The ichnological optimum: enhanced trace fossil diversity in the Cambrian offshore driven by ecology, sedimentation and outcrop

The trace fossil record provides a valuable archive of the Cambrian Explosion and its variable expressions in different environments. To characterize factors controlling such variability, we present a 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. An environment with favourable controls on all three levels results in an ichnological optimum , exemplified by environments between storm- and fairweather wave base. We illustrate this with reference to the Cambrian Stage 3 Redalen Member (southern Norway), from which we report 28 ichnogenera. Its depositional environment hosted a diverse assortment of low-diversity communities, indicating that faunal segregation and substrate variability contributed to an elevated original ichnodiversity. Further, non-erosive episodic sedimentation without intervening wave-action extended colonization windows and facilitated preservation, promoting registered ichnodiversity. Finally, thin-bedded heterolithic successions incompletely homogenized by bioturbation are conducive to ichnotaxon identification in outcrop, enhancing observable ichnodiversity. Relative to this Cambrian ichnological optimum, sedimentary controls restricted shallower-marine registered ichnodiversity, while evolutionary controls restricted original ichnodiversity in deeper-marine and tidal flat settings. Future consideration of original, registered and observable fossil records will refine how we decipher spatially heterogeneous evolutionary patterns.