Ediacaran rangeomorphs in the Mistaken Point biota, Newfoundland
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The Ediacara biota is a cosmopolitan assemblage of the first macroscopic multicellular soft-bodied fossil organisms (Late Neoproterozoic, 575 – 542 Ma). The Avalon Peninsula of Newfoundland preserves entire communities composed of hundreds of organisms in their natural life positions on decameter- to hectometer-scale bedding plane surfaces as impressions under a layer of ash. The communities are dominated by rangeomorphs, an extinct taxon with modular construction and fractal architecture. Bradgatia, a multibranched rangeomorph initially studied from 7 specimens from England, is represented by >200 specimens from 8 surfaces on the Avalon Peninsula. The Bradgatia petalage is a radial array of up to eight rangeomorph petals with four visible orders of rangeomorph branching. The petalage changes throughout ontogeny from juvenile I- or V-shaped organisms to larger U-shaped specimens to the largest and oldest O-shaped specimens. Counts of the number of petals within each taphomorph suggest that a constant number of petals was maintained throughout ontogeny. There is no significant variation in the number of frondlets with petal length suggesting that petals maintained a constant number of secondary branches throughout ontogeny. Culmofrondis new genus and species is a rangeomorph frond known from a single surface within the Mistaken Point Formation. It is constructed from non-rangeomorph primary branches which are subdivided by rangeomorph secondary and tertiary branches. The zigzagging central midline and non-rangeomorph primary branches are typical of Charnia-type branching whereas its rangeomorph secondary branches and flexible primary branches are typical of Rangea-type branching. Both Bradgatia and Culmofrondis were basally attached to the ocean floor by holdfasts suggesting they were sessile, epifaunal benthic organisms. Both were oriented parallel to contour current direction and display decreasing preservation distally along specimens. Additionally, Bradgatia overlies spindles and discs and Culmofrondis possesses a long stem further supporting an epifaunal suspension feeding lifestyle. Bradgatia’s multiple petals and lack of a stem would have maximized the surface area available for suspension feeding whereas the elevated petalodium of Culmofrondis would have accessed nutrients in a zone higher in the water column with lower competition.