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| Akrophyllas longa (Glaessner & Wade, 1966) in Grimes, Narbonne, Gehling, Trusler et Dececchi, 2024 |
Abstract
Decimeter-scale, elongate, fossil fronds from the Ediacara Range in South Australia were formally described as Rangea longa Glaessner and Wade, 1966, but the disparate nature of documented specimens has hindered their inclusion in global syntheses and has resulted in these fossils being assigned to at least five different genera in two different clades since their discovery. Detailed study of the type material from the Ediacara Range and the few specimens subsequently collected elsewhere in the Flinders Ranges reaffirms that these specimens represent a single species, with the apparent morphological variation between specimens entirely taphonomic and reflecting the obverse and reverse surfaces of these fronds coupled with the orientation of the frond axis and petaloids at different angles relative to the sea bottom on which they were preserved. The preserved architecture of these fronds constitutes three orders of branching microstructure that are strictly orthogonal to immediately higher and lower orders. This implies affinities with the arboreomorphs, but representing a new frond genus herein named Akrophyllas. Akrophyllas n. gen. differs from all other Ediacaran fronds in exhibiting a stalk that is visible only on one side of the frond and is internal to the other side where the first-order branches instead meet at a zigzag axial trace. Akrophyllas n. gen. was attached to a bulbous holdfast on the sea bottom, and evidence for current scours that formed in the lee of the fronds and for a strong current alignment of felled fronds with depositional overlap of adjacent fronds imply an upright, epibenthic lifestyle for Akrophyllas longa new combination.
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| Taphonomic variation in preservation of Akrophyllas longa n., comb. on bedding surfaces. (1) SAM P24593, the largest-known specimen of Akrophyllas n. gen., preserved in part and counterpart as a cleavage relief within a thick bed of laminated sandstone from Nilpena, with preservation of the marginal rim (mr), central stalk (cs), and two orders of branching (br) through composite molding. (2) SAM P12716 showing preservation of current-aligned and locally overlapping specimens of Akrophyllas n. gen. on an epirelief (top) surface in the Mincham-Flounders collection from Ediacara Range. Fronds (A) and (B) preserved in reverse view; frond (D) preserved in obverse view. Frond (B) partly overlies (A); frond (D) partly overlies (C), which may represent a separate frond, ... |
Systematic paleontology
Clade Arboreomorpha
Genus Akrophyllas new genus
Type species: Rangea longa Glaessner and Wade, 1966
Diagnosis: New. Elongate, gently tapering, bifoliate petalodium attached to a discoid to spheroidal holdfast via a short stem or a naked stalk. Petalodium architecture consisting of three orders of strictly orthogonal branching, with mm-scale, second-order rectangular branches at right angles to the first-order branches and submillimetric third-order rectangular branches at right ...
Etymology: Akros meaning “at the top” in Greek, in reference to its unusual preservation on the tops of sandstone beds. Phyllas meaning “leaf” in Greek, in reference to the overall lanceolate to linear, leaf-like shape of its petalodium. In combination with its original species name “longa”, it is the “long leaf at the top”.
Remarks: This material previously has been referred to at least five different genera in two different major groupings of Ediacaran fronds. Its architecture indicates that it is an arboreomorph whose construction differs from all previously described fronds.
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| Morphological reconstruction of Akrophyllas longa n., comb. (1) Complete erect frond attached to the sea bottom. The twist midway through the frond is diagrammatic to show both sides of the frond. (2) Close-up of the obverse side of the frond, which shows the first-order branches passing off a prominent central stalk. (3) Close-up of the reverse side of the frond, which shows the first-order branches meeting at a zigzag axial trace running the length of the frond. |
Kelsey F. Grimes, Guy M. Narbonne, James G. Gehling, Peter W. Trusler and T. Alexander Dececchi. 2024. Elongate Ediacaran fronds from the Flinders Ranges, South Australia. Journal of Paleontology. 98(2); 249 - 265. DOI: doi.org/10.1017/jpa.2023.45
