Cambrian Explosion

Abstract

The term “Cambrian explosion” refers to an event in the history of the biosphere when the vast majority of metazoan phyla that populate the Earth today first appeared in the fossil record. Historically, there have been two opposing ways of assessing the Cambrian explosion. One line of thought highlights the merits of the fossil record as a reliable source to reconstruct the origin and diversification of Metazoa, regarding the abrupt appearance of fossils in the stratigraphic column as evidence of the explosive Cambrian diversification. The opposing view underscores the incompleteness of the fossil record, supporting an origin and diversification of Metazoa long predating the Cambrian explosion. During the last two decades, the advent of new conceptual and methodological tools in the area of molecular genomics and comparative development, and an increased scrutiny of the fossil record (including both body fossils and trace fossils), have resulted in the emergence of a more coherent picture of animal evolutionary history. Still, the origin of many metazoan groups (e.g., arthropods) is hotly debated. Establishing a chronology of biodiversity construction is essential to dissect the Cambrian explosion. Strata of the oldest Cambrian stage, the Fortunian, are remarkably poor in body fossils and dominated by small shelly fossils, small carbonaceous fossils, and trace fossils. Cambrian Stage 2 strata are still limited in macrofossil content, but archaeocyathans, a group of heavily calcified sponges, formed the first animal-dominated Phanerozoic reefs. The full expression of the Cambrian explosion is displayed in Cambrian Series 2 rocks by the rapid appearance of groups having calcitic exoskeletons, including the oldest trilobites. The breadth of this radiation is fully revealed by the body-fossil record of Burgess Shale-type preservation, with archetypal examples spanning from Cambrian Series 2–3. Construction of biodiversity was accompanied by changes at the ecosystem scale. Ediacaran sediments were dominantly stabilized by extensive microbial binding, persisting into the Fortunian despite the appearance of a new set of organisms able to interact with these substrates. The most dramatic changes at the ecosystem scale took place during Cambrian Age 2 and Epoch 2, as illustrated by the concept of the “Agronomic revolution,” which refers to the advent of penetrative bioturbation and the replacement of matgrounds by mixgrounds. Evolutionary innovations in the water column and in the sediment resulted in the establishment of the basic ecologic principles that regulate modern ecosystem functioning and food webs. The proposed explanations for the Cambrian explosion have been historically classified into environmental, developmental, and ecological. Among the former, oxygen increase has traditionally figured prominently, but, although there are minimal oxygen requirements for large size, it is no longer considered the fundamental trigger. Developmental explanations have received significant attention during the last decades due to recent discoveries in the field of evolutionary developmental biology, and there is growing consensus that they play a significant role. Ecological explanations, although less popular for several years, have experienced a recent comeback, albeit in combination with developmental and environmental ones. Although treating these three sets of explanations separately may be useful on pedagogical grounds, links among them are apparent and complex. Notably, the intricate links between the different categories of explanations and the different factors within one set of explanations are central to the view that self-propagating feedback systems may be key to understand evolutionary dynamics of the Cambrian explosion.