Reconstructing a Lost World: Exceptional Preservation of Ediacaran Rangeomorphs from Spaniard’s Bay, Newfoundland  

Ediacaran fronds at Spaniard’s Bay on the Avalon Peninsula of Newfoundland are among the best preserved Ediacara-type fossils, with exquisite, three-dimensional preservation of morphological features less than 0.05 mm in width visible on the best preserved specimens.  All of the nearly 100 specimens are members of the rangeomorph clade, an Ediacaran group which dominated the early evolution of complex multicellular life.  Five fossil taxa are present (four of them recognizable worldwide), and the predominance of mainly juvenile forms at Spaniard’s Bay provides important information about the growth of these previously enigmatic organisms. 

Spaniard’s Bay rangeomorphs are characterized by centimetre-scale architectural elements exhibiting self-similar branching over several fractal scales.  Larger structures were constructed by using these elements as ‘modules’. Small leek-shaped fossils appear to represent current-aligned, juvenile specimens of the characteristic Avalonian fossil Bradgatia.  Complete ontogenetic studies of more than 200 juvenile and adult specimens of Bradgatia from the Avalon Peninsula of Newfoundland allows for the interpretation of the growth of this taxon as being entirely inflationary or fractal over more than an order of magnitude of growth. 

Four genera of rangeomorph fronds exhibit a bifoliate array of primary rangeomorph branches that pass off a central stalk.  These taxa form a series, ranging from symmetric rangeomorph branches that were unconstrained except at their attachment to the central stalk, and thus free to rotate and pivot relative to neighbouring branches; through “single-sided” (developmentally overfolded) branches whose movement was increasingly constrained; to single-sided branches that were fixed relative to each other.  This series provides a developmental linkage between Rangea-type and Charnia-type rangeomorphs.  It is marked by a shift from mainly inflationary growth to growth mainly by branch addition at the apex of the frond.  The combination of architectural and ontogenetic characters represents a fundamental shift in Ediacaran taxonomy; it forms scaffolding from which a higher-level classification of these enigmatic organisms may be supported.