PRELIMINARY CHARACTERIZATION OF THE MIDDLE EOCENE KISHENEHN BASIN INSECT FAUNA OF MONTANA

Although discovery of fossil insects in shale deposits of the Kishenehn Basin (46.2 +/- 0.4 Ma) was first reported in 1989, no subsequent investigation of the Basin’s fossil insect fauna has occurred. During the last 2 years, however, the NMNH Paleobiology Department has collected nearly 2,800 specimens of shale in an ongoing study of the fossil insects and the habitat of this Middle Eocene deposit. Collections were acquired along the Middle Fork of the Flathead River between Paola and Coal Creeks at 15 different sites in accordance with USFS Authorization HUN281.

The sedimentation planes of the oil shale are very well defined and discrete, varve-like layers of lacustrine sediment as thin as 50 µ. The shale splits into layers with smooth-surfaced mats that range in color from dark red to light yellow that provide exceptional preservation. Given the thin nature of the sediment layers, we found an expected strong bias toward the preservation of small insects. In a comparative study of the sizes of weevils (Curculionidae) in the Kishenehn and Green River collections, we observed that fewer than 1% of Green River specimens were < 2 mm in length, whereas greater than 27% of the Kishenehn specimens were < 2 mm in length. Similarly, nearly 20% of all Hymenoptera were miniscule members of the superfamily Chalcidoidea. Large insects such as Lepidoptera, Orthoptera and Odonata are extremely rare and, when present, are usually represented by isolated wings and legs. Although a number of unknown taxa exist and need to be studied in detail, representatives of only 9 insect orders presently have been found, perhaps a consequence of the bias described above.

To date, 2,746 specimens of shale containing 7,432 insect fossils have been collected. The majority (55%) of the fossil insects are Diptera (true flies), with Chironomidae (midges) comprising 84% of the insects collected in this family. The remarkable qualities of the Kishenehn Basin fossil insects include the often very high quality of preservation, preservation of color, and preservation of relatively large numbers of extremely small (< 1.0 mm) insects . Exceptional preservation of this unique insect fauna will allow studies designed to identify new species of Eocene insects, resulting in more accurate definition of the ecological niches that existed along the shores of Lake Kishenehn.