Predation as a Source of Fish Fossils in Eocene Lake Sediments

Abstract

The relative contribution of predation to the fossil record is usually difficult to determine because of the multitude of possible causes of death and because of overprinting of the evidence by post-mortem disturbances. Small lakes offer important advantages because, in these relatively simple systems, there are fewer potential predators. As well, in the fossil record offish, predation can be recognized primarily by identification of coprolites, and coprolites are more easily identified in the relatively undisturbed sediments that are common in lakes. Criteria for identification of coprolites include: similarity to gut contents in place, distinctive shape, and brown, amorphous matrix. Aggregations of fish bones that have either passed through a predator's gut or been regurgitated differ from scavenged carcasses and from fragments of floated carcasses by packing and arrangement of bones, by inclusion of multiple prey individuals, and by presence of dissolved and broken bones. Birds are likely predators because the feces offish seldom contain recognizable bones, but gastric residues (pellets) of predaceious birds commonly do. Moreover, in small modern lakes at least, birds eat about as many fish as do other fish. In 25 Eocene assemblages, up to 69% of fish remains consist of presumed fossil pellets, but on the average they are less common than either articulated fish, isolated bones, or scales. Pellets with recognizable bones are seldom abundant in deepwater assemblages where articulated skeletons of larger fish are common, but are more often found together with numerous disarticulated fish bones in shallow-water assemblages, where fish are mostly smaller, and both decay and scavengers more prevalent