THE CAMBRIAN SUBSTRATE REVOLUTION AND THE EVOLUTION OF FORAGING BEHAVIOR: INSIGHTS FROM EXPERIMENTAL NEOICHNOLOGY

The trace fossil record shows a rapid, early increase in complexity and diversity, from simple, random traces to tightly meandering and spiraling tracks during the Cambrian period. The Cambrian substrate revolution changed the ancient seafloor from a relatively homogeneous environment of algal and bacterial matgrounds to a heterogeneous environment of resource patches separated by unexploitable substrate. Foraging behavior in early bilaterian animals would have been strongly influenced by these changes in the spatial structure of the seafloor. In particular, this behavior should have evolved to increase the efficiency of resource exploitation and the expenditure of energy. The apparent evolution of trace fossil patterns may result from organisms interacting with an increasingly heterogeneous environment. The behaviors that lead to complex trace fossils, including the spiraling and meandering patterns, may be phylogenetically primitive, and thus widespread throughout the animal kingdom.

A patchy environment should lead to two general types of movement: between resource patches, and within or near a patch. As an organism travels between patches, it engages in searching behavior to detect, locate and evaluate resources. Searching movement consists of longer, straight movement paths, and less frequent and larger turns. Movement that occurs as an organism exploits a resource patch includes fewer forward movements, more turns and a smaller average turning radius. The two different types of movement make searching for patches more energy efficient, and ensure more efficient exploitation of a resource patch. These concepts were tested by experimental analyses of the foraging behavior of a variety of invertebrates including mollusks, arthropods, and the soil nematode Caenorhabditis elegans. Patch size, density and shape were manipulated and the specific movements of each forager both within and outside of the patches were monitored. The similarity of the movement patterns and foraging behaviors within and between the different groups mirror what is seen in the trace fossil record.