TAPHONOMY OF DIPTERA IN THE LACUSTRINE DEPOSITS OF THE FLORISSANT FORMATION (LATE EOCENE), COLORADO

Here we examine the taphonomy of fossil flies (Diptera) preserved in the lacustrine deposits of the Florissant Fossil beds. Specifically, we examined whether preservation quality correlated with the ability to identify specimens to various taxonomic levels, and we tested whether specimen size or orientation influenced a specimen's preservation quality. In addition, we examined specimens from two sites, one that is near shore and the other off-shore, to see the influence of depositional environment on the size, orientation and preservation quality of specimens.

We examined 326 fossil diptera that were collected from two localities; UCM 83013 and UCM 83014 (also known as Cockerell 13 and 14, respectively). These sites are among the most insect-rich fossil sites in the Florissant deposits. Site 13 is located near shore and Site 14 is more offshore.

We found that preservation quality was important for the identification of fossils to the species level, but that specimens that were of low preservation quality were still identifiable to the family and genus levels. The highest preservation quality was found among the largest specimens and orientation of specimens did not affect the quality of preservation.

Specimens preserved at sites 13 and 14 did not differ in the quality of preservation nor in the orientation of specimens. However, they did differ in the size of specimens preserved at each site, with larger specimens being found in the offshore deposit (site 14). In addition, there were significantly more specimens found in the offshore site. This is the opposite of expectations generated from previous studies which have examined the taphonomy of insects in different depositional environments. As such, it appears that depositional trends in the preservation of insects are less straightforward than previously thought and that size sorting can bias the taxonomic composition of assemblages. Care should be taken to document depositional environment when examining insect assemblages, especially if these data are to be used in larger paleonenvironmental interpretations.