‘shelly’ eral key taphonomic windows across the Ediacaran–Cam- fossil record approximately corresponds with an increase brian transition obscures the precise tracking of in the degree and complexity of (Jensen taxonomic ranges from this crucial interval. At present, et al. 2006; Herringshaw et al. 2017), substantial shifts in only a handful of taxa known from body fossils are con- the of biogenic sediments (Davies et al. 2019, vincingly shown to span the boundary (e.g. Narbonne fig. 1), a disappearance of macroscopic Ediacara-style et al. 1997; Crimes & McIlroy 1999; Hagadorn et al. preservation (Butterfield 2003), and major changes in the 2000; Narbonne 2005; Laflamme et al. 2013; Moczy- composition of assemblages (Moczydłowska dłowska et al. 2014; Darroch et al. 2015; Budd & Jensen

© 2020 The Authors. doi: 10.1111/pala.12484 733 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. 734 PALAEONTOLOGY, VOLUME 63

2017; Simon 2018). The apparent disconnect in the body More recent reports of Cochleatina, recovered among fossil record is contrasted by the relatively unbiased trace acritarch preparations, have expanded its known geo- fossil record, which instead documents a signal of conti- graphic range beyond Baltica and to and nuity between late Ediacaran and earliest Cambrian ben- (e.g. Sabouri et al. 2013; Palacios et al. 2018). thic bilaterian behaviour (e.g. Jensen 2003; Mangano & Attempts to pin Cochleatina to the have been Buatois 2017; Kesidis et al. 2019). Before a precise wide-ranging. Several authors have proposed a metazoan description of the magnitude, timing and nature of this affinity (among or molluscs; Butterfield & Har- transition can reasonably be achieved, there is a pressing vey 2012), a premise which would clearly have significant need for an improved accounting of non-biomineralizing implications if confirmed or refuted. taxa in order to discriminate genuine macroevolutionary Here we describe new material of Cochleatina from patterns from localized signals or taphonomic shortfalls. Ediacaran sediments of Estonia (Kotlin Formation) and Small carbonaceous fossils (SCFs) offer one means of Ukraine (Krushanovka Formation). We further discuss tracking the Ediacaran–Cambrian transition without the the broader significance of this SCF taxon in light of its associated biases of mineralization. Even under relatively status as a credible Ediacaran–Cambrian ‘survivor’, in the indifferent taphonomic circumstances, cell walls, cuticle, context of recently revised (Meidla 2017), and other recalcitrant components of non-biomineralizing and its emerging palaeobiogeographical distribution organisms can be recognizably preserved (Butterfield & (Fig. 2). We further examine and test previous hypotheses Harvey 2012). The widespread preservation of SCFs has for the biological affinity of Cochleatina, and propose new recently been demonstrated from regions and time-inter- models for its possible mode of life. vals where other, more ‘exceptional’ evidence of non-bio- mineralizing taxa is lacking (Slater et al. 2017a–b, 2018). In this study, we focus on an enigmatic SCF taxon, GEOLOGICAL SETTING Cochleatina, a distinctive and widely distributed taxon that appears to span the Ediacaran–Cambrian divide. Estonia Cochleatina is especially interesting in that it is preserved in substantially different depositional environments to The Kotlin Formation (Fig. 3) is widely developed across iconic boundary-spanning taxa such as Cloudina (Penny the Baltic States on the , and et al. 2014; Warren et al. 2014; Yang et al. 2016). Despite equivalent strata occur from Poland in the west, to the this, Cochleatina has so far been neglected from discus- margin of the Baltic craton in the east (Moczydłowska sion of Ediacaran ‘survivors’, and so warrants renewed 1991; Pirrus 1993; Mens & Pirrus 1997). In Estonia, the attention, particularly in the context of recent debate on Kotlin Formation is known exclusively from subsurface rates of turnover, extinction and the nature of the Edi- drillcore material, the nearest outcrop being on Kotlin acaran–Cambrian transition (Budd & Jensen 2017; Dar- Island (Russia) in the Gulf of Finland. The Kotlin Forma- roch et al. 2018; Tarhan et al. 2018; Wood et al. 2019). tion comprises a relatively homogeneous package of sedi- Cochleatina is a coiled carbonaceous fossil formed as a ments composed predominantly of finely laminated grey, spiral-shaped ribbon ornamented with fine serrations illite–smectite mixed-layer clays, with occasional interbeds (Fig. 1). Examples of this fossil were first figured among of fine-grained and siltstone (Mens & Pirrus acid-extracted material from the Ediacaran of the Ukraine 1997; Raidla et al. 2006). Due to a relatively shallow bur- by Aseeva (1974), but were initially interpreted as simple ial depth and quiescent regional tectonic history, Kotlin coiled filaments and ascribed to the filamentous form- strata have experienced negligible thermal alteration over taxon Volyniella (albeit as a new species). Three further their more than half a billion history (Raidla et al. species were later added based on material from the 2006). In Estonia, the Kotlin Formation conformably Rovno (latest Ediacaran or earliest Cambrian) and Lon- overlies the coarser-grained sandy sediments of the Gdov tova (Cambrian) formations in Belarus, Lithuania and Formation, and is in turn overlain by the correspondingly Latvia (Paskeviciene 1980), but remained assigned to sandstone-rich Voronka Formation (Fig. 3; Mens & Pir- Volyniella until Aseeva (1983a) established Cochleatina as rus 1997; Meidla 2017). Together, this package of Edi- a new genus to circumscribe these morphologically dis- acaran sediments rests unconformably on a weathered tinct fossils. Several succeeding studies mentioned or fig- crystalline basement (Puura et al. 1983; Nielsen & ured Cochleatina from sediments in Baltica and Siberia Schovsbo 2011; Meidla 2017). (e.g. Velikanov et al. 1983; Aseeva 1988; Rudavskaya & Despite its relative homogeneity, the Kotlin Formation Vasilyeva 1989), but with no substantial revision until a in Estonia is partitioned into three subdivisions (Mens & major redescription and analysis by Burzin (1995), in Pirrus 1997; Meidla 2017). The lowermost Jaama and which the four currently accepted species were amended: uppermost Laagna members comprise relatively - C. canilovica, C. rara, C. rudaminica and C. ignalinica. genous grey clays, whilst the middle Merikula€ Member can SLATER ET AL.: EDIACARAN– CAMBRIAN SURVIVOR COCHLEATINA 735

