Hymenoptera of the Kishenehn Formation: A 46-million-year-old Lagerstätten

The Kishenehn Formation is a lacustrine and riverine deposit from the middle Eocene, located in western Montana and southern Alberta, Canada. Lake surfaces periodically supported algal mats that sank on die-off, were covered by sandy sediments, and became a varved (laminated) oil shale. Many insects alighted on the algal mat and became entangled, unable to extricate themselves. To date, the Coal Creek Member of the Formation has yielded more than 15,000 fossil insect specimens, about 6% of which are Hymenoptera (the third-most plentiful order recovered, after Diptera and Hemiptera). Complete articulated exoskeletons show details such as mouth parts, wing venation, antenna segments, legs, genitalia, and sensilla. Some specimens also preserve original molecular components (e.g., a heme molecule found in an engorged mosquito abdomen). Most specimens are quite small, measuring at most only a few millimeters long. This extraordinary level of detail in minute insect fossils offers insights into the organisms as well as the ecosystem that preserved them. Examples include specimens from the superfamilies Chalcidoidea, Ichneumonoidea (including both Ichneumonidae and Braconidae), Platygastroidea, Proctotrupoidea, Diaprioidea, Vespoidea (Formicidae), Cynipoidea, Ceraphronoidea, and Chrysidoidea (Bethylidae), and at least one specimen from the suborder Symphyta. Further work will better define the population and diversity of at least a subset of the Hymenoptera within this ecosystem.