Scientists from the Florida Coastal Everglades Long Term Ecological Research Program sampling old macroalgal mats in the Everglades.
Taphonomy (from "taphos" [Greek] = tomb or burial and "nomia" [Greek] = method) is the study of the processes involved in the preservation of dead organisms and their conversion to fossils. Fossilization is a complex process and, to a large degree, not well understood. But we do have a wealth of data that document some of the events involved and, importantly, we have theories that we can test.
The fossiliferous oil shales of the Kishenehn Formation are often weathered to very thin layers of rock as depicted below. When we look closely at a 1 mm thick piece of that shale, we can see that it consists of several layers. Each layer is called a varve and consists of the lake sediments that slowly accumulate over the period of a single year. Each varve is itself composed of different layers. The top layer is black in color and very thin. When we do elemental analysis of this layer, it too is found to contain layers, in this case several wavy layers of carbon-rich material (the yellow layers in the figure on the right below.) It is within this thin black carbonaceous layer that the shale naturally splits. This is also where we find fossil insects.
The very thin fossiliferous oil shale of thenKishenehn Formation. The ruler is 6 inches long.
A close-up of the oil shale showing annual layers of carbon-rich material (yellow) derived from algal mats that covered Lake Kishenehn.
We think that the layers of carbonaceous material represent fossil remnants of algal (actually cyanobacterial) mats. When growing on the surface of a lake, Cyanobacteria are known to produce large amounts of sticky mucins at the surface of the mat (the yellow layer in figures A and B below. One can imagine that tiny insects will get stuck in the mucin while larger insects may more easily escape. This would explain the bias that favors preservation of very tiny insects in the shales of the Kishenehn Formation. As the mat continues to grow, tiny insects are completely enveloped by the mat (B). When the mat dies and falls to the bottom of the lake, it becomes buried in sediments and subsequent layers of mats. Over time, the buried mats are compressed, as are the insects, and both are eventually fossilized (C). Scientists from the Florida Coastal Everglades Long Term Ecological Research Program sampling old macroalgal mats in the Everglades.
The surface of a lake covered by a green cyanobacterial mat.
A green algal mat covered by a thin layer of sticky secretions captures small insects; larger insects escape. The small insects are entrapped and eventually become fossils.
An algal bloom of Cyanobacteria covers the surface of a lake, secretes mucins and entraps small insects.
Our proposed model for the mechanism by which the insects that lived along the shores of Lake Kishenehn were preserved predicts that only small insects could be effectively fossilized. It predicts that there should be a size bias in the insects found in the shale along the Middle Fork of the Flathead River compared to those found at other sites (e.g. the Green River and Florissant Formations). The preservation of many parasitic wasps 0.5 to 2 millimeters in length - see photographs below - documents this bias. There are no butterflies or intact dragonflies preserved in the Kishenehn.
This adult female proctotrupid (Hymenoptera: Proctotrupidae) is less than 2 mm in length. It may be the smallest species of this family of wasps, extinct or extant, known to science.
The two tiny wasps pictured above are fully articulated, with antennae, legs and wings all intact - and full of the morphological detail required to identify them as chalcidoids.
Publications on Kishenehn Formation taphonomy
Constenius, K.N., Dawson, M.R., Pierce, H.G., Walter, R.C., and Wilson, M.V.H. 1989. Reconnaissance paleontologic study of the Kishenehn Formation, northwestern Montana and southeastern British Columbia. In: D.E. French and R.F. Grabb (eds.), 1989 Field Conference Guidebook: Montana Centennial Edition, Vol. 1, 189–203. Geological Resources of Montana, Billings.
Greenwalt, D.E., Rose, T.R., Siljeström, S.M., Goreva, Y.S., Constenius, K.N., and Wingerath, J.G. 2015. Taphonomic studies of the fossil insects of the Middle Eocene Kishenehn Formation. Acta Palaeontologica Polonica, 60(4):931-947. Link to pdf
Greenwalt, D.E., Rose, T.R., and Chatzimanolis, S. 2016. Preservation of mandibular zinc in a beetle from the Eocene Kishenehn Formation of Montana. Canadian Journal of Earth Science, 53:614-621.