Anatomy of the Ediacaran rangeomorph Charnia masoni

Abstract

The Ediacaran macrofossil Charnia masoni Ford is perhaps the most iconic member of the Rangeomorpha: a group of seemingly sessile, frondose organisms that dominates late Ediacaran benthic, deep‐marine fossil assemblages. Despite C. masoni exhibiting broad palaeogeographical and stratigraphical ranges, there have been few morphological studies that consider the variation observed among populations of specimens derived from multiple global localities. We present an analysis of C. masoni that evaluates specimens from the UK , Canada and Russia, representing the largest morphological study of this taxon to date. We describe substantial morphological variation within C. masoni and present a new morphological model for this species that has significant implications both for interpretation of rangeomorph architecture, and potentially for existing taxonomic schemes. Previous reconstructions of Charnia include assumptions regarding the presence of structures seen in other rangeomorphs (e.g. an internal stalk) and of homogeneity in higher order branch morphology; observations that are not borne out by our investigations. We describe variation in the morphology of third and fourth order branches, as well as variation in gross structure near the base of the frond. The diagnosis of Charnia masoni is emended to take account of these new features. These findings highlight the need for large‐scale analyses of rangeomorph morphology in order to better understand the biology of this long‐enigmatic group.

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

“…Of these groups, rangeomorphs are not only the most diverse but display the greatest anatomical variation (Shen et al, 2008;Xiao and Laflamme 2009). Some rangeomorphs are preserved as single fronds (e.g., Charnia), but others were bushy (e.g., Bradgatia), spindle-shaped (e.g., Fractofusus) or arborescent (e.g., Primocandelabrum) (Gehling and Narbonne 2007;Bamforth et al, 2008;Flude and Narbonne 2008;Bamforth and Narbonne 2009;Dunn et al, 2019). Rangeomorph branches differentiated directly from one another or from a central stalk (Dunn et al, 2019) and some rangeomorphs additionally exhibited a naked stem that elevated the frond into the water column (Laflamme et al, 2012).…”

Survival and selection biases in early animal evolution and a source of systematic overestimation in molecular clocks

“…[10,[44][45][46][47][48]; a pattern that is consonant across the small skeletal, carbonaceous and trace fossil records [47]). To wit: at around 571 Ma or so appear fossils that may be assigned to stem or early crown group animals [36,37,[49][50][51]; at around 566 Ma comes the first evidence of eumetazoan-grade organisms in the form of trace fossils [52]; no later than around 555 Ma are found complex (i.e. bilaterian) trace fossils and taxa assignable to stem cnidarian, ctenophore and bilaterian grades [14,36]; by around 545 Ma near the end of the Ediacaran comes a diversification of large trace fossils (assignable to late stem and perhaps early crown group bilaterians [53]); and rather early in the Cambrian, perhaps 535 Ma come the first definitive crown group bilaterians (e.g.…”

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

“…1D), implying that, at some point following death, the branches must have been "inflated" and presumably interconnected to allow sediment to circulate without baffling. A boundary appears to have existed between second-order branches at the time of infill [e.g., Dunn et al (11), figure 10C], demonstrated by the ease with which individual second-order branches can be separated into discrete units with smooth faces. Tomographic data confirm these patterns (Fig.…”

“…The function of the interconnected compartments is unclear, but Butterfield (10) advanced a hypothesis that each individual secondorder branch had its own gastrovascular cavity, with higher-order branches functioning as mesenteries. However, previous work has shown the independent mechanical flexibility of such structures [e.g., (11)], which is incompatible with a mesentery-like function. Furthermore, our XTM data show no evidence for internal strut-like projections, despite sufficient resolution in fossil preservation and x-ray tomography (Fig.…”