Newsletter 98 85

Opening a new window onto the Cambrian Explosion of animal life

Ben J. Slater Department of Earth Sciences (Palaeobiology), Uppsala University

Introduction The radiation of bilaterian animals near the beginning of the Cambrian Period is the evolutionary revolution that divides the ecologically ‘modern’ Phanerozoic world from that of the preceding microbial-dominated biosphere. Tracing how this radiation unfolded has to a large extent relied on the records of biomineralized taxa, simply because these organisms are more readily fossilized. The misleading picture afforded by the shelly fossil record is most famously revealed in Cambrian Burgess Shale-type Lagerstätten, where the diversity of unmineralized organisms greatly outnumbers that of biomineralizers (Butterfield 2003). Cambrian Burgess Shale-type Lagerstätten, however, do not appear until Cambrian Stage 3, post-dating the rapid radiation of biomineralized taxa and trace fossils recorded in the earliest Cambrian. This temporal restriction of Burgess Shale- type Lagerstätten and their absence from the Cambrian Terreneuvian Series (~541–521 Ma) severely limits our picture of earliest Cambrian ecosystems (Budd 2003).

Small carbonaceous fossils (SCFs) are a polyphyletic assortment of organic microfossils preserved in siliciclastic sediments, encompassing the (usually) fragmented remains of metazoans, algae and protists (Butterfield and Harvey 2012). Often, SCFs capture elements of the biota that are otherwise only found among macrofossil-bearing Burgess Shale-type Lagerstätten. By contrast with these rare and restricted Lagerstätten sites, SCFs are preserved in a much broader array of sedimentary rocks, likely as a result of the relaxed biostratinomic demands on their fossilization. Consequently, SCFs offer the opportunity to greatly expand the fossil record of non-biomineralizing and cuticle-forming organisms in the Cambrian, particularly into parts of the Cambrian System which currently lack any known Burgess Shale-type Lagerstätten (Budd and Jensen 2003).

Recent identification of a rich SCF record in sediments from eastern Scandinavia (southeast Sweden and the Baltic islands, see Figure 1) has shed new light on the non-biomineralized constituents of early Cambrian ecosystems from this region (Slater et al. 2017a; Guilbaud et al. 2018). In eastern Scandinavia, the record of these SCF-bearing sediments begins in Cambrian Stage 4 (approximately 514 Ma or younger). In western Scandinavia (e.g. in southern Norway, Figure 1), however, comparable siliciclastic sequences encompass more-or-less the full range of the early Cambrian. The problem with constructing a similar SCF record in western Scandinavia is not so much one of discontinuity, but of diagenesis – the significant thermal metamorphism encountered in strata closer to the epicentre of Caledonian orogenic deformation is liable to have volatilized any original organic remains in the worst affected rocks (Slater et al. 2017b). Fortunately, substantial packages Newsletter 98 86 of early Cambrian sediments in southern Norway have escaped such heating. This project focuses on sediments from the Mjøsa Lake region of southern Norway, alongside several other localities in southern Scandinavia. The aim is to extend the nascent SCF record in Scandinavia into the earliest Cambrian.

Figure 1. Map of Scandinavia showing localities where Cambrian SCF assemblages have been recovered in the east (red stars), and new target regions for this study (blue stars).

Methodology During fieldwork to key localities around the Mjøsa Lake region, samples were systematically collected from targeted members and formations, spanning the latest Ediacaran to early Cambrian Stage 4 (Figure 2).

Figure 2. Field localities. A: River section at Lauselva showing gneissic Proterozoic basement overlain by Cambrian sediments. B: Cambrian section at Skyberg, (Sam Slater and Graham Budd in the foreground). C: Ediacaran sediments at Biri. D: early Cambrian mudstones and sandstones at Sollerud. >>Grant REPORTS Newsletter 98 87

An interesting aspect of some of the targeted sections is that fine-grained sediments bearing carbonaceous remains are intimately associated with shelly fossil deposits, permitting close spatial and temporal comparison of these contrasting preservation modes: this is unlike the situation for the majority of SCF-productive sediments in eastern Scandinavia (Slater et al. 2017a), which preserve only a limited variety of shelly fossils. Where permitted by the lithology, both mudstones (SCF-bearing) and interbedded limestones (bearing shelly fossil remains) were sampled. Collected mudstone samples were subsequently processed for organic remains using a low-manipulation procedure optimized for the recovery of SCFs (see Butterfield and Harvey 2012), while limestones were subject to treatment with weak formic and acetic acid to recover phosphatic shelly remains.

Future work

Preliminary results show that several of the target sections are productive for both SCFs and acritarch assemblages. Ongoing processing and preparation of samples aim to identify the most productive horizons within these sections for further analysis and imaging of microfossils using SEM. Since the data on earliest Cambrian SCFs is currently limited, the question remains as to whether initial reports of Terreneuvian SCFs assemblages (see Slater et al. 2018) give a representative picture from this time window more widely, or if they are the product of a more localized facies-controlled signal – further investigations of the sediments recovered in this study will help to resolve how widespread these seemingly characteristic assemblages are.

Acknowledgements

I sincerely thank the Palaeontological Association for funding this research (grant PA-RG201702) which allowed me to conduct fieldwork to collect essential data for this project. Thanks also to Graham E. Budd, Sam M. Slater and Dasiel Borroto Escuela for their support with fieldwork and logistical aspects of the project.

REFERENCES

BUDD, G. E. 2003. The Cambrian fossil record and the origin of the phyla. Integrative and Comparative Biology, 43, 157–165. BUDD, G. E. and JENSEN, S. 2003. The limitations of the fossil record and the dating of the origin of the Bilateria. Systematics Association Special Volume, 66, 166–189. BUTTERFIELD, N. J. 2003. Exceptional fossil preservation and the Cambrian Explosion. Integrative and Comparative Biology, 43, 166–177. BUTTERFIELD, N. J. and HARVEY, T. H. P. 2012. Small carbonaceous fossils (SCFs): A new measure of early Paleozoic paleobiology. Geology, 40, 71–74. GUILBAUD, R., SLATER, B. J., POULTON, S. W., HARVEY, T. H. P., BROCKS, J. J., NETTERSHEIM, B. J. and BUTTERFIELD, N. J. 2018. Oxygen minimum zones in the early Cambrian ocean. Geochemical Perspectives Letters, 6, 33–38. SLATER, B. J., HARVEY, T. H. P., GUILBAUD, R. and BUTTERFIELD, N. J. 2017a. A cryptic record of Burgess Shale-type diversity from the early Cambrian of Baltica. Palaeontology, 60, 117–140. SLATER, B. J., WILLMAN, S., BUDD, G. E. and PEEL, J. S. 2017b. Widespread preservation of small carbonaceous fossils (SCFs) in the early Cambrian of North Greenland. Geology, 46, 107–110. SLATER, B. J., HARVEY, T. H. P. and BUTTERFIELD, N. J. 2018. Small carbonaceous fossils (SCFs) from the Terreneuvian (lower Cambrian) of Baltica. Palaeontology, 61, 417–439.