EVIDENCE OF ORIBATID MITE DETRITIVORY IN ANTARCTICA DURING THE LATE PALEOZOIC AND MESOZOIC

Journal of Paleontology, Nov 2004 by Kellogg, Derek W, Taylor, Edith L

Based on paleogeographical information and floral diversity, the Triassic locality at approximately 70�0 -72�S is hypothesized to have been located at a paleolatitude approximately 10 degrees north of the Permian locality (Powell and Li, 1994; Grunow, 1999; Taylor et al., 2000). Based on the presence of tree rings and the depauperate flora found throughout Gondwana, the Late Permian climate is thought to have been cool-temperate and humid (Collinson, 1997). The more diverse flora and extensive fauna in the Middle Triassic suggest a warm temperate climate (Collinson, 1997). Thus, the hypothesized paleoclimate of the two fossil localities fits well within the range in which both insect and mite wood borers are found today.

Another possibility is that the lack of insect wood borers in the Antarctic peats is due to the environment of deposition and/or preservation type. Mite wood borings in the Paleozoic are primarily known from coal ball permineralizations and silicified peat which was deposited in swampy environments (e.g., Cichan and Taylor, 1982; Labandeira et al., 1997). However, Mesozoic examples of insect wood boring occur in petrifactions, many of which were deposited in drier, more riparian habitats (Walker, 1938; Linck, 1949; Tidwell and Ash, 1990). The logical extension of this hypothesis would be that insects are more prolific wood borers in dry environments and oribatid mites are dominant in swampy environments. The record of the Permian, Triassic, and Jurassic coprolites from Antarctica provides some support for this hypothesis. All three localities yield permineralized peat, which initially formed in a swampy environment. The environment of deposition of the Antarctic peats differs from that of northern hemisphere coal balls, however, since there is no coal directly associated with the Antarctic peats and they were deposited in fluvial sequences (Taylor et al, 1989). Unfortunately, there is not enough information available about the modern xylophagous mite fauna to determine whether these ecological relationships hold true today.

Considering their important position as decomposers in today's ecosystems, the discrepancy between the frequency of mite borings in the Late Permian and Middle Triassic has implications for the decomposition rates at the two localities. Since oribatid mites are often the most abundant part of the xylophagous fauna, a decrease in their abundance would have drastic implications for the decomposition rates of wood in an ecosystem. Some oribatid mites are known to play at least five pivotal roles in the decomposition process. They manually break apart plant tissues and hollow out cavities which increases the surface area for colonization by fungi and other microbes (Kevan, 1962; MacFayden, 1964). Their fecal pellets stimulate the growth of microbial spores. Microbes appear to be limited in their ability to disperse on their own; thus when the available energy in a certain area has been expended, the microbial biomass shuts down until new resources become available. The intestines of the mites provide sufficient nutrients to activate ingested microbial spores (Shereef, 1971; Luxton, 1972; Ponge, 1984; Lavelle et al., 1994; Lavelle, 1997). In some cases, fecal pellets are preferentially colonized by microbes over undigested plant material. The microbial activity of fecal pellets, combined with the wood-boring activity of the oribatid mites, serves to introduce microbes into the inner tissues of plant organs faster than fungi or bacteria would be able to do on their own. The presence of microbes increases the nutritive content of the fecal pellets which are often then reingested by other arthropods, breaking the plant material down even further and functioning somewhat like an "external rumen" (Mason and Odum, 1969; Crossley, 1970). Finally, oribatid mites and other microarthropods help to move organic material deeper into the soil (Saichuae et al., 1972). Thus, a low level of mite boring activity as seen in the Permian locality could translate to a decrease in overall decomposition.