EXCEPTIONAL DIVERSITY AND PRESERVATION: INSECTS AND PLANTS OF THE MIDDLE MIOCENE STEWART VALLEY ASSEMBLAGE

Initial findings on the diversity and preservation quality of insects and plants preserved at Stewart Valley are presented. The fine-grained, lacustrine shales from Stewart Valley are within the Middle Miocene, Savage Canyon Formation and have been dated at approximately 14.1 Ma (Perkins et al. 1998; Schorn and Gooch 1994). Preserved in these shales are exquisite specimens of fossil insects and plants (with traces of insect feeding damage).

Fossil insects have been collected from two main localities and identified to seventy families within eleven orders. Most abundant are Diptera and Parasitic Hymenoptera. Coleoptera are less abundant, but when found have the best preservation quality. Insects are found preserved in various taphonomic states, from poor to excellent. There are many disarticulated wings, some which have preserved the three dimensional folding that occurs along wing veins. Other specimens are complete and articulated, often with compound eyes preserved in three dimensions.

Numerous plant taxa have been identified from macrofossils, including twenty-four angiosperm species, from twelve families. These leaves are also preserved in exquisite detail and are abundant at a number of collecting localities. The most abundant leaves are members of the Fagaceae, Rosaceae and Salicaceae. Because of the high quality of preservation, leaves were examined for the traces of insect feeding damage. Of 1206 leaves analyzed, 32% had insect damage, including a number of galls and leaf mines. Quercus leaves appeared to have the greatest amount of feeding damage (45% of leaves damaged). These levels are higher than what has been reported for Eocene fossil leaf damage, but significantly lower than modern levels of damage.

Future studies of the Stewart Valley assemblage will focus on insect taxonomy, paleoenviromental indicators and plant-insect interactions. In addition, the Stewart Valley assemblage will be compared to other exceptionally preserved Tertiary assemblages, like the Florissant Fossil Beds and the Green River shale deposits.