Plants and animals go through certain changes after death. If not immediately
preserved in a medium, they will usually decay or be degraded through the
feeding action of bacteria or scavenging animals or through chemical action.
If the organism becomes imbedded in a preserving medium, fossilization occurs.
This process of fossilization is termed taphonomy (Efremov, 1940).
This catalogue lists the following forms of preservation or media in which fossil Diptera have been found: body fossils including amber, compression and impression, copal, permineralization, sediment recovery, and tar pit recovery; and trace fossils.
Amber and Copal:
In amber and copal specimens, an organism becomes entrapped over a relatively short period of time in the various plant resins [exuded from trees such as Agathis, Araucaria (Araucariaceae), Bursera, Protium (Burseraceae), Hymenaea (Fabaceae), Liquidambar (Hamamelidaceae), Pinus (Pinaceae), Shorea (Dipterocarpaceae), and various Taxodiaceae-see Langenheim (1969) for a detailed breakdown of plant sources] that form copal and amber [Schlüter (1990) and Henwood (1992a) give detailed accounts of the imbedding process for specimens in amber and Pike (1993) and Henwood (1993) discuss taphonomy and collecting bias]. Though what is found in these amber and copal media is the actual specimen that died from thousands to millions of years ago, in many specimens, some amount bacterial action may have taken place over time and membranous material supporting the exoskeletal sclerites is often missing. Thus, when one attempts to recover the entrapped specimen by dissolving the amber or copal in a solvent, the result is often a disappointing slurry of floating chitinous plates. However, in some specimens preserved in this fashion, little or no decay has taken place and some muscle tissue complete with mitochondria survives and DNA can still be recovered (Henwood, 1992b). This had led to the exciting field of paleomolecular entomology in which the DNA of fossil organisms over 100 million years old can be studied and comparisons made with putative extant relatives.
Compression and Impression Fossils:
Two-dimensional compression fossils normally encounter some sort of mineral substitution during the fossilization process. Most often, solution and other chemical action under water transform the composition of insect tissue and exoskeleton into a thin film of carbon. When this type of alteration of the composition of the hard parts of insect tissue occurs (called carbonization), normally only major features such as the head, thorax, abdomen, and larger appendages are visible, but in two dimensions. In addition to the difficulty in identifying a specimen in two dimensions, the compression process as well as the process of mineral replacement can lead to a great deal of distortion of various body features. Heads, thoraces, and abdomens can often look like parts of insects that have been squashed on cards or abdomens will be bloated due to a long period of submersion in liquid before any amount of sedimentation took place to preserve the specimen. Wings and their venation generally are the parts of the insect least subjected to this distortion and, if in good condition, are the major identifiable features of Diptera compression fossils.
When the original organic or replaced mineral material exfoliates from the fossil leaving only an imprint on the sedimentary matrix, this is called an impression.
In some cases, chemical replacement occurs of an entire specimen in which all parts of the insect are visible in three dimensions after acid extraction of the specimen from the mineralized nodule in which it had become imbedded. One type of permineralization, silicification, is found in fossils from Böttinger Marmor in Switzerland and the Calico Mountains and other desert and semi-desert montane environments in southern California where organisms in heavily mineralized Miocene sinter environments became embedded in silicified nodules over time, the mineral of which replaced the chitinous portions of the insect. This form of fossilization results in a remarkably well preserved specimen in which even the smallest setae (now silicified) can still be observed on various portions of the insect exoskeleton. Palmer & Bassett (1954) and Pierce (1960) discuss the process and resulting faunal discoveries in more detail.
Other examples of permineralization include hematitization (Parachute Creek, Colorado-Eocene), and pyritization (Bognor Regis, England-Lower Eocene).
Trace fossils, ichnofossils, or "Lebenspurren", are structures in sediment or other biological materials (e.g., wood, leaves, roots, etc.) left by living organisms. Trace fossils give evidence that a particular type of organism was occupying a specific medium at one time. Most trace fossils are examples of the work of an organism. They are of two types: 1) sediment-entombed burrows and galleries, dwelling structures such as larval cases, motion trails and tracks, and 2) galls, leaf mines, grazing trails, external foliage feeding, borings, etc. These structures and tracks can easily be destroyed through wave or wind action, but under the right circumstances can be preserved and undergo fossilization. Quick drying or cementation of the sedimentary medium before being overlain by sediments allows for preservation of many trace fossils [see Häntzschel (1975) for more details and a world catalogue of trace fossils]. In the case of Diptera, trace fossils listed in this catalogue include leaf mines, galls, and trails.
Other Forms of Preservation or Recovery:
Some forms of preservation of fossil insects do not encounter deterioration or mineral substitution of any kind. Those listed in this catalogue include subfossil and tar pit or brea recovery. The ages of these types of fossils listed in this catalogue are relatively young (most from the Pleistocene or Pliocene). In these fossils, all or parts of the insect specimen are visible in three dimensions much the same as those imbedded in amber or copal. Subfossil recoveries of Diptera have been made in peat bogs, caves, and ancient middens of small mammals.