The developmental biology of Charnia and the eumetazoan affinity of the Ediacaran rangeomorphs

Abstract

Molecular timescales estimate that early animal lineages diverged tens of millions of years before their earliest unequivocal fossil evidence. The Ediacaran macrobiota (~574 to 538 million years ago) are largely eschewed from this debate, primarily due to their extreme phylogenetic uncertainty, but remain germane. We characterize the development of Charnia masoni and establish the affinity of rangeomorphs, among the oldest and most enigmatic components of the Ediacaran macrobiota. We provide the first direct evidence for the internal interconnected nature of rangeomorphs and show that Charnia was constructed of repeated branches that derived successively from pre-existing branches. We find homology and rationalize morphogenesis between disparate rangeomorph taxa, before producing a phylogenetic analysis, resolving Charnia as a stem-eumetazoan and expanding the anatomical disparity of that group to include a long-extinct bodyplan. These data bring competing records of early animal evolution into closer agreement, reformulating our understanding of the evolutionary emergence of animal bodyplans.

Orientations of Mistaken Point Fronds Indicate Morphology Impacted Ability to Survive Turbulence

“…The Ediacaran macrobiota is a polyphyletic assemblage of organisms (Darroch et al, 2018) which appear in the fossil record ∼575 million years ago and contain some of the oldest animals in the fossil record (Xiao and Laflamme 2009;Budd and Jensen 2017;Bobrovskiy et al, 2018;Dunn et al, 2018Dunn et al, , 2021Hoyal Cuthill and Han 2018;Wood et al, 2019). The morphologies of Ediacaran organisms from Newfoundland and the United Kingdom have few clear points of homology with living animal lineages or Phanerozoic fossil groups, which has historically limited our understanding of their phylogenetic affinities and hampers our understanding of the functional ecology of these organisms (Laflamme et al, 2013;Liu et al, 2015).…”

“…The Ediacaran communities of Eastern Newfoundland are dominated by the perhaps most distinct members of the Ediacaran macrobiota-the sessile, frondose rangeomorphs (Narbonne and Gehling 2003;Narbonne 2005). Rangeomorphs are characterised by a "fractal" branching architecture (Narbonne 2004;Hoyal Cuthill and Conway Morris 2014), and which increasing data supports as a clade of stem-group eumetazoans (Hoyal Cuthill and Han 2018;Dunn et al, 2021). Rangeomorphs numerically dominate these late-Ediacaran sea floors, but they lived alongside a number of different groups, the most abundant of which are the arboreomorphs (Clapham et al, 2003;Xiao and Laflamme 2009).…”

Renewed perspectives on the sedentary-pelagic last common bilaterian ancestor

“…574-579 Ma recently was indicated to consist of "potentially colonial stem-group cnidarians" (Butterfield, 2020: 1) or a stem-eumetazoan group (figs. 1 and 2; Dunn et al, 2021).…”

Metacommunity analyses show an increase in ecological specialisation throughout the Ediacaran period

“…One of the most dramatic events in the history of Earth is the sudden appearance of animals in the fossil record during the Ediacaran period (635 to 541 Ma), after billions of years of microbial life [ 1 – 3 ]. Ediacaran anatomies are particularly difficult to compare to modern phyla, which has hampered our understanding of Ediacaran evolution and how Ediacaran organisms relate to the Cambrian Explosion and extant animal phyla [ 4 ].…”