Morphological and behavioural evolution through the Ediacaran and basal Cambrian of the Mackenzie Mountains, NW Canada
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The Mackenzie Mountains of NW Canada contains a superb record of biotic evolution through the late Ediacaran-early Cambrian that is ideal for studying the biological, ecological, behavioural, and environmental innovations that occurred during the Ediacaran and basal Cambrian. Newly discovered Ediacara-type megafossils in the uppermost Blueflower Formation at Sekwi Brook include tubes possibly attributable to suspension-feeding annelids, the preserved top of a large frond holdfast, and several problematica. These fossils represent the youngest and shallowest Ediacaran fossils known from NW Canada, and differ significantly from the communities of deep-water rangeomorphs preserved lower in the succession. Behavioural evolution of the infauna through the Ediacaran and earliest Cambrian can be observed in the rich trace fossil records of the Blueflower and Ingta formations. Trace fossils in the lower part of the Blueflower Formation are characterized by millimeter-diameter, simple, horizontal burrows of microbial grazers and deposit feeders that demonstrated very primitive and inconsistent two-dimensional avoidance strategies. Upper Blueflower trace fossils additionally include three-dimensional avoidance burrows and oblique burrows of filter-feeders or predators, reflecting new behavioural innovations and increased three-dimensional use of the substrate. The Cambrian strata of the Ingta Formation further include probing, U-shaped, and radiating burrows, irregular networks, and arthropod trails. These new feeding strategies were accompanied by increasingly more systematic grazing burrows. The development of more diverse feeding styles upwards through the succession both caused and reflected the spatial and temporal disappearance of Proterozoic matgrounds and their replacement by Phanerozoic mixgrounds. Avoidance strategies among grazing burrows became more consistent and complicated upward throughout this succession, increasingly resembling the guided meanders of Phanerozoic trace fossils. This implies that, while the first avoidance burrows probably reflected the responses of individual burrowers to individual stimuli, genetically-coded programmed behaviour developed and became dominant in the earliest Cambrian. These observations imply that increases in sensory and neural capacity accompanied skeletonization as a major factor in the Cambrian explosion of animal life.