PECTINATES: AN EARLY EXPERIMENT IN MULTICELLULAR LIFE FROM THE EDIACARAN OF NEWFOUNDLAND

The Ediacara biota is a distinctive, worldwide, Late Neoproterozoic assemblage of soft-bodied macrofossils representing the world's first experiments in multicellular metazoan life. The Mistaken Point assemblage (575-560Ma), from localities on the Avalon Peninsula of Newfoundland, consists of the oldest Ediacaran fossils yet discovered. Fossils from this deepwater locality, well below the photic zone and storm wave base, are representative of in situ paleocommunities preserved by instantaneous deposits of volcanic ash. This assemblage is dominated by rangeomorphs; an extinct higher level taxon of unknown affinity restricted to the Ediacaran period and characterized by a modular construction of fractally branching ‘rangeomorph elements'.

Comb-shaped rangeomorphs or ‘pectinates', a species endemic to the Avalon Peninsula, are composed of an elongate, curved pedicle rod from which multiple struts branch perpendicularly at regular intervals along one side. Taphonomic data suggests that in life, the pedicle rod was a curved, tubular structure and that the struts, originally bearing rangeomorph elements, branched from it in two alternating rows. Graphical analysis of 104 specimens from four localities suggests that pectinates grew by adding struts incrementally throughout its lifetime to both ends of the pedicle rod. This finding lends supporting evidence to the hypothesis that two independent growth strategies – growth by inflation and by strut/ branch addition – existed contemporaneously in different groups of Avalon Ediacarans. Although pectinates unquestionably all belong to the same species, each of the pectinate localities displays a unique specimen size range, preservational grade, and proportion of pectinates in its census population. We suggest that each locality represents a different age cohort within the pectinate lifecycle, similar to those found in modern macrobenthos that generate spatfalls of pelagic larva as a method of reproduction. Due to their unique taphonomy, structure and distribution, pectinates may prove to be an important tool in understanding how rangeomorphs functioned, grew, and were related to one another.