A HARD ROCK LIFE: THE GUILD CONCEPT IN UNDERSTANDING ORGANISMS THAT EXPLOIT THE ENDOLITHIC HABITAT

A wide variety of marine organisms reside in live and dead hard substrates, ranging from carbonate skeletons to rock to wood. The exploitation of the endolithic habitat is an excellent example of ecological partitioning, and it is recorded primarily by trace fossil cavities, such as destructive borings and constructive bioclaustrations. Modern endolithic organisms have evolved many specialized anatomies, physiologies and behaviors to accommodate their lifestyles. The cavities they produce record conservative morphologies that often are archetypical of the activity of particular biological clades.

The guild concept offers an ecological framework in which endolithic trace fossils may be used to understand the timing and factors that control habitat partitioning. In addition to sharing a hard substrate habitat, many (but not all) modern endolithic organisms share a simple bauplan with reduced hard parts, although some mechanical borers are exceptions. The most important ecological difference among endolithic organisms is in their nutritive strategies. With few exceptions, endoliths are either suspension feeding heterotrophs or photosynthetic autotrophs. This nutritive divide corresponds largely with the size-based categories used in the field, including microborings (autotrophic endolithic guild, excluding fungal borings) and macroborings + bioclaustrations (heterotrophic endolithic guild).

Current understanding of the Phanerozoic record of hard substrate trace fossils shows distinct patterns among the different endolithic guilds. The phylogenetic-independence of the guild concept, as applied to endolithic trace fossils, argues for extrinsic controls, such as predation pressure or competition for space and nutrients, in driving organisms to exploit the endolithic habitat.