Taphonomy of a mass-death layer of fishes in the Paleocene Paskapoo Formation at Joffre Bridge, Alberta Canada

Abstract

A layer of silty mudstone only a few centimetres thick, in the lower part of the mainly fluvial Paskapoo Formation, contains thousands of complete skeletons of fossil fish and is Tiffanian (Late Paleocene, Ti3) in age. The diversity represented in the layer is low, with thousands of specimens of a species in the trout-perch family (Percopsidae), tens of specimens of a species in the smelt family (Osmeridae), and just two specimens of a species of osteoglossomorph in the family Osteoglossidae recovered to date. Although the fishes occur without other kinds of fossils in the "Fish Layer," other layers in the same outcrop have yielded abundant remains of plants, insects, mollusks, and mammals. Taphonomic features of the Fish Layer are used to test the hypothesis that it represents a mass-death event and to evaluate mechanisms of formation. There is little disarticulation and no sign of scavenging, fish occur in distinct size classes, individuals of the smelt species demonstrate breeding tubercles, fish tend to be clumped together on a scale of a few metres or less, and fish tend to lie with their heads facing one or perhaps two preferred directions. These findings suggest that the deposit represents a mass-death event, probably occurring during spring or early summer high water while the fish were attempting to spawn in shallow water, and that the layer itself was deposited under the influence of gentle currents and wave action. Such conditions could have occurred during or following a flood that trapped the fish in an overbank, backswamp, or abandoned-channel pond or shallow lake, as has recently been suggested on sedimentological and stratigraphic evidence. Rapid burial is probably the reason for the lack of scavenging and disarticulation. This deposit shows that well-preserved fish skeletons can be found in large numbers in fluvial formations, representing paleoenvironments and time intervals for which our knowledge of fish diversity has been poor in the past.