FIG. 1. Examples of Cochleatina canilovica from the Ediacaran of the Volyn region of Ukraine. Courtesy of M. Burzin. Scale bar represents 100 lm.

be distinguished by its visible fine-scale intercalations of Ukraine sand, silt, and clay (‘varve-like’ appearance; Pirrus 1992), abundance of sapropel films, and macroscopic ‘vendo- Ediacaran sediments of the Krushanovka Formation taenid’ fossils on bedding (Mens & Pirrus 1997). (Kanilovka ) from Ukraine represent broadly coeval The Kotlin Formation was deposited in a shallow-mar- deposits, also belonging to the Kotlin regional stage ine pericratonic basin (Poprawa et al. 1999). Some (Fig. 3; Sokolov & Fedonkin 1985; Velikanov 1990; Iosi- authors have proposed brackish (Bityukova & Pirrus fidi et al. 2005). Note that the Kanilovka Series of Podil- 1979) or even freshwater conditions within a basin with lya (alternatively Podolia) is not to be confused with the restricted circulation, based on suggestive boron concen- Kanilovka Formation of Volyn from which specimens of trations in mudstones, localized absence of ‘Ediacara-type’ Cochleatina have been reported elsewhere in Ukraine , and a paucity of trace fossils. Certain (Burzin 1995). The Krushanovka Formation is widely regions where the Kotlin Formation developed, however, known from drillcore in the Podillya region of Ukraine, show clear evidence of marine (see Burzin and comprises a series of fine-grained, greenish-grey to 1996), and the extent of freshwater/brackish influence white with substantial interbeds of reddish remains controversial. siltstones and claystones in its upper parts (Iosifidi et al. The Kotlin Formation shares its name with the regional 2005). The formation rests conformably on the Zhar- chronostratigraphic Kotlin stage, which in Estonia encom- novka Formation (a sequence of coarse to fine-grained passes the Gdov, Kotlin and Voronka formations (Fig. 3). sandstones) and is capped by the overlying Studenitsa Although once placed relatively deep within the Ediacaran Formation (predominantly coarse to fine-grained sand- (e.g. Sokolov 2011), the Kotlin Formation is now stones with occasional siltstones). thought to have been deposited during the terminal There are two recognized subdivisions of the Krusha- ~10 myr of Ediacaran time, based on correlation with novka Formation: a lower (~45 m thick) Kryvchany strata from the Lublin Slope (Poland), Podillya (Ukraine), Member, and an upper (~15 m thick) Durnyakovka Urals and White Sea region (Russia) where U–Pb Member. The Kryvchany Member is generally coarser, dates from volcanic tuff horizons have yielded lower with a larger proportion of sandstones, while the Durnya- boundary ages in the range of 551–548 Ma (Moczy- kovka Member is dominantly composed of distinctive red dłowska 1991; Grazhdankin et al. 2011; Meidla 2017; Sol- siltstones with occasional coarse sandstone beds (Sokolov datenko et al. 2019). & Fedonkin 1985; Iosifidi et al. 2005). Deposition 736 PALAEONTOLOGY, VOLUME 63

FIG. 2. Palaeogeographic distribution of fossil occurrences of Cochleatina sp. A, localities in Baltica where Cochleatina sp. have been recovered; 1, outcrop, Finnmark, Norway; 2, Toila 77 and Merikula€ F169 drillcores, Estonia (this study); 3, Ludza drillcore, Latvia; 4, Vishki drillcore, Latvia; 5, Butkunay drillcore, Lithuania; 6, Svedasay drillcore, Lithuania; 7, Drukshyay drillcore, Lithuania; 8, Tverecius drillcore, Lithuania; 9, Stradech-17 drillcore, Belarus; 10, various cores from Volyn, Ukraine (e.g. drillcore No. 1562, Il’pan); 11, various cores and outcrops from Podillya, Ukraine (drillcores: Bolotino, Vapnyarka No. 18, Malaya Sloboda No. 4, Bagov- itsy No. 3, Pechora No. 2, Krushanovka No. 1, Zarechanka No. 11664; outcrops: Studenitsa village No. 202, Bakota village No. 238); 12, drillcore No. 700, Podillya, Ukraine (this study); (for locality data, see Fig. 9; reconstruction of Baltica after Cocks & Torsvik 2005). B, distribution of palaeocontinents during the Ediacaran–Cambrian transition showing reported occurrences of Cochleatina sp., mainly from Baltica, but also Siberia, Avalonia and peri-Gondwanan terranes (continental distribution after various sources, e.g. McKerrow et al. 1992; see Fig. 9 for details of occurrence data). occurred in a shallow-marine basin with storm influence Estonia (Merikula€ Member of the Kotlin Formation), we (Iosifidi et al. 2005). processed a total of 31 samples: 11 from the Maidla 75A drillcore; 2 from the Maidla F-238 drillcore; 6 from the Toila 77 drillcore; and 12 from the Merikula€ F-169 drillcore. From Sampling the Podillya region of Ukraine, a total of 5 samples were pro- cessed from the Durnyakovka Member of the Krushanovka Sampling for targeted the most fine-grained Formation, drillcore No. 700. Estonian cores are housed at lithologies (mudstones and siltstones) from both areas. In the University of Technology Institute of Geology SLATER ET AL.: EDIACARAN– CAMBRIAN SURVIVOR COCHLEATINA 737

FIG. 3. Ediacaran–Cambrian stratigraphy of Estonia and Ukraine (Podillya region). Red stars indicate position of samples analysed in this study.

