Frond orientations with independent current indicators demonstrate the reclining rheotropic mode of life of several Ediacaran rangeomorph taxa

Abstract

Fossils from the deep-sea Ediacaran biotas of Newfoundland are among the oldest architecturally complex soft-bodied macroorganisms on Earth. Most organisms in the Mistaken Point–type biotas of Avalonia—particularly the fractal-branching frondose Rangeomorpha— have been traditionally interpreted as living erect within the water column during life. However, due to the scarcity of documented physical sedimentological proxies associated with fossiliferous beds, Ediacaran paleocurrents have been inferred in some instances from the preferential orientation of fronds. This calls into question the relationship between frond orientation and paleocurrents. In this study, we present an integrated approach from a newly described fossiliferous surface (the “Melrose Surface” in the Fermeuse Formation at Melrose, on the southern portion of the Catalina Dome in the Discovery UNESCO Global Geopark) combining: (1) physical sedimentological evidence for paleocurrent direction in the form of climbing ripple cross-lamination and (2) a series of statistical analyses based on modified polythetic and monothetic clustering techniques reflecting the circular nature of the recorded orientation of Fractofusus misrai specimens. This study demonstrates the reclining rheotropic mode of life of the Ediacaran rangeomorph taxon Fractofusus misrai and presents preliminary inferences suggesting a similar mode of life for Bradgatia sp. and Pectinifrons abyssalis based on qualitative evidence. These results advocate for the consideration of an alternative conceptual hypothesis for position of life of Ediacaran organisms in which they are interpreted as having lived reclined on the seafloor, in the position that they are preserved.

Morphology shapes community dynamics in early animal ecosystems

“…67), all from the Mistaken Point Ecological Reserve (MPER), Newfoundland, Canada; Bishop's Cove and Green Point 68 Bay Roberts area in Newfoundland, Canada; H5, H14, H26 and H38 (ref. 69), the Capelin Gulch site near Melrose 70 and Goldmine, all in the Discovery Geopark in Newfoundland, Canada; and North Quarry Bed B from Charnwood Forest, Leicestershire, United Kingdom 71,72 (Extended Data Fig. 1).…”

Hydrodynamic insights into the paleobiology of the Ediacaran rangeomorph Fractofusus misrai

“… 17 Fractofusus misrai dominates the D and E surfaces at Mistaken Point 20 , 21 and the “Melrose Surface” at Capelin Gulch in the Discovery UNESCO Global Geopark. 22 These organisms inhabited deep marine paleoenvironments and are preserved by obrution and smothering beneath volcanic ashes or tuffites. 3 , 5 , 23 , 24 , 25…”

“…Early considerations of the mode of feeding in the Rangeomorpha focused on filter feeding by comparison with morphological analogues in the Cnidaria 26 and also through inferences based on the tiered community structure. 20 , 27 Earlier inferences of suspension feeding (relying on particulate organic matter [POM]) 26 , 27 and absorption of dissolved organic matter (DOM) 13 as the primary mode of feeding in the Rangeomorpha have been contested, 6 , 8 , 22 , 28 , 29 , 30 and phagocytotic/chemosymbiotic feeding at the organism-substrate interface has been proposed for feeding in reclining organisms. 5 , 30 , 31 , 32 , 33 Determining the feeding strategies for rangeomorphs like Fractofusus is problematic due to the lack of direct modern analogues, 23 their unique body plan, 8 , 14 and incomplete (mostly 2D) preservation (but see Narbonne and other studies 34 , 35 , 36 , 37 , 38 ).…”

“… 18 , 31 Preferential orientations of Fractofusus misrai , oblique to a paleocurrent, have however been reported at Capelin Gulch near Melrose in the Catalina Dome ( Figure 1 ). 22 This fossiliferous surface is significant because paleocurrents at the site can be determined from current ripples associated with the fossiliferous bedding plane. 22 Using Ediacaran fossil orientations to infer paleocurrents is problematic owing to debate concerning the mode of life of many frondose taxa, particularly the Rangeomorpha.…”

“… 22 This fossiliferous surface is significant because paleocurrents at the site can be determined from current ripples associated with the fossiliferous bedding plane. 22 Using Ediacaran fossil orientations to infer paleocurrents is problematic owing to debate concerning the mode of life of many frondose taxa, particularly the Rangeomorpha. 30 The determination of non-random Fractofusus orientation trends oblique to a paleocurrent at Capelin Gulch poses new questions regarding the paleobiology and paleoecology of F. misrai that we address herein ( Figure 2 ).…”