Let Us Prey: Simulations of Grazing Traces in the Fossil Record

Trace fossils are the only direct record of the behavior of ancient organisms. They thus provide critical indications of the early evolution of sensory systems and of the behavioral response to environmental heterogeneity. An important category of traces preserve the foraging behavior of organisms on or just below the seafloor. Previous conceptual and numerical models of such traces have been based on either random walks or highly stereotypical movement patterns, with an implicit assumption of environmental homogeneity. Different trace morphologies have been interpreted as representing different behaviors. We have developed a individual based, spatially explicit movement model for trace formation that incorporates environmental heterogeneity, as well as contact and distance chemoreception. This model produces a wide range of trace morphologies from the same basic behavior. Changes in the occurrence of trace fossil types over time, in particular during the Precambrian-Cambrian transition, may thus be largely a consequence of the development of spatial heterogeneity on the ocean floor, rather than the development of new and more complex behaviors.