Orientations of Mistaken Point fronds indicate morphology impacted ability to survive turbulence

Abstract

The Ediacaran organisms of the Mistaken Point E surface have provided crucial insight into early animal communities, including how they reproduced, the importance of Ediacaran height and what the most important factors were to their community dynamics. Here, we use this iconic community to investigate how morphological variation between eight taxa affected their ability to withstand different flow conditions. For each of Beothukis, Bradgatia, Charniodiscus procerus, Charniodiscus spinosus, Plumeropriscum, Primocandelabrum and Fractofusus we measured the orientation and length of their stems (if present) and their fronds. We statistically tested each taxon's stem and frond orientation distributions to see whether they displayed a uniform or multimodal distribution. Where multimodal distributions were identified, the stem/frond length of each cohort was tested to identify if there were differences in size between different orientation groups. We find that Bradgatia and Thectardis show a bimodal felling direction, and infer that they were felled by the turbulent head of the felling flow. In contrast, the frondose rangeomorphs including Beothukis, Plumeropriscum, Primocandelabrum, and the arboreomorphs were felled in a single direction, indicating that they were upright in the water column, and were likely felled by the laminar tail of the felling flow. These differences in directionality suggests that an elongate habit, and particularly possession of a stem, lent greater resilience to frondose taxa against turbulent flows, suggesting that such taxa would have had improved survivability in conditions with higher background turbulence than taxa like Bradgatia and Thectardis, which lacked a stem and which had a higher centre of mass, which may have fared better in quieter water conditions.

The rangeomorph Pectinifrons abyssalis: Hydrodynamic function at the dawn of animal life

“… 31 , 69 —which proposes that rangeomorph tissues possessed sulfur-oxidizing ectosymbionts, that in turn took advantage of sulfidic porewaters generated in the sediment underneath organisms. Although many of the reconstructions favored by these authors apparently require frondose rangeomorphs to be reclining on the seafloor—something at odds with the majority of fossil evidence 12 , 13 , 32 , 51 , 70 —our study does suggest that chemoautotrophy requires more detailed testing. It has been hypothesized that the holdfasts of some rangeomorphs may have played a role in nutrient acquisition, 66 , 71 a suggestion that may ultimately be more plausible, but would require both new anatomical and geochemical data to adequately test.…”

“…lyra model (geometries identical for orientations of 0° and 180°). This was done to account for the apparently random orientations exhibited by Pectinifrons specimens on bedding surfaces 11 , 32 and uncertainty regarding the preferred orientation of C . lyra specimens to flow.…”

“…We do, however, consider this model in greater detail in the Discussion section. A key observation is that Pectinifrons appears not to have—in contrast to other taxa 32 —adopted a preferred orientation on the Mistaken Point Ecological Reserve bedding planes 12 (although see Ichaso et al. 13 for a counter example in the Spaniard’s Bay area), indicating that it must have been capable of feeding regardless of orientation to current.…”

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

“…It has been previously noted that Bradgatia sp. are commonly bimodally oriented in both the inferred up-current and down-current directions on the "G" Surface at MPER (Flude and Narbonne 2008), while at other localities ("E" and "D" Surfaces at MPER) there are also abundant specimens oriented perpendicular to the paleocurrent (Flude and Narbonne 2008;Vixseboxse et al 2021;McIlroy et al 2022). Orientation data from Bishop's Cove potentially show unimodal orientation, with several specimens oriented orthogonally and against the paleocurrent direction (Flude and Narbonne 2008).…”

“…The poorly supported felling model has been used to propose an erect mode of life for all the fossil organisms in the Mistaken Point assemblages, except Fractofusus spp. (e.g., Wood et al 2003;Vixseboxse et al 2021). An alternative model invoking rheotropic epifaunal growth of reclining taxa has been proposed (McIlroy et al 2022), which calls into question the causal relationship between frond orientation and paleocurrents.…”