THE EFFECTS OF PREDATION ON THE PRESERVATION OF ONTOGENETICALLY YOUNG INDIVIDUALS

Size-frequency distributions are an integral part of understanding species interactions and population dynamics. Numerous studies have observed that the mode of the size-frequency distribution of fossil assemblages is shifted toward the larger size classes, relative to the mode of live assemblages, suggesting there is a loss of individuals from the smaller size fractions in fossil assemblages. The loss of these individuals is often attributed to size-selective taphonomy, however, selective predation on younger, smaller individuals may also play a role. This study analyzes the effect of predation on the preservation of ontogenetically young individuals by examining predation traces relative to size of biconvex and concavo-convex brachiopods from the Ordovician, Devonian, and Pennsylvanian. We constructed two size-frequency distributions: one based on the size of the brachiopod preserved in the fossil record, and another based on the size of the brachiopod when it was attacked, as indicated by repair scars. For the concavoconvex brachiopods, the size-at-attack and fossil death assemblage curves are different; repairs occurred early in ontogeny, at sizes at which fossils were not preserved. Thus, predation on concavo-convex brachiopods focused on small individuals, the same size that is missing from the fossil record. Furthermore, as the repair frequencies of concavo-convex brachiopods increase, so does the difference between the medians of the distributions, i.e., as repair frequency increases, the fewer ontogenetically young individuals are preserved (Spearman’s rank correlation [r = 0.828, p = 0.033]). If size-selective taphonomy had been solely responsible for the loss of ontogenetically young individuals, then there should have been a preferential loss of all small bodied individuals, regardless of taxon or age, but this pattern was not observed. The results presented herein suggest that predation plays a larger role on the preservation of ontogenetically young individuals than has been previously thought.