Ediacaran survivors in the Cambrian: suspicions, denials and a smoking gun

Abstract

The relative timing of extinctions and originations is a foundation for reconstructing evolutionary causes. However, there has been a tendency to dismiss reported Ediacaran holdovers in favour of effective extinction around the Cambrian boundary. Here, focusing on the classically Ediacaran frondose biota (Petalonamae), I suggest four main reasons why proposed Ediacaran survivors have previously been denied the acceptance they deserve: denials based on mistaken identity, doppelgängers, a last gasp or dead clades walking. I then point to the lower Cambrian species Stromatoveris psygmoglena as a key example which simultaneously meets these objections. Collectively, Cambrian survivors are a ‘smoking gun’ showing that extinction of the classically Ediacaran frondose biota did not occur until at least 30 Ma after the end of the Ediacaran period, registered by phylogenetic petalonamid Thaumaptilon from the Burgess Shale. Therefore, to paraphrase Mark Twain, reports of their earlier demise have been greatly exaggerated. Causes of their ultimate extinction should instead be sought in their total range and diversity dynamics. Overall, the Ediacaran–Cambrian transition shows extremely low numbers of recorded survivors, but diversity dynamics are dominated by the Cambrian explosion. In this context, recorded occurrences for the classically Ediacaran frondose biota are compatible with at least two extinction events, one within a possible mass extinction near the Cambrian boundary, and later, their ultimate extinction in, or after, the middle Cambrian (Miaolingian Series, Wuliuan Stage). There is, however, no correlative basis for a causal link between the Cambrian transition and the effective or final demise of the classically Ediacaran soft-bodied biota.

Decline and fall of the Ediacarans: late‐Neoproterozoic extinctions and the rise of the modern biosphere

“…Therefore, the presence of cnidarians in the Avalon assemblage likely indicates that bilaterians were also present. Coupled with the evidence for ‘Cambrian Ediacarans’ (Hoyal Cuthill, 2022), this suggests a possible ~50 million year (Myr) (or longer) coexistence of Ediacarans and total‐group Bilateria.…”

“…Taken together, the taxonomic, ecological, and macroevolutionary continuities among Ediacaran assemblages and the Cambrian world appear increasingly robust with enhanced sampling and novel analytical techniques (e.g. Eden et al ., 2022; Bowyer et al ., 2022; Hoyal Cuthill, 2022; Turk et al ., 2022). This does not mean that the possible roles of extrinsic catastrophic events such as flood basalt volcanism (Hodgin et al ., 2021) in sharpening or accelerating late‐Ediacaran turnover should be dismissed altogether.…”

“…Similarly, ecospace takeover by Nama eumetazoans may be taken to imply a competitive advantage over the comparatively 'ineffectual' and 'static' niche occupancy of Ediacarans (Schiffbauer et al, 2016). However, recent fossil discoveries demonstrate the coexistence of Ediacarans and crown-group eumetazoans as early as the Avalon (Dunn et al, 2022) and perhaps as late as the mid-Cambrian (Zhao et al, 2022;Hoyal Cuthill, 2022). Meanwhile, other findings dramatically expand their known range of interactions in the White Sea assemblage, at the ecological apex of the Ediacaran Biota (Tarhan et al, 2018(Tarhan et al, , 2015Droser, Tarhan & Gehling, 2017;Bowyer et al, 2022).…”

“…Classic Ediacaran soft‐bodied macrofossils, henceforth informally referred to as ‘Ediacarans’, comprise disparate frondose, modular, and lobate body plans (Fig. 1) conventionally thought to have become extinct, or at least to have faded from ecological dominance, in the terminal Ediacaran (Darroch et al ., 2018; Hoyal Cuthill, 2022). Due to a scarcity of obvious homologies with living taxa, their constructional morphology and phylogenetic placement have proved notoriously controversial (e.g.…”

Organic-walled microfossils from the lower Cambrian of North Greenland: a reappraisal of diversity

“…While a few Ediacaran macrofossil and microfossil taxa can be recognised as parts of true boundary-crossing lineages based on their complex combination of characters (e.g. Cochleatina , Swartpuntia, Thaumaptilon ; Hagadorn et al 2000; Budd and Jensen 2017; Slater et al 2020; Cuthill 2022), the bulk of the organic-walled microfossil record consists of likely convergent, simple morphologies that do not allow phylogenetic connections to be drawn across the Proterozoic–Cambrian boundary. Despite these limitations, the acritarch record remains one of the few statistically robust, continuous, and widespread fossil records available to palaeontologists (Butterfield 2003), and as such has been used intensively in large-scale studies of species richness and assemblage compositions in deep time (e.g.…”