THE MIDDLE EOCENE COAL CREEK MEMBER OF THE KISHENEHN FORMATION - AN EMERGING INSECT KONSERVAT-LAGERSTÄTTE

The Kishenehn Formation consists of nonmarine Cenozoic sediments that represent a wide variety of depositional environments. Exposures of the formation occur along the North and Middle forks of the Flathead River, which form the southwestern and western borders of Glacier National Park in northwestern Montana. The oldest deposits, containing fragmentary mammalian taxa that range in size from tiny insectivores to immense titanotheres, are from middle Eocene Coal Creek member exposures (46.2 ± 0.4 Ma by 40Ar/39Ar analysis) along the Middle Fork. The early arboreal primate Tarkadectes montanensis, recently assigned to the extinct family Omomyidae was originally described from this member. Younger fossils, ranging through the early Miocene, have been found along the North Fork and its tributaries. Molluscan remains are relatively common and extremely diverse with over 55 taxa in the North Fork region and 62 taxa in the Middle Fork region. Plant fossils, most often found in siltstone and sandstone, are relatively rare although a variety of very small seeds are present in the oil shale of the Coal Creek member. Most prominent however, both in their diversity and their high degree of preservation, are the fossil insects. Fifteen different orders of insects have been identified; the majority of individual fossils are the non-biting midges (Chironomidae). There is a strong taphonomic bias towards the preservation of very tiny insects not found in other North American Lagersättten. These include fossils of the Ptiliidae (Coleoptera) only 670 mm in length and the very first compression fossils of Mymaridae (Hymenoptera), often less than 1 mm in length. Over 40 specimens of Culicidae have been collected from the Coal Creek member including the first fossil of a blood-engorged fossil. This latter specimen was shown to contain hemoglobin-derived heme in its abdomen. The preservation of these insects appears to have occurred in a shallow nearshore lacustrine depositional environment as evidenced by a preponderance of small autochthonous aquatic insects and low levels of insect disarticulation. Coal Creek Member shale varves contain seasonal accumulations of non-diatomaceous, possibly cyanobacterial, microbial mats which appear to have been integral to the taphonomic processes involved in preservation of the insects.