BIOTIC AND ENVIRONMENTAL CONTROLS ON LONG-TERM TRENDS IN INSECT TAPHONOMY

Insect taphonomy is a topic that has drawn interest because of its potential biases on diversity and ecological information recorded by ancient insect faunas. To enter the compression fossil record, insects must be transported from their original habitat, break water surface tension and sink, and avoid degradation and predation while in the water column and prior to burial.

We assembled a database of more than 9000 Carboniferous-Pliocene insect compression fossils from the primary literature to test biotic and environmental controls on preservation quality. We grouped the fossils into 10 Myr bins and scored preservation quality as either articulated bodies or isolated wings; specimens with a body implied high quality of preservation. Paleozoic and Triassic insect fossils are known overwhelmingly from isolated wings (only 10% articulated bodies), but our database shows a significant increase in the percentage of specimens preserved as articulated bodies beginning in the Early Cretaceous, about 120 Myr ago. This transition could reflect variations in the robustness of different insect orders and shifts in the taxonomic composition of insect faunas, but all but three of the 11 most abundant groups in the database exhibit significant increases in articulation (two of the others, Grylloblattodea and Early Paleoptera, are not recorded after 120 Ma in our database). Instead, a shift in the frequency of insects preserved in each of the two main environments, lacustrine and marine, could explain the trend. Lacustrine sediments do contain a greater proportion of articulated bodies but there is not a clear trend towards increasing contribution by lacustrine localities. One additional factor that has been proposed to increase preservation in insects is the presence of diatom mats, which increase the rate at which individuals break the surface tension and sink in the water column. Diatoms first evolved in the Late Jurassic, coinciding with the beginning of the increase in articulated insect preservation. In addition, many Cenozoic localities, which have the highest preservation quality, preserve insects in diatomaceous sediments, suggesting that the fundamental shift in insect preservation was driven by extrinsic evolutionary factors.