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The 'Cambrian Explosion' describes the rapid increase in animal diversity and abundance, as manifest in the fossil record, between ~540 and 520 million years ago (Ma). This event, however, is nested within a far more ancient record of macrofossils extending at least into the late Ediacaran at ~571 Ma. The evolutionary events documented during the Ediacaran-Cambrian interval coincide with geochemical evidence for the modernisation of Earth's biogeochemical cycles. Holistic integration of fossil and geochemical records leads us to challenge the notion that the Ediacaran and Cambrian worlds were markedly distinct, and places biotic and environmental change within a longer-term narrative. We propose that the evolution of metazoans may have been facilitated by a series of dynamic and global changes in redox conditions and nutrient supply, which, potentially together with biotic feedbacks, enabled turnover events that sustained multiple phases of radiation. We argue that early metazoan diversification should be recast as a series of successive, transitional radiations that extended from the late Ediacaran and continued through the early Palaeozoic. We conclude that while the Cambrian Explosion represents a radiation of crown-group bilaterians, it was simply one phase amongst several metazoan radiations, some older and some younger.

The first known diverse, complex, macroscopic benthic marine ecosystems (late Ediacaran, ca. 571-541 Ma) were dominated by the Rangeomorpha, an enigmatic group of extinct frondose eukaryotes that are candidate early metazoans [1, 2]. The group is characterized by a self-similar branching architecture that was most likely optimized for exchange, but nearly every other aspect of their biology is contentious [2-4]. We report locally enhanced, aberrant growth ("eccentric branching") in a stalked, multifoliate rangeomorph-Hylaecullulus fordi n. gen., n. sp.-from Charnwood Forest (UK), confirming the presence of true biological modularity within the group. Random branches achieve unusually large proportions and mimic the architecture of their parent branch, rather than that of their neighbors (the norm). Their locations indicate exceptional growth at existing loci, rather than insertion at new sites. Analogous overcompensatory branching in extant modular organisms requires the capacity to orchestrate growth at specific sites and occurs most frequently in response to damage or environmental stress, allowing regeneration toward optimum morphology (e.g., [5-7]). Its presence in rangeomorphs indicates a hitherto unappreciated level of control to their growth plan, a previously unrecognized form of morphological plasticity within the group, and an ability to actively respond to external physical stimuli. The trait would have afforded rangeomorphs resilience to fouling and abrasion, partially accounting for their wide environmental tolerance, and may have pre-adapted them to withstand predation, weakening this argument for their extinction. Our findings highlight that multiple, phylogenetically disparate clades first achieved large size through modularity., (Copyright © 2018 Elsevier Ltd. All rights reserved.)