The Cambrian: Sedimentary controls on marine ichnology and stratigraphic time at outcrop
The Cambrian Period represents a pivotal chapter in Earth’s history, marked by the expansion of vast epeiric seas and major evolutionary innovations associated with the Cambrian Explosion. Evolutionary reconstructions can be influenced by sedimentary processes, which were heterogeneous across Cambrian seas and variably altered the record that is observable today. The ichnological record provides a valuable archive for understanding this dynamic: trace fossil diversity (ichnodiversity) reflects ecological diversity and evolutionary trends, while also providing insight into how erosion, deposition, and sedimentary stasis affected the registration of such signals within the stratigraphic record. Here, sedimentological and ichnological data are presented from a series of field-based case studies that examine the classic Cambrian succession of North Wales and the archetypal epeiric sea successions that characterize the Cambrian record. These successions represent littoral, offshore, and deepwater depositional settings and their respective environment-specific controls on the ichnological record. Findings are interpreted within a new framework that deconstructs local ichnodiversity into 1) ‘original ichnodiversity’, reflecting ecology and behavioural diversity; 2) ‘registered ichnodiversity’, reflecting colonization windows and substrate preservation; and 3) ‘observable ichnodiversity’, reflecting present-day outcrop quality. In offshore environments, controls on original, registered, and observable ichnodiversity favourably aligned to create a diverse ‘ichnological optimum’. Sustained low ichnodiversity in deepwater strata is interpreted to represent evolutionary limitations, given the lack of systematically unfavourable sedimentary controls. Littoral ichnodiversity was restricted by a combination of sedimentary and time-stratigraphic factors that were accentuated in epeiric settings, implying that littoral communities were more complex than their known record would suggest. These findings illustrate how holistic consideration of original, registered, and observable records can refine our understanding of spatially heterogeneous evolutionary patterns within Cambrian seas.
Cambrian, Sedimentology, Stratigraphy, Ichnology
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