Following the logic behind biological interpretations of the Ediacaran biotas

Abstract

For almost 150 years, megascopic structures in siliciclastic sequences of terminal Precambrian age have been frustratingly difficult to characterize and classify. As with all other areas of human knowledge, progress with exploration, documentation and understanding is growing at an exponential rate. Nevertheless, there is much to be learned from following the evolution of the logic behind the biological interpretations of these enigmatic fossils. Here, I review the history of discovery as well as some long-established core members of widely recognized clades that are still difficult to graft on to the tree of life. These ‘orphan plesions’ occupy roles that were once held by famous former Problematica, such as archaeocyaths, graptolites and rudist bivalves. In some of those cases, taxonomic enlightenment was brought about by the discovery of new characters; in others it required a better knowledge of their living counterparts. Can we use these approaches to rescue the Ediacaran orphans? Five taxa that are examined in this context are Arborea (Arboreomorpha), Dickinsonia (Dickinsoniomorpha), Pteridinium plus Ernietta (Erniettomorpha) and Kimberella (Bilateria?). With the possible exception of Dickinsonia, all of these organisms may be coelenterate grade eumetazoans.

Phyllozoon and Aulozoon: key components of a novel Ediacaran death assemblage in Bathtub Gorge, Heysen Range, South Australia

“…Unsurprisingly, we reject Retallack's scenario for reasons discussed elsewhere (Xiao et al 2013;Runnegar, 2021). Seilacher's idea that Phyllozoon was a benthic organism that lived below the mat is similar to our proposal that Phyllozoon was a prostrate inhabitant of the matground but, by analogy with other erniettomorphs that seem to have lived in the water column (Runnegar, 2021), we prefer the scenario of a mat top lifestyle (Table 1; Fig. 11c).…”

“…Some individuals of Dickinsonia costata grew to a substantial size (Runnegar, 2021, fig. 4b) but nowhere near the size of the largest individuals of D. rex (online Supplementary Fig.…”

Renewed perspectives on the sedentary-pelagic last common bilaterian ancestor

“…Therefore, even if some Ediacaran organisms display some degree of a mobility and even if a lesser part of its body has been compared to bilaterians (Dzik & Ivantsov, 1999), their basic morphological patterns still included ancient developmental mechanisms not common with bilaterians, but with sedentary vendobionts. In this respect, with all the potential reservations and findings of Ediacaran taxa with variable symmetries (Waggoner, 1996;Xiao & Laflamme, 2008;Runnegar, 2021), the main symmetry patterns in the majority of vendobionts basically demonstrate non-bilaterian features (fig. 2).…”

“…1 and 2; Hoekzema et al, 2017;Ivantsov et al, 2019a;Evans et al, 2021;Xiao et al, 2021). An animal relationship for the Ediacaran taxa is currently preferred (Gold et al, 2015;Budd & Jensen, 2017;Bobrovskiy et al, 2018) and Dickinsonia can be evaluated as a putative "stem-bilaterian" (Runnegar, 2021). However, Dickinsonia and other "proarticulates" without true segmentation (Ivantsov, 2013) "likely do not represent crown group bilaterians" (Evans et al, 2020: 7848).…”

“…Compared to those supposed Ediacaran predecessors of "mobile bilaterians", Dickinsonia, Kimberella, Ikaria and Yilingia, which do not possess any attributable characters of true bilaterians (fig. 2; Runnegar, 2021), the sedentary Ediacaran Cloudinidae bear significant similarity to several bilaterian phyla.…”