EXCEPTIONAL PRESERVATION AND DIVERSITY OF INSECTS FROM THE PALEOBURN LOCALITY OF THE EOCENE GREEN RIVER FORMATION OF COLORADO

The Paleoburn locality of the Parachute Creek Member of the Green River Formation, located in the Piceance Creek Basin near Parachute, Colorado, is known for its characteristic red coloration and insect fossils that can exhibit varying degrees of three dimensional preservation. The main fossiliferous unit grades from sediments that are bright red to those that are gray in color. Fossil specimens were sampled from two end-member sites along the gradient to study whether lithology is correlated with the diversity and quality of insects preserved. In the field, samples were standardized by collecting time. Specimens were identified to family and taphonomic data (specimen size, disarticulation, relief) were recorded. Sedimentological data (matrix color, texture) were also collected for each site.

A total of 2,700 specimens were collected and a subset of 400 specimens (200 per site) was examined. An estimated family level richness of 20 was found for both sites combined. The number of families sampled was not significantly different between the two sites, but there was a significant difference (P=0.005) in the relative abundance of insects preserved. While both sites included a majority of Curculionidae (68% at the gray site, 45% at the red site), the red site also included more Carabidae, Scarabaeidae, and several Diptera and Orthoptera. The average specimen size was ~5 mm, and most specimens showed some level of disarticulation as only 10% were found to be complete. Although three-dimensional preservation has been well documented for the Paleoburn site, the majority of specimens (74%) were preserved in two dimensions. Lithological characteristics were not found to be correlated with insect size, disarticulation levels, or overall specimen relief. In general, these results support the finding of other studies, which have shown that lithology does not strongly affect the diversity and quality of insects preserved in lake settings. Additionally, color differences likely reflect chemical alterations from secondary fluid flow, and this appears to influence neither the diversity, nor the preservation quality of insect specimens. Thus differences in relative abundance of insect families may reflect real differences in the distribution of the living assemblage as opposed to taphonomic filtering.