(GIT) core-storage at S€arghaua (Estonia), and samples from drillcore, 180 m depth in Maidla F-238 drillcore, 153 m drillcore No. 700 (Podillya, Ukraine) are hosted at the Insti- in the Toila 77 drillcore, and 119.4 m from the Merikula€ tute of Geology and Geochronology of the Rus- F-169 drillcore, whilst those from the drillcore No. 700 in sian Academy of Sciences in Saint Petersburg. SCF Podillya, Ukraine were sourced from a productive layer at processing and examination followed a gentle, low-manipu- 184 m depth. Both the Estonian and Ukrainian samples lation hydrofluoric acid maceration procedure aimed at the of Cochleatina exhibit substantial taphomorphic variation. recovery of larger, delicate forms, otherwise destroyed by In the Estonian samples, all Cochleatina-bearing horizons standard palynological processing (see techniques outlined produced masses of sapropel sheets, alongside occasional in Butterfield & Harvey 2012). vendotaenids and filamentous microbes. Productive sam- ples from Ukraine were also associated with sapropel sheets, but at substantially lower levels. RESULTS

Our processing recovered a total of 103 individual New material Cochleatina specimens, of which 70 are from the Estonian Kotlin Formation (Figs 4, 5), and 33 come from the Specimens from the new Estonian Kotlin assemblage Ukrainian Krushanovka Formation (Fig. 6). The majority (Figs 4, 5) are preserved as flattened spirals or incomplete of specimens were recovered from a small number of sections of a spiral fused to sapropel films (sheets of rela- highly productive samples; Estonian specimens were tively featureless organic matter, sometimes with identifi- recovered from a depth of 186–187 m in the Maidla 75A able filaments superimposed and variably fused together). 738 PALAEONTOLOGY, VOLUME 63

These sapropel films are interpreted as compacted and though these serrations are often obscured by the under- variably fused sedimentary organic material and/or ben- lying organic sheet (e.g. Figs 4L, 5E). Other zones are dis- thic mats (Figs 4, 5). Specimens consist of a coiled rib- cernible by their thicknesses (Fig. 7; see Systematic bon; coils reach 540 lm in maximum width (x = 246, Palaeontology, below). Basal portions are either broken SD = 83, n = 70) and display a continuum of morpholo- (e.g. Fig. 4I), or alternatively, where fused to a sheet, the gies, ranging from tightly wound bobbin-like configura- ribbons have no obvious termination but instead fade tions (Figs 4A, 5G–J) to more open spiral forms (e.g. into the sheet material (e.g. Fig. 5D–F, H, L). Fig. 5D, F, R, S, T). The ribbon narrows towards the cen- The new Ukrainian Cochleatina (Fig. 6) occur as indi- tre of the spiral and is a complex of four distinct longitu- vidual isolates (with the possible exception of Figure 6J, dinal zones running the entire ribbon length (Fig. 7). no clusters were recovered) and were never found in Thin, sharply pointed serrations project from the first attachment to larger organic sheets (note the absence of inner zone, directed away from the centre of the coil, organic material in the central opening of the bobbin;

FIG. 4. Cochleatina from the Kotlin Formation, north-east Estonia. Specimens A–F, H–J, L–O, Q–S from 153 m depth in Toila 77 drillcore; G from 180 m depth in Maidla F-238 drillcore; K and P from 187 m in Maidla 75A drillcore. Tallinn University of Technol- ogy acquisition numbers (GIT): A, 831; B, 842; C, 837; D, 838; E, 836; F, 843; G, 850; H, 841; I, 828; J, 842; K, 851; L, 841; M, 829; N, 833; O, 838; P, 851; Q, 841; R, 839; S, 832. Scale bar represents 100 lm. SLATER ET AL.: EDIACARAN– CAMBRIAN SURVIVOR COCHLEATINA 739

FIG. 5. Cochleatina from the Kotlin Formation, north-east Estonia. D–L, specimens adhered to large sapropel sheets; D, F, H, K, and L are clustered Cochleatina, note that within each cluster coils are at approximately the same size, shape, and thickness. Specimens A, B, D, F, K, Q–S from 189 m depth in Maidla 75A drillcore; C, E, G, H, J, L–P, T from 153 m depth in Toila 77 drillcore; I from 180 m depth in Maidla F-238 drillcore. Tallinn University of Technology acquisition numbers (GIT): A, 845; B, 846; C, 840; D, 848; E, 832; F, 853; G, 838; H, 835; I, 850; J, 852; K, 849; L, 854; M, 829; N, 842; O, 834; P, 830; Q, 845; R, 847; S, 844; T, 852. Scale bars represent: 100 lm; (A–F, M–T); 200 lm(G–L).

