38 The Ediacaran–Cambrian transition: the emerging record from Small Carbonaceous Fossils (SCFs) Ben J. Slater1, Thomas H.P. Harvey2, Romain Guilbaud3, Sebastian Willman1, Graham E. Budd1, Nicholas J. Butterfield4. 1Department of Earth Sciences (Palaeobiology), Uppsala University, Villavägen 16, SE-75236 Uppsala, Sweden ([email protected]) 2School of Geography, Geology and the Environment, University of Leicester, Leicester LE1 7RH, UK 3Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK. 4Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK Abstract.––The most profound change in the fossil record is centered on the Ediacaran–Cambrian transition. Sediments deposited near to this boundary record the first complex trace fossils, the first animal biomineralizers and the earliest fossils of the major animal groups that came to define the subsequent Phanerozoic fossil record. Here we discuss how the emerging record of small carbonaceous fossils (SCFs) is shedding new light on unmineralized aspects of the biota from this critical evolutionary transition. The new SCFs record complements currently established records from trace fossils and biomineralized shelly-remains in reconstructing the broader scale macroevolutionary patterns of this transition, but via an entirely different taphonomic window. SCFs extend the record of multiple major animal groups, improve our picture of protistan diversity from this interval, and reveal new data on elements of the biota that span the Ediacaran–Cambrian boundary. Key words: small carbonaceous fossils; bilateria; Burgess Shale-type preservation; Ediacaran; Terreneuvian. The dramatic changes in the fossil record across subsequent Cambrian radiation (Butterfield, the Ediacaran–Cambrian transition seemingly 2003; Slater et al., 2018a). In particular, capture the initial diversification of bilaterian Burgess Shale-type Lagerstätten sites are animals. This radiation is principally noticeably absent from near to the Ediacaran– recognised by the signal from mineralized Cambrian boundary, and from the earliest skeletal fossils (Budd & Jensen, 2003; Cambrian Terreneuvian Series (~541–521 Ma). Butterfield, 2007), along with the conspicuous Consequently, our knowledge of unmineralized emergence and diversification of complex trace biota from this critical timespan is severely fossils (Jensen, 2003). However, the existing impaired. Recently, the newly identified record record from shelly fossils captures only a small of ‘small carbonaceous fossils’ (SCFs) has fraction of the original diversity, as revealed at emerged as an alternative means of detecting Konservat-Lagerstätten sites such as the and tracking unmineralized taxa, otherwise Burgess Shale, where the vast majority of absent from the conventional shelly fossil fossilised taxa produce no mineralized skeletal record (Butterfield & Harvey, 2012). SCFs parts (Butterfield, 2003). Despite the comprise a polyphyletic assortment of organic exceptional snapshots afforded by such remains sourced from metazoans, protists, Lagerstätten sites, their inherent rarity limits algae and other organisms. While SCFs their contribution to tracing the overall patterns typically preserve only incomplete flattened of the Ediacaran–Cambrian transition and fragments of the original organism, these can 39 Figure 17.––Metazoan SCFs from the early Cambrian of Baltica. A-B. Simple spinose elements; C–D. Priapulid-derived cuticular sclerites; E. Palaeoscolecid plate; F. Sclerite sourced from a Wiwaxia. A, B, from the Fortunian of western Finland. C-F, from the early Cambrian of southern Sweden. Scale bars: A-E = 100 μm; F = 100 μm. frequently capture exquisitely detailed cuticular major groups, yet these older assemblages tend components and distinctive microstructures to contain only a moderate diversity of that betray the affinity of their producer, even at metazoans usually dominated by spines with the specific level (e.g., Harvey et al., 2012; simple morphologies. However, the fine-scale Smith et al., 2015). characteristics of these spines including their shape, texture and thickness can distinguish A series of new SCF biotas identified different categories of metazoans even here. from Ediacaran–Cambrian successions on The abundance of ‘large’ benthic non-animal Baltica (Slater et al., 2017, 2018a; Slater & SCFs in the Fortunian is further evidence for the Willman, 2019), Laurentia (Slater et al., 2018b) persistence of widespread microbial and other regions is adding a new dimension to matground-type substrates in the earliest part of the fossil record across this transition. Initial the Cambrian. Preliminary investigations of investigations have shown that the distribution latest Ediacaran SCF assemblages reveal a of oxygen was a major factor in determining the largely protistan-dominated record, including distribution of animals and preservation of evidence for multiple boundary-crossing SCFs (Guilbaud et al., 2018). Taken together, elements of the assemblage. In spite of the these studies have also revealed a characteristic substantial differences in mode of preservation temporal signal: early Cambrian (Stage 3–4) and taxonomic coverage, the pattern emerging SCF assemblages produce a rich diversity of from this new SCFs record is therefore one that recognisable metazoan remains, many of which still coarsely resembles that recognised among are assignable to Cambrian taxa known from the more conventional records of mineralized Burgess Shale-type Lagerstätten elsewhere. hard-parts and trace fossils. Earliest Cambrian (Terreneuvian) SCF assemblages push back the records of several 40 Figure 18.––Assortment of non-animal SCFs from the earliest Cambrian of Baltica. A. Large acritarch (Lontohystrichosphaera grandis), reminiscent of forms known from Proterozoic strata; B. Large sheet- like problematica (Retiranus balticus); C. Filamentous Polythrichoides-like SCF; D. Problematic annulated tubular fossil (Sokoloviina costata); E. Large Leiosphaeridia; F, G. Palaeolyngbya‐and Rugosoopsis-like pseudo‐segmented filaments. A–D, G, from the Fortunian of Estonia; E, F, from the Fortunian of western Finland. Scale bar: A, C–G = 200 µm; B = 400 µm. References Budd, G.E. & Jensen, S. (2003). The limitations of the fossil record and the dating of the origin of the Bilateria. Slater, B.J., Harvey, T.H.P., Guilbaud, R. & Butterfield, N.J. Systematics Association Special Volume, 66: 166-189. (2017). A cryptic record of Burgess Shale‐type Butterfield, N.J. (2003). Exceptional fossil preservation and the diversity from the early Cambrian of Baltica. Cambrian Explosion. Integrative and Comparative Palaeontology, 60: 117-140. Biology, 43: 166-177. https://doi.org/10.1111/pala.12273 Butterfield, N.J. (2007). Macroevolution and macroecology Slater, B.J., Harvey, T.H.P. & Butterfield, N.J. (2018a). Small through deep time. 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