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Guy Narbonne, Queen's University (Canada)
Marc Laflamme, Queen's University (Canada)
Emily Bamforth, Queen's University (Canada)
Lija Flude, Queen's University (Canada)
Jim Gehling, South Australia Museum (Australia)
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Abundant Ediacaran fossil impressions in the Mistaken Point area and other localities on the Avalon Peninsula of Newfoundland date to 575-560 Ma, and thus are the oldest-known large and complex eukaryotes and the oldest representatives of the Ediacara Biota worldwide. Most of the species were rangeomorphs, probable stem-group animals consisting of cm-scale fractaly branched elements that were used as modules to construct dm- to m-scale organisms. Communities of Mistaken Point organisms were preserved in place when they were catastrophically buried under beds of volcanic ash to produce tennis court-sized surfaces littered with hundreds to thousands of fossils. All life stages from a few millimeters to more than a meter in size were preserved, making Mistaken Point an ideal locality to recreate the growth of the earliest Ediacaran organisms.
Our growth series from Mistaken Point supports the previously published view that the Ediacaran frond Charnia masoni grew by insertion of new branches at its tips, and the same mode of growth would also seem to typify our specimens of Charnia wardi. Multi-branched pectinate rangeomorphs also grew by insertion of branches, probably at both ends of the organism, accompanied by inflation of previous branches. However the vast majority of species at Mistaken Point appear to have grown entirely by inflation. Specimens of the fronds Charniodiscus procerus and C. spinosus and the spindle-shaped rangeomorphs Fractofusus andersoni and F. misrai show an invariant, species-dependent number of branches/elements despite an order of magnitude difference in size between the smallest and largest specimens of each species. Similarly, Newfoundland specimens of the multi-branched, bush-shaped rangeomorph Bradgatia show a gradual opening of the bush from I- to V- to U- to O-shaped specimens with increasing ontogenetic age, but there is no ontogenetic difference in the number of primary branches or the number of secondary branches on each primary branch.
Mistaken Point shows that developmental series can provide significant insights into Ediacaran paleobiology. However, the presence of different developmental strategies within a single group (the multi-branched rangeomorphs) and of similar developmental strategies within different groups (the frond Charniodiscus and the rangeomorph spindle Fractofusus) suggests that it must be combined with the traditional tools of comparative morphology and taphonomy in order to yield biologically meaningful conclusions.
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