Fig. 6). The coils reach 320 lm in maximum width. Like spiral (Fig. 7). The ribbons are optically darker than their the Estonian specimens, the ribbons are divided into four counterparts from the Kotlin Formation, especially the discernible zones which narrow towards the centre of the first and third zones of the ribbon which are opaque in 740 PALAEONTOLOGY, VOLUME 63

FIG. 6. Cochleatina from the Krushanovka Formation, Podillya, Ukraine. Specimens sourced from a productive layer at 184 m depth within drillcore No. 700. Tallinn University of Technology acquisition numbers (GIT): A–G, 855; H–J, 856. Scale bar represents 100 lm. SLATER ET AL.: EDIACARAN– CAMBRIAN SURVIVOR COCHLEATINA 741

FIG. 7. Schematic diagram of Cochleatina canilovica, including terminology of ribbon morphol- ogy used here. The ‘first zone’ comprises the dark innermost part of the coil, and is fringed with marginal serrations that point away from the centre of the spiral. The ‘second zone’, where pre- served, is a thin, filmy part of the ribbon which is typically overlain by the spines emanating from the first zone. The ‘third zone’ is of similar construction to the first zone (dark, sclerotized) but lacks any serrations and may be sepa- rated from the second zone by a ‘perforation zone’ toward the basal portion of the ribbon. The ‘fourth zone’ (frequently damaged or missing) is a thin, filmy region, similar to the second zone.

most specimens (Fig. 6). Serrations emanating from the The new assemblages of Cochleatina from Estonia and inner first zone of the ribbon are also prominently visible Ukraine differ in a number of aspects. For example, ser- in the majority of specimens (e.g. Fig. 6A–C, E, G, J). rations appear more pronounced in the Ukrainian speci- The ribbon tip has a brush-like termination of fibrous mens. This, however, appears to be purely taphonomic; projections between 5 and 15 lm in length (e.g. Fig. 6C– serrations are present in all well-preserved Kotlin G). Cochleatina, but are simply less prominent due to the obscuring presence of the underlying/fused organic sheet. Cochleatina from the Krushanovka Formation exhibit Comments darker ribbons (particularly in zones one and three), however, this can be explained by variations in local The new specimens from Estonia and Ukraine are post-depositional burial histories (e.g. different degrees assigned to C. canilovica on the basis of their consistent of thermal alteration). When these taphonomic consider- spinose serration, ribbon oriented perpendicular to the ations are taken into account, it is clear that both bobbin axis, and four broad ribbon zones, features which assemblages of Cochleatina exhibit the same underlying are lacking in other taxa (see Systematic Palaeontology, morphology. below). Both the Estonian and Ukrainian assemblages are consistent with the currently known range of C. canilovica which is reported from the Kotlin regional stage of the Clustered forms late Ediacaran, and the lowermost part of the Rovno regional Ediacaran/Cambrian stage. Although Cochleatina Among the more complete specimens of Cochleatina has been reported from elsewhere in the Baltic region recovered from the Kotlin Formation are a notable sub- (e.g. Paskeviciene 1980), these are the first reports from set that occur as clusters, consisting of three coils Estonian strata. adhered to the same carbonaceous sheet (N = 6). The 742 PALAEONTOLOGY, VOLUME 63 sheets are interpreted as the compacted remains of ben- ejectosomes of helicosporidial cysts are somewhat similar thic organic material. No more than three coiled ele- in form to Cochleatina, but are less than ten microns in ments per cluster are seen, even on more extensive size (Fig. 8B). sheets. Within clusters, some coils are incomplete Cochleatina specimens have been reported in rare (Fig. 5H, K), and some partially overlap (Fig. 5D, F, H, instances adhering to the macroscopic fossil ‘alga’ Kanilovia L). Clusters can comprise tightly-wound bobbin-like and insolita (Ischenko 1983) from the ‘Kotlin’ regional stage of uncoiled forms, but within each cluster the coils are Ukraine (e.g. Gnilovskaya 1988, pl. 17.26). This association always of the same (potentially ontogenetic) stage/type. with Kanilovia insolita (itself a problematicum) is intrigu- The asymmetry of the ribbon zones, in particular the ing, but whether the relationship is truly biological is diffi- overlap of the serrations, reveals that the coils occur as cult to ascertain; even if fortuitous superposition could be enantiomorphs (both right-handed and left-handed ruled out, there is the possibility that the Cochleatina were forms/chirality), which can co-occur in the same cluster derived from epibionts or some other organism in associa- (e.g. Fig. 5D, F). Occurrence as triplet clusters is an tion with Kanilovia insolita. Similarly, though the triplet unexpected and novel insight into Cochleatina morphol- associations of Cochleatina (Fig. 5) are probably biological, ogy. It is possible that the ‘individual’ Cochleatina the attachment of Cochleatina to organic sheets (e.g. Esto- reported in previous studies have been selectively disag- nian material in this study) may or may not be biological. gregated during more intensive, conventional palynologi- It is common among SCF-style preservation for multiple cal processing; indeed, low-manipulation processing overlapping organic constituents to become fused into a appears to be essential for recovery of these delicate single layer during diagenesis (Martı Mus 2014). The sheets clusters. Since these Cochleatina are all at the same stage themselves preserve little discernible morphology, and or type within a cluster, they are unlikely to represent although they could represent fragments of thalli (some fortuitous superposition via currents or fall-out from the have regular margins), they could alternatively be regarded water column. Either these clusters represent groups of as sheets of degraded and depolymerized organic matter three similar individuals from a population with a ben- (sapropel), to which the more recalcitrant Cochleatina are thic ecology, or were clustered prior to sinking from fused. The consistent within-cluster similarity of Cochlea- suspension, or are the recalcitrant components of a sin- tina in these instances would at least suggest the coils them- gle organism that has otherwise decayed away. selves represent structures from a single individual, or individuals from a single population (Fig. 5D, F, H, K). Elsewhere among the fossil record, some of the more DISCUSSION densely coiled Cochleatina a superficial resemblance to sheet-like fossils preserved in (lower Biological affinities Cambrian) hydrothermal cherts from South China, which can exhibit a tightly enrolled coil-like habit, the coils even Previous suggestions for the biological nature of Cochlea- occurring in ‘clusters’ (Fig. 8G–H; see Yin et al. 2017, tina have been broad ranging, reflecting the dearth of figs 5A, C–E, 6A, E–F, 7A). These sheet-like fossils (inter- suitable fossil or modern analogues (a problem shared preted as cuticles by the authors) also bear a fine with many Ediacaran fossils). Proposed affinities have surface covering of hair-like or dentate projections (Yin included the coiled ‘elaters’ of bryophyte-grade et al. 2017, fig. 4E, F). A more precise structural compar- spores (Fig. 8A; Ischenko 1983; Gnilovskaya 1988), defen- ison to Cochleatina, however, is problematic; the surface sive ejectosomes of Cryptophyta (Fig. 8C; Burzin 1995) spines on these silicified sheets are sparsely distributed and subcomponents of a macroscopic alga (Burzin 1995). hollow projections, quite unlike the regular rows of Homology with the elaters of liverwort, and -like serrations in Cochleatina. Moreover, Cochlea- spores (Fig. 8A) can be ruled out on both tina is never found as distended, sinuous sheets or loops, functional grounds (the ribbons of Cochleatina are solid but only occurs as regular coils. In instances where speci- with no internal cavity, and therefore unsuitable for mens are found on sheets (e.g. Figs 4A, B, D, G, J, L, M, extension and retraction via hygroscopic turgor), and the Q, S; 5D–K, P) there is no basal connection to a sheet- fact that spores assignable even to stem- are margin, indicating that Cochleatina cannot be the flat- not otherwise known until the (Wellman & tened enrolled margin of such a sheet or cuticle. Gray 2000; Edwards et al. 2014). The coiled ribbon-like ejectosomes of Cryptophyta bear a superficial resemblance to Cochleatina (Fig. 8C; cf. Hausmann 1985, fig. 132) but Cochleatina as a feeding structure are intracellular organelles, orders of magnitude smaller than Cochleatina, making even an analogous function Although only a few of the previously proposed affinities improbable. Similarly, the serrated filamentous for Cochleatina can be rejected outright, none offers a SLATER ET AL.: EDIACARAN– CAMBRIAN SURVIVOR COCHLEATINA 743

