The term "fossil" is used by different authors in different senses. Malacologists call a shell a "fossil" if it was found below the surface of the earth, no matter how slightly covered by soil or rock it may have been when found. This can lead to confusion when considering whether or not a particular "fossil" shell is extant or extinct. In paleontology the use of the term "fossil" is usually restricted to remains occurring before the Holocene (i.e., before ~12,000 years b.p.), with Holocene remains termed "subfossil".
So then, just what makes a fossil a fossil? Pierce (1950) discussed this very question. There is no clear answer. Since definitions by researchers in different disciplines vary, the term "fossil" as used in this catalogue will be chiefly limited to those taxa that have been preserved in various media after death over a period of more than 50,000 years. The period of time necessary to make something fossil can be much less than 50,000 years and many fossil Diptera exist that are this young. For the purposes of the entries in this catalogue, this age limit requires the exclusion of a large number of lacustrine taxa uncovered during studies on Quaternary paleoenvironments and those from historic or prehistoric middens. For the most part, these taxa are the same as extant taxa at least at the generic level and in many cases the specific level as well (Elias, 1994). However, where new taxa have been described from these strata and have not been listed in any previous regional catalogue, they are treated here.
Entries in this catalogue, with the exception of just a few, have not been recorded in other regional catalogues dealing with extant species of Diptera. Some extant taxa have been recorded as fossils and most of these have been listed if no subsequent study has been found to correct an identification. The reader must bear in mind that the older paleontological works, which date mainly from the eighteenth and early nineteenth centuries, often included fossils with the same names as contemporary taxa due chiefly to the fact that the concept of evolution was either not known or not a common concept in those days. Many of the taxa that have been determined with names of extant species and date from that range of years should be reexamined.
No attempt was made in this catalogue to make any taxonomic changes to genera or species except for families within the author's specialty (Bombyliidae and Mythicomyiidae). Where necessary for nomenclatural reasons, new combinations and synonymies are listed.