Scratching the discs: evaluating alternative hypotheses for the origin of the Ediacaran discoidal structures from the Cerro Negro Formation, La Providencia Group, Argentina

Abstract

In the Ediacaran marine succession of the Cerro Negro Formation (Tandilia System, NE Argentina), abundant microbially induced sedimentary structures indicate general conditions of substrate biostabilization. Numerous discoidal structures in this succession were previously interpreted as moulds of soft-tissue holdfasts of sessile organisms, within the form genus Aspidella. In this study, we performed a detailed re-analysis of some of these features and discuss two alternative hypotheses to explain their genesis: (1) as the result of soft-sediment deformation and fluid injection structures; and (2) as structures of active animal–sediment interaction (i.e. trace fossils). We show that the dome-shaped discs are internally laminated, with a cylindrical to a funnel-shaped vertical tube at their central region. The presence of these downwards vertical extensions and other intricate internal arrangements cannot be explained under the taphonomic spectrum of discoidal fossils, but shows striking similarities to Intrites-like structures and other sand-volcano-like pseudofossils (e.g. Astropolithon). However, some structures are hard to distinguish from vertical dwelling burrows with funnel-shaped apertures and thick-lined walls, commonly produced by suspension- and detritus-feeding invertebrates (e.g. Skolithos isp., Monocraterion isp. and, less likely, Rosselia isp.). Since reliable age constraints are unavailable, and further investigation concerning other palaeobiological indicators is needed, the most parsimonious hypothesis is that of a structure derived from fluid-escape processes. Our study demonstrates the importance of detailed investigation on discoidal structures in either upper Ediacaran or lower Cambrian strata.

Simple Structures and Complex Stories: Potential Microbially Induced Sedimentary Structures in the Ediacaran Serra De Santa Helena Formation, Bambuí Group, Eastern Brazil

“…Type 3 discoidal structures (Fig. 11A) are characterized by a central depression, which may represent the collapsed conduit through which fluid escaped during sediment compaction (Dornbos et al 2007; Callow et al 2011; Menon et al 2015, 2016; Inglez et al 2022). Although this interpretation needs to be further tested with petrographic observation in thin sections, as has been done for Type 2 discs (Figs.…”

“…Due to the influence of viscous microbial substances such as extracellular polymeric substances (EPS), MISS often develop and preserve intricate wrinkled patterns such as Kinneyia, “Old Elephant Skin” and arumberiamorph textures (Hagadorn and Bottjer 1997; Gehling and Droser 2009; Kolesnikov et al 2012, 2017; Kumar and Ahmad 2014). In addition, other sedimentological features might be important indicators of microbial activity in ancient environments, including but not limited to: (1) microbial gas entrapping structures or gas domes (Bosak et al 2010; Mata et al 2012; Knoll et al 2013); (2) discoidal structures (Gerdes et al 2000; Hagadorn and Miller 2011; Taj et al 2014; Menon et al 2016; Inglez et al 2022); (3) “sand chips” (Pflüger and Gresse 1996; Bouougri and Porada 2002); (4) sinusoidal cracks (Bouougri and Porada 2002); (5) multidirectional ripple marks (Noffke 1998); and (6) erosional remnants and pockets (Noffke 1999).…”

“…Also, several simple plug-shaped structures, attributed to resting or shallow dwelling burrows (e.g., Bergaueria and Conichnus ) can be preserved as discoidal features on the bedding plane (Pemberton et al 1988; McIlroy et al 2005). Finally, discoidal structures can be generated through abiotic processes, such as concretion formation (Anderson et al 2016; Schwid et al 2021) and fluid escape (Menon et al 2016; Inglez et al 2022).…”

Renewed perspectives on the sedentary-pelagic last common bilaterian ancestor

“…It was already remarkably noted that without trace fossils "Given the simple morphology of Ikaria, it is unlikely that we would be able to confidently assign it to the Bilateria, or even Metazoa …" (Evans et al, 2020: 7849). In this respect, despite that bilaterian trace fossils are currently considered to be a part of at least some Ediacaran communities (Mángano & Buatois, 2008: 8), we may underestimate complications caused by different factors (Runnegar, 2021: 15;Warren et al, 2020;Inglez et al, 2021). Given also the character loss (Jenner, 2004a), widespread simplification via paedomorphosis in marine soft sediments (Martynov et al, 2020), and in the light of the presence of complex metazoans in the Ediacaran period (Butterfield, 2020;McIlroy et al, 2021;Dunn et al, 2021), it is implausible to consider simple Ikaria as the ancestor of all bilaterians.…”

“…Instead, the ancestral state of the non-segmented benthic sedentary (adult) polyps without elaborate coloniality but with the ability for clonal reproduction ('solitary to semi-colonial') for all cnidarians is confidently supported, although records of cnidarians in the Ediacaran are controversial (Park et al, 2012;Van Iten et al, 2014;Han et al, 2016;Mendoza-Becerril et al, 2016;McFadden et al, 2021). The disk-shaped Aspidella is one of the earliest sedentary organisms with cnidarian-like behaviour at least at 565 Ma (Menon et al, 2013;Liu et al, 2015) and potentially predates the Ediacaran period (Meert et al, 2011), although it is also possible to interpret some disc-shaped fossils as fluid-escape processes (Inglez et al, 2021). Despite these uncertainties, the earliest confirmed sedentary metazoan Ediacaran assemblage ca.…”