FIG. 8. Comparative extant and fossil analogues for Cochleatina. A, coiled elaters found in triplets on Elaterites triferens plant spores () (see also: Good & Taylor 1974, figs 1–8; Baxter & Leisman 1967, figs 1–18). B, SEM of dehisced helicosporidial cyst (parasitic green ) showing uncoiled filamentous cell bearing barbed serrations. C, reconstruction of the ribbon-like ejectosome of Cryptophyta algae (intracellular scale). D–E, SEM of the protozoan trapping structure of the corkscrew plant Genlisea repens (an- giosperm); E, close-up of D showing serrated coils through which prey enters. F, Redkinia spinosa from the Ediacaran of north-west Russia, inset shows enlargement of serrations. G–H, coiled organic sheets found in early Cambrian (Terreneuvian) cherts. I, paired coiled of the extant mollusc Plawenia sphaera. J, coiled anterior region of the ciliated Stentor. Images from: A, Taylor et al. (2009); B, Boucias et al. (2001); C, based on diagram from Biocyclopedia (Cryptophyta); D–E, Rutishauser (2016); F, Golubkova et al. (2018); G–H, Yin et al. (2017); I, Scheltema & Schander (2000); J, Lanzoni et al. (2019). Scale bars represent: 225 lm (A); 7.5 lm (B); 1 mm (D, F); 100 lm (E); 20 lm(G–H), 200 lm (I); 50 lm (J). 744 PALAEONTOLOGY, VOLUME 63 convincing basis for assigning it to any particular biologi- et al. 2019, fig. 1). These cilia are fused into flat, trian- cal taxon. Nevertheless, there are other extant and fossil gular plates and borne on a coiled basal membranellar examples that serve to elucidate at least some of the char- band. Environmental shocks can to the membranel- acteristics that set Cochleatina apart. Notably, Cochleatina lar band being sloughed off and detached from the main can be usefully compared to a variety of feeding struc- body of the Stentor (Tartar 1961; Sood et al. 2017, tures seen in extant and fossil heterotrophs, from protis- fig. 1). When shed, the membranellar band does not dis- tan to eumetazoan grade. aggregate, but remains fused as an isolated ribbon which Comparisons have been made between Cochleatina contracts in the transverse direction to form an even and another serration-bearing carbonaceous fossil, Red- more tightly wound coil (Tartar 1961). The microanat- kinia (Fig. 8F; Sokolov 1977; Burzin 1995), which also omy of Stentor (particularly S. coeruleus) has been stud- occurs in Ediacaran deposits, both as microfossils (Veli- ied in detail for its ability to regenerate, during which kanov et al. 1983,pl.18,images8–9) and as bedding- clusters of ciliary bands can form (e.g. Tang & Marshall plane visible mesofossils (Golubkova et al. 2018, fig. 2A). 2017, figs 2, 4). Similar clustering can occur naturally It was initially proposed that Redkinia represented a dis- during reproduction or during the sessile rest state, articulated jaw (i.e. a ; Sokolov when numerous individual Stentor can attach adjacently 1977) and later, the mandible-like jaws of a stem-arthro- to a substrate via their posterior holdfast (Tartar 1961). pod (Conway Morris 1993a); if the connection to Red- The main obstacle to analogy with Cochleatina is tapho- kinia was established, it would potentially support a nomic. Without any obvious robust macromolecular bilaterian affiliation for Cochleatina. Burzin (1995) high- extracellular components to the ciliary band, it is diffi- lighted the shared characteristics of Redkinia and cult to envisage how such a structure could produce the Cochleatina, principally the first and second order serra- recalcitrant SCF Cochleatina. It is possible that relatively tions (inset in Fig. 8F), which are somewhat similar to labile structures could fuse to more resistant organic those seen in C. ignalinica, and considered the possibil- materials during diagenesis, forming a composite struc- ity of the latter evolving from the former based on their ture (Martı Mus 2014), and it is worth noting that stratigraphic relationships (but questioned the ability of seemingly decay-prone tissues are occasionally captured Cochleatina to have functioned as a feeding apparatus). in Burgess -type Lagerst€atten (e.g. ctenophores; Fu It is also questionable whether the two structures et al. 2019, fig. 2C). Regardless of taphonomic issues, (Cochleatina and Redkinia)arehomologous;serrations these similarities with Stentor demonstrate that complex are a deeply convergent morphological feature, and SCF structures like Cochleatina could, in principle, other than their carbonaceous habit, this is the only derive from protists. shared character which promotes any useful comparison. Another intriguing possibility is that the coils of Cochlea- A further likeness to metazoan mouthparts was raised tina functioned as a spiral protozoan trap, analogous with by Butterfield & Harvey (2012), who remarked on the the protistan traps of extant Genlisea, the corkscrew plant broad similarity of Cochleatina to certain molluscan radu- (Fig. 8D–E; Barthlott et al. 1998). In Genlisea, specialized lae. In particular, the simple pairs of coiled radulae borne spiral rhizophylls with a narrow serrated slit serve to trap by certain (Fig. 8I) are somewhat Cochlea- motile protists in the manner of an ‘ trap’ (Rutishauser tina-like in overall appearance (see: Scheltema & Schander 2016). Progressively narrowed spirals or coils are prevalent 2000, fig. 19F; Scheltema 2014, figs 3, 4). Cambrian radu- among such traps in the broadest sense, including those of: lae are known from SCFs (Butterfield 2008) and from the ciliated predatory protists (e.g. Stentor), helical bryozoans radula-like mouthparts of and (McKinney & McGhee 2003), coiled graptolites (e.g. Cyr- (Smith 2012); Cochleatina substantially predates these tograptus and Monograptus turriculatus; Linnarsson 1881; occurrences. However, Cochleatina also lacks any belt-like Williams & Zalasiewicz 2004), the spiral traps constructed arrangement of individual tooth-elements; the ribbon is a by (Minter et al. 2006) and even the bubble- solid structure, with no joints or segments. Moreover, traps of whales (Leighton et al. 2007). Viewed in this light, one of the species of Cochleatina (C. rudaminica) does the multi-spiral and bobbin shaped forms of Cochleatina not possess any serrations at all, making a radula-like may represent multiple traps under continuous rejuvena- function or homology unlikely. tion. Movement is key to ; in a pre-muscular Among extant organisms, a particularly useful com- world (as also seen in plant and fungal predators), passive parison is with the giant (>1 mm) single-celled sit-and-wait trapping is expected to have been the standard Stentor (Tartar 1961; Slabodnick & Marshall 2014). feeding technique, with protozoans as the primary target. Specifically, the coiled anterior region of oral cilia in Whereas Ediacaran may have extracted food Stentor is strikingly reminiscent of Cochleatina and via passive suspension, Cochleatina may represent a next- reaches a similar size (Fig. 8J; Foissner & Wolfl€ 1994, step in luring self-propelled prey (perhaps aided by attrac- figs 1–6, 11–15; Zinskie et al. 2015, figs 2, S4; Lanzoni tive chemotaxis as in Genlisea (Barthlott et al. 1998) and SLATER ET AL.: EDIACARAN– CAMBRIAN SURVIVOR COCHLEATINA 745

