PHANEROZOIC TRENDS IN THE DEPTH OF MARINE BIOTURBATION

The sedimentary transition layer is defined by the maximum depth of utilized burrows and is a key metric of seafloor colonization. Understanding how the transition layer varies across different seafloor environments and intervals of Earth’s history provides an important window into the ecosystem engineering behaviors of infaunal deep-burrowing animals. Here we reconstruct sedimentary transition layer depths across the Phanerozoic with a quantitative dataset of >400 measurements of six common deep-penetrating vertical or near-vertical ichnotaxa: Arenicolites, Diplocraterion, Ophiomorpha, Skolithos, Thalassinoides, and Zoophycos. This dataset allows us to constrain the maximum burrow depths reported from different geologic successions as well as to characterize trends in mean burrow depth and variance across time and in different marine environments.

We observe no significant trend in overall maximum burrow depths over the Phanerozoic, supporting an early establishment of a well-developed transition layer. However, when viewed at the system and series levels, the means and median of the reported deepest burrow depths of sampled ichnotaxa increase steadily beginning in the Permian, except for a short-lived Lower Triassic decrease which we attribute to the well-sampled interval bracketing the Permo-Triassic mass extinction. This positive trend in transition layer depths is apparent across near-shore, shelf, and deep-water environments.

Change-point analyses identify a pronounced increase in average burrow depths of deep transition-layer ichnotaxa in the Lower Jurassic, supporting an intensification of deep marine burrowing associated with the Mesozoic Marine Revolution. We observe subsequent ephemeral shifts in burrow depths, not associated with a state change, in the middle Cretaceous, lower Eocene, and middle Miocene. These data jointly indicate that while the seafloor was colonized by deep-burrowing infauna early in animal’s evolutionary history, the ability to burrow at depth became substantially more ecologically and environmentally widespread during the Mesozoic.