Pleistocene and Holocene Traps

The accumulation of insects in sediments that formed during the Quaternary (1.7 mya to present) provides unique insight on climate change and the duration of species, a subject treated thoroughly by Elias (1994). Most Quaternary remains occur in peats from mature successional stages of bogs, which are the edges. Here, the thickly sclerotized, durable elytra, pronota, and heads of beetles predominate. Fortunately, the gross structure and microsculpturing of beetle sclerites allow detailed matches with modern species. Scudder (1877, and various papers thereafter) was among the first to carefully study Quaternary insects, of which he was mainly preoccupied with deposits from Scarborough, Ontario. He described 50 beetle species from Scarborough, all but two presumed to be extinct. It was not until the work of Carl Lindroth (1948), a coleopterist, that Quaternary insects were revealed to be generally extant, not extinct. Russell Coope, in Britain, systematically challenged the dogma that Pleistocene insects were largely extinct species like mammals. He essentially developed the study of Quaternary insects and was the first to document contractions in the distributions of modern insect species, some of which are dramatic. For example, fossils of the scarab Aphodius holderei and the staphylinid Tachinus caelatus occur in Britain, but these today are found in the Himalayas and Mongolia, respectively. Their present day distribution is a relict vestige of a time when cooler climates embraced most of Europe. Because dozens, even hundreds, of insect species can occur in a Quaternary site, these deposits provide abundant evidence of past climates along with fossil pollen and leaves.

Besides beetles, heavily sclerotized remains of other insects are also preserved, such as ant heads, oribatid mites, and the larval cases of caddisflies. The sclerotized head capsules of otherwise soft-bodied midge larvae are extremely abundant in lake sediments. Intricate structures on the head capsules allow species identification of these remains, some hundreds of thousands of years old. The tarpits of La Brea, California, are famous for the impressive mammals that were trapped and preserved there, but insects were also victims (Miller, 1983) (Figure 2.35). Mammoths and remains of other Pleistocene mammals frozen in tundra permafrost occassionally yield parasites (Dubinin, 1948; Grunin, 1973) (Figure 2.36), as do human mummies (Figure 2.37). Perhaps the most intriguing Quaternary fossils come from pack rat middens of the American southwest, sheltered amidst rock overhangs and caves. As the name indicates, pack rats stock their nest with gathered materials, like pebbles, cactus spines, and vertebrate bones. These materials form a conglomerate mass with feces and urine - the midden - from

2.35. Remains of predatory dytiscid beetles from the Pleistocene tar pits at La Brea, near Los Angeles, California. The beetles may have been living in water that pooled on top of the tar pits. AMNH; length of middle beetle 23 mm.

naturally extinct, but for insects it isn't always certain if a species is extinct. Several species of scarab beetles from the La Brea tarpits, for example, are unknown among the living fauna (Miller, 1983). It is plausible they became extinct with the sloths, mammoths, and other mammalian fauna on whose dung they depended. Or, the species may persist, say in remote areas of Mexico. A similar situation pertains to a species of ptinid beetle found in 12,000- and 30,000-year-old pack rat middens, but unknown from anywhere else (Spilman, 1976). A more practical and perhaps equally valid definition is that a fossil is the remains or workings of any species, living or extinct, that have been naturally preserved for several thousand years or more.

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