FIG. 9. Global stratigraphic range of body-fossils known to span the Ediacaran–Cambrian boundary compared to the range of Cochleatina sp. Temporal ranges for Cochleatina sp. from: 1, Estonia (this study); 2, Podillya, Ukraine (this study; Aseeva 1974, 1976, 1983a–b, 1988; Velikanov et al. 1983); 3, Volyn, Ukraine (Keller & Rozanov 1979; Burzin 1995, 1996); 4, Belarus (Paskeviciene 1980); 5, Lithuania (Paskeviciene 1980); 6, Latvia (Paskeviciene 1980); 7, Finnmark, Norway (Hogstr€ om€ et al. 2013); 8, Burin Peninsula, (Palacios et al. 2018); 9, Alborz Mountains, northern Iran (Sabouri et al. 2013; Etemad-Saeed et al. 2016); 10, Anabar Uplift, eastern Siberia (Rudavskaya & Vasilyeva 1989). Note that ‘Redkino’, ‘Kotlin’, and ‘Rovno’ are informal regional stages of Edi- acaran–Cambrian chronostratigraphy used in Baltica and Siberia. Temporal ranges for other boundary-crossing body fossils compiled from various sources (Narbonne et al. 1997; Jensen et al. 1998; Hagadorn & Waggoner 2000; Hagadorn et al. 2000; McIlroy et al. 2001; Winchester-Seeto & McIlroy 2006; Kontorovich et al. 2008; Zhuravlev et al. 2012; Moczydłowska et al. 2014; Yang et al. 2016; Budd & Jensen 2017; et al. 2017; Palacios et al. 2018; Simon 2018; Wood et al. 2019).

