Taphonomic bias in fish diversity from cenozoic floodplain environments

Abstract

The fossil record of Cenozoic floodplain fishes increases from few species in the Paleocene and Eocene to about 5-15 species per locality in the Pliocene and Pleistocene. Modern floodplain habitats usually have more than 5-10 times this many species. The trend could be interpreted as an evolutionary increase, except that there seem to be no ecological or evolutionary reasons to expect ancient floodplains to have fewer species than modern floodplains.The alternate hypothesis is that ecological and fluvial processes destroy most fish bones before they are finally buried. Although floodplain depositional environments trap many fishes, these are subjected to extensive predation and scavenging, thereby reducing the opportunities for bones of small fishes, which make up most of the diversity, to be preserved in the fossil record. Abrasion in bedload probably destroys most small bones that are reworked. Surface collecting methods exaggerate the bias further because fish bones from fluvial rocks are fragmentary, difficult to discover, and difficult to identify. Screen washing for fossils from fine-grained sedimentary lenses should increase the known diversity from floodplain deposits.