Environmental and evolutionary controls in animal-sediment interactions at the onset of the Cambrian explosion

The Cambrian explosion was a time of groundbreaking ecological shifts related to the establishment of the Phanerozoic biosphere. Trace fossils, which are the products of animals interacting with their substrates, provide a key record of the diversification of the benthos and the evolution of behavioural complexity through this interval. The Chapel Island Formation of Newfoundland in Canada hosts the most extensive trace-fossil record from latest Ediacaran to Cambrian Age 2, spanning about 20 million years continuously. In order to elucidate the relative roles of environmental changes as opposed to evolutionary trajectories, we gathered the largest trace-fossil dataset to date and designed fourteen high-resolution time-environment matrices on bioturbation intensity, burrow width and depth, tiering (i.e., the vertical partitioning of trace fossils within the substrate), ichnodiversity, ichnodisparity (i.e., the development of novel architectural designs in ichnotaxa), ecospace utilization (i.e., the development of ecological niches by benthic animals), and other trends related to specific trace-fossil types. Ecosystem engineering by early animals resulted in three stages identified in the Chapel Island Formation that are probably global – an Ediacaran matground ecology, a Fortunian matground/firmground ecology, and a latest Fortunian/Cambrian Age 2 mixground ecology. Time-environment matrices further imply that the lower offshore was the cradle of diversification for animal behaviour, later expanding inshore and leading to a novelty evolutionary event, which refine our comprehension of the early steps of the Cambrian explosion.