carnivorous fungi (Barron 1981)). Trapping of protistan Cochleatina are recovered from Fortunian strata of the prey may be seen as part of a broader stepwise escalation of regional Baltic Lontovan Stage (Paskeviciene 1980), which eukaryovory and predation running from the to the probably corresponds to the latter half of Fortunian time Cambrian (Porter 2011; Cohen & Riedman 2018; Antcliffe based on its acritarch and contents (in partic- et al. 2019). (and angiosperms and fungi) also dis- ular the appearance of the acritarchs Granomarginata play rare instances of trap-based carnivory (Vacelet & prima and Asteridium tornatum along with trace fossils Boury-Esnault 1995), but this style of hunting would have such as pedum, Gyrolithes and Monomorphich- declined in importance in a world of increasingly motile nus; Moczydłowska 1991; Jensen & Mens 2001; Palacios eumetazoan predators. et al. 2017, 2018; Slater et al. 2018). The majority of reports, however, are sourced from the intervening ‘Rovno’ regional Baltic/Siberian stage. In the older litera- An Ediacaran ‘survivor’ ture (e.g. Burzin 1995), the Rovno was generally regarded as forming the uppermost division of the ‘Vendian’ Sys- The oldest known Cochleatina are found in rocks of the tem. It is currently unclear whether the Ediacaran–Cam- Kotlin regional Baltic/Siberian stage (this study; Burzin brian boundary actually resides within the Rovno stage 1995, 1996; Golubkova & Raevskaya 2005). Under all (Mens et al. 1990; Moczydłowska 1991; Jensen & Mens schemes, the Kotlin is regarded as Ediacaran in 1999), however, in places the upper part of the Rovno (Grazhdankin et al. 2011; Meidla 2017). The youngest Formation is clearly Fortunian (Treptichnus pedum and 746 PALAEONTOLOGY, VOLUME 63 other typically basal Fortunian ichnofossils are found in accounting for at least some of the disconnect between the Rovno; Paliy 1976; Fedonkin 1983). While some Ediacaran and Cambrian biotas. recent schemes regard the entire Rovno stage as of earliest Fortunian origin (Meidla 2017), the scheme of Moczy- dłowska (1991) places the lower parts of the Rovno in the CONCLUSIONS Ediacaran and the upper portion, in which trace fossils of Cambrian aspect appear, in the Fortunian. Regardless of Cochleatina persisted for some ~15–20 myr, from the lat- which scheme is used, Cochleatina ranges across the Edi- est Ediacaran to the latter part of the Cambrian Fortunian acaran–Cambrian boundary (Fig. 9). Stage. The range of Cochleatina encompasses possibly the The majority of Cochleatina specimens have been found most dramatic biotic transition in Earth history, spanning in Ediacaran–Cambrian sediments of the Baltic Basin and the close of the Proterozoic until their apparent disap- Ukraine (Fig. 2). Rare reports from beyond these sedi- pearance in concert with the classical Cambrian ‘explo- mentary basins occur elsewhere on the palaeocontinent sion’ of shelly metazoans towards the end of the Baltica (Finnmark; Hogstr€ om€ et al. 2013), as well as from Fortunian. The Ediacaran was clearly a time of enormous the palaeocontinent Siberia (Rudavskaya & Vasilyeva experimentation in multicellularity, ecology and preda- 1989), with isolated reports from Avalonia (Palacios et al. tion; an expansion of bilaterians in the Cambrian may 2018) and Iran (Sabouri et al. 2013; Etemad-Saeed et al. have marginalized previously successful modes of preda- 2016). The current pattern is liable to change with tion, perhaps accounting for the disappearance of forms increased exploration of undersampled regions, but taken such as Cochleatina. Shelly and trace fossil records proba- at face value, the distribution of Cochleatina is centred on bly represent a relatively reliable account of when various the margins of the Ægir Ocean (Torsvik & Rehnstrom€ taxa and behaviours first appeared or disappeared during 2001), as well as adjacent peri-Gondwanan terranes this part of the record; the same is not true for records (Fig. 2). from Lagerst€atten, which are time-restricted and largely Cochleatina demonstrates how SCFs can contribute to absent from this time-window (Butterfield 2003). The the emerging fossil record of Ediacaran–Cambrian ‘sur- challenge at the Ediacaran–Cambrian boundary is to dis- vivors’ (Fig. 9). Although all Cambrian taxa are neces- tinguish fossil taxa that are taphonomically recalcitrant sarily derived from lineages that survived from the enough to preserve outside Lagerst€atten conditions, and Ediacaran, the current picture of the Ediacaran–Cam- so stand a chance of exhibiting a global range in the first brian boundary remains one of widespread fossil range place. SCFs appear to fulfil these criteria, at least during truncation. Closer scrutiny, however, reveals a more the latest Ediacaran and early Cambrian (Guilbaud et al. complex pattern. ‘Terminal Ediacaran’ Cloudina, for 2018; Slater et al. 2018; Slater & Willman 2019). Clearly example (Amthor et al. 2003), is now known to range the emerging distribution of Cochleatina reveals how SCFs into the Cambrian (e.g. Zhuravlev et al. 2012; Yang can supplement a crucial geographical dimension to the et al. 2016; Han et al. 2017; Simon 2018), as do the problem of the Ediacaran–Cambrian biotic transition ‘Ediacaran macrofossils’ (see; Narbonne (Figs 2, 9). Cochleatina is now known from four palaeo- et al. 1997; Jensen et al. 1998; Hagadorn & Waggoner and ten formations. Given this distribution, 2000; Hagadorn et al. 2000; Budd & Jensen 2017) and Cochleatina begins to enter the select realm of readily pre- (see; Narbonne et al. 1997; Budd & Jensen served, morphologically complex and widely distributed 2017), while the Cambrian foraminiferan Platysolenites is fossils from this time window, alongside iconic taxa such documented in terminal Ediacaran strata (Kontorovich as Cloudina. et al. 2008). These are joined by a small but increasing number of Cambrian taxa which, on morphological grounds, appear to be examples of ‘Ediacara-biota’, but SYSTEMATIC PALAEONTOLOGY have thus far only been described from Cambrian rocks; e.g. (Conway Morris 1993b) and Stroma- INCERTAE SEDIS toveris (Shu et al. 2006; Hoyal Cuthill & Han 2018). The Genus COCHLEATINA Aseeva, 1983a emend. Burzin, 1995, current roster of ‘Ediacaran survivors’ is modest, but emend nonetheless significant. When combined with the conti- nuity seen among the trace fossil record (e.g. McIlroy & Type species. Cochleatina canilovica Aseeva, 1974 emend. Logan 1999; Jensen et al. 2000, 2006; Gehling et al. Aseeva, 1983a, emend. 