Building the eukaryotic planet: a view from marginal marine settings

Marginal marine settings – the deltaic, estuarine, and mudflat habitats at the interface of land and sea – offer exceptional taphonomic windows on the rise of eukaryotic ecologies. Organic microfossils from tidally influenced horizons point to pre-Cryogenian origins for major eukaryotic groups, including red algae (Butterfield 2000), putative fungi (Butterfield 2003, 2005), and amoebae (Porter et al. 2003; Dehler et al. 2012). Meanwhile, an absence of comparable records even in those supratidal settings offering exceptional preservation conditions (e.g., in early diagenetic silica) suggests that Precambrian eukaryotes were essentially confined to subaqueous environments. Yet, these windows onto early eukaryotic history are vanishingly rare and temporally restricted. Efforts to place them within a broader record, spanning the Precambrian-Cambrian transition and its Phanerozoic aftermath, have been frustrated by a lack of similar organically preserved biotas from Cambrian marginal marine settings. New ichnofossils and Small Carbonaceous Fossils (SCFs; Butterfield & Harvey, 2012) from mudcracked horizons of the Middle Cambrian Pika Formation (Western Canada) offer a comprehensive view on an early Palaeozoic fauna from a periodically emergent mudflat. The wiwaxiids, priapulids, stem- and crown-annelids, and burrow traces of the Pika biota show that both classic Burgess Shale-type metazoans and ecosystem engineers from modern classes ventured into Cambrian tidally influenced settings, where they coexisted with members of derived living orders. This attests to an early influence of animal ‘pioneer taxa’ on dysoxic, intermittently desiccating marginal habitats. These findings push the limits of metazoan ecological tolerance to dehydration, UV exposure and salinity and redox fluctuations (e.g. Sagasti et al., 2001; Blewett et al., 2022), complementing the Precambrian record to suggest shallow-marine settings as cradles of eukaryotic innovation across the Neoproterozoic-Cambrian boundary. ReferencesBlewett, T. A., Binning, S. A., Weinrauch, A. M., Ivy, C. M., Rossi, G. S., Borowiec, B. G., ... & Norin, T. (2022). Physiological and behavioural strategies of aquatic animals living in fluctuating environments. Journal of Experimental Biology, 225(9), jeb242503.Butterfield, N. J. (2000). Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Paleobiology, 26(3), 386-404.Butterfield, N. J. (2005). Probable proterozoic fungi. Paleobiology, 31(1), 165-182.Butterfield, N. J. (2005). Reconstructing a complex early Neoproterozoic eukaryote, Wynniatt Formation, arctic Canada. Lethaia, 38(2), 155-169.Butterfield, N. J., & Harvey, T. H. P. (2012). Small carbonaceous fossils (SCFs): a new measure of early Paleozoic paleobiology. Geology, 40(1), 71-74.Dehler, CM, SM Porter, and JM Timmons (2012) "The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon", in JM Timmons and KE Karlstrom, eds., pp. 49–72, Grand Canyon Geology: Two Billion Years of Earth's History. Special Paper no. 489, Geological Society of America, Boulder, Colorado.Porter, S. M., Meisterfeld, R., & Knoll, A. H. (2003). Vase-shaped microfossils from the Neoproterozoic Chuar Group, Grand Canyon: a classification guided by modern testate amoebae. Journal of Paleontology, 77(3), 409-429.Sagasti, A., Schaffner, L. C., & Duffy, J. E. (2001). Effects of periodic hypoxia on mortality, feeding and predation in an estuarine epifaunal community. Journal of Experimental Marine Biology and Ecology, 258(2), 257-283.