Abstract The diversity, abundance, distribution, and depth of trace fossils in the Cambrian–Ordovician deposits in Laurentia, from California and Nevada to New York (United States) and Quebec (Canada), are a series of biozones that record the early evolution and radiation of metazoans in shallow-marine environments. The Neoproterozoic–Paleozoic transition (NPT) plays a significant part in understanding the diversity, timing, rate, circumstance of first appearances and subsequent metazoan radiations, and trends in ecospace utilization through the Cambrian–Ordovician as recorded in carbonate and mixed carbonate-siliciclastic deposits. The first burrows with spreiten and complex branching, designated as the Phycodes (Treptichnus) pedum Zone, delineate the base of the Cambrian. This zone overlies the uppermost Neoprotero-zoic Harlaniella podolica Zone and is composed of relatively simple horizontal burrows. The Rusophycus avalonensis Zone, characterized by the occurrence of more complex burrow architectures, overlies the Phycodes (Treptichnus) pedum Zone and represents the last pretrilobite biozone. As recorded by ichnofabric through the Cambrian–Ordovician, trends in the depth and extent of bioturbation illustrate the spatial and temporal change in ecospace utilization. With the onset of the substrate (media) revolution across the NPT, animals adapted, and evolved new innovations to penetrate microbial-mediated sedimentary environments. This change reflects ongoing Cambrian–Ordovician evolution and radiation of metazoans from shallow inner-shelf environments to middle-shelf environments with increasing biogenic reworking through time. Nonetheless, a mixing depth of 6 cm (2.4 in.) was not surpassed until later in the Ordovician. This pattern ismirrored by the first appearances of trace-fossil ichnotaxa in shallow-water environments that later gradually moved offshore to shelf environments. The ichnological patterns are debated, however, asevidence of deep burrowing (i.e.,>6cm[>2.4in.]) has been described from the Cambrian and the Ordovician deposits in the Mackenzie Mountains (western Canada) and the Great Basin (western United States). Evidence for the early evolution of continental ecosystems does not exist in Laurentian deposits until the Late Ordovician, although some evidence for the invasion of land in the Early Cambrian and the Early Ordovician exists.

Abstract The base of the Cambrian System is recognized by a characteristic (marine) trace fossil suite assigned to the Treptichnus pedum Biozone, which signals increasing complexity of animal behaviour and demarcates the Cambrian from the (older) Ediacaran System (Proterozoic Eonathem). Ichnotaxa of the T. pedum Biozone are not the earliest trace fossils, and are preceded in the latest Proterozoic by a progressive increase in the diversity of trace-producing organisms and the communities they comprised, the structural and behavioural complexity of the trace fossils, and even the depth of burrowing in sediments. Parallels can be drawn with the increasing complexity of subsurface structures associated with human cities, which also reflect evolution of an increasingly complex community. Before the nineteenth century, these structures were limited and simple, but beginning with the development of London in the mid-nineteenth century as the world’s first megacity, subsurface structures have become increasingly complex, reflecting the technology-driven behaviour of twentieth- and twenty-first-century humans.