2001; Jensen 2003; Jensen & Runnegar 2005; McIlroy & Brasier 2017), an increasing case can be made for differ- Emended diagnosis. Coiled carbonaceous ribbon display- ential preservation, rather than purely extinction, ing a continuum of morphologies, ranging from tightly SLATER ET AL.: EDIACARAN– CAMBRIAN SURVIVOR COCHLEATINA 747 wound bobbin-like configurations to more open-coiled Emended diagnosis. A species of Cochleatina with a ribbon forms. The ribbons comprise a carbonaceous strap, widest flattened perpendicular to the bobbin axis and subdivided at the ‘base’ (outermost terminus of the coil), narrowing into four lateral zones running the entire length of the rib- toward the centre of the bobbin and terminating in a thin bon. The innermost zone (with respect to the centre of the film of fibrous projections at the ‘tip’. Ribbon is divided spiral) is optically dark and fringed with fine (5–20 lm into a complex of three to four lateral zones running the length) marginal serrations that point away from the centre entire ribbon length, the zones varying in degree of thick- of the spiral. This first zone is usually the widest, and can ening, possession of jagged or smooth margins, and pres- reach up to approximately half of the total ribbon width (ex- ence or absence of serrations. If present, serrations run cluding serrations). The second zone, where preserved, is a entire length of ribbon, project away from the centre of thin, relatively translucent layer that is usually ~20% the the bobbin and increase in size towards the base. Coils total width of the ribbon. The third zone mirrors the darker, may occur as overlapping or adjacent clusters. No dis- sclerotized construction of the first zone but lacks serrations. cernible basal attachment structure. It is of intermediate width between the first and second zones, but can be as wide as the first zone in open-coiled Remarks. We apply principles of form to the specimens. The outermost fourth zone is the narrowest (typ- classification of Cochleatina, however, the distinctive and ically ~10% of the total ribbon width) and consists of a thin, complex morphology of Cochleatina is sufficient to sug- filmy layer. It is often missing or has a ragged outer margin. gest true biological significance (i.e. Cochleatina probably At the tip, the ribbon structure is tightly bound with serra- forms a natural taxonomic group). Nevertheless, individ- tions abutting or overlapping the second and third zones of ual or clustered Cochleatina could in principle be sub- the ribbon. Towards the basal portion, the ribbon frequently components of an as yet unknown organism. Of the five tends to ‘unzip’, creating a parting (or ‘perforation zone’) described species, four are considered valid here based on between the serrated margin and remainder of the ribbon. their distinct ribbon morphologies (C. canilovica, C. rara, The ribbon exhibits a continuum of tightly wound to open- C. rudaminica and C. ingnalinica). coiled forms. Coils may occur in clusters. The concept of distinct morphological zones across the ribbon (Fig. 7) was introduced by Paskeviciene (1980), Acknowledgements. We thank Heikki Bauert, Ursula Toom, Olle ~ and later modified by Burzin (1995, see fig. 3). Differences Hints (Tallinn University of Technology, Estonia) and Tonis in the morphology of these zones forms much of the basis Saadre (Estonian Geological Survey) for their help in sampling at the superb TUT drillcore facilities and collections in Estonia, for the specific taxonomy of Cochleatina. For example, and Victor Podkovyrov (IPGG, Russian Academy of Sciences) C. rudaminica and C. ingnalinica can be distinguished for access to samples from the drillcore No. 700, Ukraine. We from other Cochleatina by their possession of sculpture on would like to thank Vojtech Kovar (Charles University, Prague) their outermost ribbon zone, and can be distinguished for help with picking the cluster in Figure 5L, and So¨ren Jensen from each other by the presence (C. ingnalinica)or (Universidad de Extremadura, ) for helpful literature. We absence (C. rudaminica) of serrations. Cochleatina canilo- thank Teodoro Palacios, Małgorzata Moczydłowska and Sally vica and C. rara both have pronounced serrations on the Thomas for constructive reviews. This research was funded by first ribbon zone. Cochleatina rara, however, exhibits a the Natural Environmental Research Council, UK, grant NE/ narrow, tightly coiled ribbon with no fourth zone, and a K005251/1 (BJS, THPH, NJB). second zone which has a jagged sclerotized margin. Cochleatina rara differs from other Cochleatina in the ori- Editor. Javier Alvaro entation of the ribbon, which is coiled in a cylindrical fashion with respect to the bobbin axis. Cochleatina con- centrica (Kolosov 1984) was a species initially assigned to REFERENCES Volyniella before assignment to Cochleatina by Jankauskas € et al. (1989). This species, however, was rejected by Burzin AMTHOR, J. E., GROTZINGER, J. P., SCHRODER, S., (1995), since it appears to be a segmented filament BOWRING, S. A., RAMEZANI, J., MARTIN, M. W. (Cochleatina are not segmented), and unlike Cochleatina it and MATTER, A. 2003. Extinction of Cloudina and can be found as a tangled mass, rather than a coil. at the Precambrian-Cambrian boundary in Oman. Geology, 31, 431–434. 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