Abrasion Corrosion and Encrustation

It is often difficult to distinguish between trilobite skeletons that have been physically abraded and those that have been corroded by biogeochemical processes. As a generalization most trilobite remains are too fragile to withstand prolonged abrasion, and this fragility may account for the rarity of trilobite skeletons in some nearshore, sandy environments in which trace fossils indicate that trilobites were common. It is well known that clay-sized sediment is ineffective as an abrasive agent. Hence, truly abraded fossils are rare in mudrocks and, if found, might indicate a much more complex history to the deposit in which shells were transported into a quieter water environment by a turbulent event.

Corroded trilobite remains tend to occur in offshore, low-energy environments, and bioerosion, likewise, tends to predominate over physical abrasion in these offshore settings (Kidwell and Bosence 1991; Parsons and Brett 1991). Many apparently abraded shells in mudrocks probably have been chemically etched or acted on by microboring organisms.

Even in life, trilobite exoskeletons may become encrusted with epibiontic organisms, such as bryozoans and even brachiopods (Tetreault 1992; Kloc 1993; Taylor and Brett 1996). Postmortem trilobite remains may be encrusted both externally and internally. Internal encrustation provides an excellent indication that skeletons have lain disarticulated for a period of time on the sea bottom. The extent of encrustation may provide an indicator of exposure time, as well. Conversely, some shells in condensed deposits show few, if any, epibionts. Microborings of forms such as endolithic algae may be recognized and may provide particularly useful indicators of exposure and, in some cases, of relative depth (Golubic et al. 1975; Vogel et al. 1987). Endolithic algae of various sorts, for example, are confined to various portions of the euphotic to upper dysphotic zone. Algal microborings in Dicra-nurus specimens have been used to suggest that these trilobites lived in the euphotic zone. Kloc (1997) suggested that abundant encrusters on the cephalic spines of these trilobites may have settled during the life of the trilobite and served as camouflage against visual predators.

0 0

Post a comment