Ediacaran fronds

摘要

Leaf-shaped Ediacaran fronds such as Charniodiscus, Charnia, Rangea and Swartpuntia are among the most widespread and readily recognized elements of the soft-bodied Ediacara Biota (late Neoproterozoic). Recent papers have proposed a variety of possible lifestyles for Ediacaran fronds, but functional morphological analysis supports the traditional view that Ediacaran fronds gathered nutrients from the water column through filter feeding or direct nutrient absorption, resulting in the onset of Phanerozoic-style ecological competition complete with denizens occupying distinct tiers in the water column. Within the standard Ediacaran frond morphoshape, four distinct branching architectures have been isolated: Arborea-type branching, characterized by pea-pod like primary branches which house several secondary branches within the protective sheath; Charnia-type branching, resulting in a zigzagging central axis due to the overlapping of alternating sigmoidal primary branches which overly an inferred internal stalk; Rangea-type branching, consisting of petaloids composed of several overlapping primary branches which are self-similar (fractal) over three orders of branching and attached to a straight central stalk; and Swartpuntia-type branching, consisting of petaloids with unornamented tube-like primary branches. Traditional Ediacaran taxonomy has emphasized unity of fronds as a high-level taxon, but the frond morphology most likely represents convergent evolution resulting from competition for nutrients in the water column. It is recommended that any new, higher-order classification schemes highlight the differences in branching architecture as a way of recognizing clades within the Ediacara biota. Crown Copyright (C) 2007 Published by Elsevier B.V All rights reserved.