|Themes > Science > Paleontology / Paleozoology > Paleozoology > Fossil Invertebrates > An Introduction to Fossil Beetle Research|
Whenever one talks about the subject of paleo-entomology, even in academia, one is met with blank stares and a faintly bemused expression. Further elaboration of the topic signifying that it is the study of fossil insects (or, in the case of our own studies, fossil beetles) still elicits comments which usually revolve around the former four from Liverpool. To most paleontologists, beetles rank almost at the bottom of a scale headed by the better known zonal fossils; the ammonites, trilobites and graptolites. Everyone has seen a beetle; the active field geologist perhaps under rocks or logs somewhere in the Rockies or Arctic, more sedentary persons may have observed them squashed on side-walks, or under debris in the garden. If you are really unfortunate they may inhabit the old wood in your home, shelter in your carpets and furnishings, or even infest your food supplies. Beetles are ubiquitous; they surround us in practically every environment except extremely high latitudes and altitudes, and in the oceans of the world.
Beetles, or more scientifically, Coleoptera, come in a bewildering variety of shapes, sizes, colours and numbers. One in five living things is an insect, and well over a third of all insect species are beetles, or, put in another way, of the over 1,300,000 described species on the planet, about one quarter belong to the family Coleoptera.
The differences in beetles are enormous. In size alone, the longest beetle, Xylorycites satyrus is over 1,000 times the length of the smallest member of the family Trichopterygidae (Ptiliidae). Individuals of this family would probably have a hard time in a confrontation with some protozoans, since many of the unicellular organisms are larger than the beetles themselves. The largest beetles in terms of volume are the "Goliath" beetles (Goliathus spp.), ranging up to 100 g. Still others have remarkable abilities; a capability to produce "cold light" as in the case of the Lampyridae (the so-called "fireflies"); chemical defense methods, particularly those of Brachinus which produce quinones in a defensive spray at almost 100 C; or the pupation of some Buprestids, recorded at a minimum of 47 years.
Beetles have a reasonable pedigree in the geological column, being first recorded in the Permian (Moore et al., 1952). Thus, they have not yet reached the longevity of the class Trilobita, but they have outlived the last dinosaurs by about 65 million years and have a 50 million year head start on the mammals.
Despite their immense diversity, fossil beetles have received little study to date. The reason for the lack of interest is fairly clear. Insect remains are not particularly common in the fossil record with the exception of well-known localities such as Florissant, the Green River Shales, some mid-Tertiary deposits in Germany, Montana and British Columbia, and various amber deposits in Europe and North America. The vast majority of fossil remains (except for the amber deposits) consist of compressed and sometimes carbonized impressions of fragments or, more rarely, whole insects. Morphological details are poorly preserved and colour patterns usually obscured or non-existent in these fossil arthropods. In addition, the phylogeny of many mid-Tertiary and older insect groups is poorly understood.
The formidable problems associated with the study of older insect remains are somewhat reduced in the examination of Late Tertiary and Quaternary insects. The latter consist of the original chitinous parts, thus colour patterns, hairs and scales can be well preserved, even on specimens which date back two or more million years.
Almost any sediment containing organic debris is likely to preserve insect remains. The nature of the preservation depends upon the initial environment of deposition, the rapidity with which the organism was buried, the depth of burial, post-depositional changes in the sediment and the position of past and present water tables in respect to the stratigraphic position of the site. Probably the two most important factors are oxidation of the organic horizon containing the fossils and the degree of compression undergone by the sequence. Insect fragments are remarkably resilient to chemical attack, but they are relatively brittle, and stresses caused by overburden pressures or cyclic wetting and drying tend to fragment specimens to the point where they become almost unidentifiable.
Materials which are known to preserve arthropod remains include silts and organic horizons within littoral facies of lacustrine sequences, organic lenses in fluvial deposits, and, to a lesser degree, southern California (Churcher, 1966; Angus, 1973; Miller, 1982). Drier environments do not preclude the preservation of arthropods since the chitinous exoskeletons remain unaltered, and insect fossils have been described from cave deposits (Bain and Morgan, in preparation), and wood rat middens in the arid southwestern United States (Ashworth, 1973; Spilman 1976; Morgan et al., 1983). Equally well-preserved remains have been recovered from organic horizons in permafrost of the Arctic (Matthews, 1968, 1974; Matthews et al., 1986). Human influences have also provided numerous environments favourable for preservation in such features as hand dug wells and ditches, refuse pits, grave sites and even granaries (Kenward, 1976). Anthropogenic modification of the natural environment can also be detected by fossil insect faunas well beyond the modern (post-industrial revolution) time frame normally thought of in respect to such activity (Osborne,1988).
Although sites with potential for paleo-entomological research are quite common, it is surprising that relatively few workers have ventured into this field. The reasons for the apparent lack of enthusiasm are varied. Initially, there appeared to be a widespread (but fallacious) belief that beetles, or other insect remains, could not be identified from disarticulated fragments. The nature of paleoentomological research is almost a deterrent in itself, being a blend of paleontology, stratigraphy, entomological taxonomy and zoogeography, so it is rare that any prospective researcher starts in this field fully qualified in all aspects. Other factors detrimental to beetle studies involve the tedium of sorting through large quantities of sediment to extract the fragments, lack of well-organized collections and taxonomic keys, uncertainty about the ecology and zoogeography of many species and the time taken to process a single site (about one site/year).
In spite of the problems, individuals or small research groups have become established in Britain, Canada, Germany, The Netherlands, Sweden, the United States and the Soviet Union; and paleoentomology, in terms of Quaternary studies, is becoming a more generally recognized subject.