The endolithic guild: an ecological framework for residential cavities in hard substrates

Endolithic organisms that live inside rocks, shells and wood use two processes, bioerosion and embedment, to produce boring and bioclaustration residential cavities. In the trace fossil record, borings and bioclaustrations preserve the distinctive and often stereotypic activity of particular endolithic organisms, thereby linking important biological and ecological information to this special group of trace fossils. In order to use these fossils to study aspects of evolutionary paleoecology, an ecologically-significant framework is needed to logically group and subdivide hard substrate trace fossils. A modified version of the guild concept can be applied to borings and bioclaustrations, which groups these cavities and their producers on the basis of biological affinity (or behavior), trophic level and habitat. In general, the endolithic guild is unified as a group of mostly soft-bodied organisms that produce shallow infaunal, stationary domichnia in hard substrates. A major subdivision of the endolithic guild roughly corresponds to the commonly used size-based divide between microborers and macroborers, but it is founded on the ecologicallysignificant criterion of trophic group. The autotrophic endolithic guild includes the photosynthetic microboring cyanobacteria, chlorophytes and rhodophytes, and the heterotrophic endolithic guild is dominated by macroscopic invertebrate borers and embedders, and includes bacteria, fungi and other microscopic fauna. Primary observations on the endolithic guild suggest that (1) morphological diversity of cavities generally increased through geologic time for all endoliths, with a major Ordovician rise noted for invertebrate endoliths; (2) the class-level composition of invertebrates in the suspension feeding endolithic guild remain nearly unchanged since their rise in the Early–Middle Paleozoic until today, despite a major shift in dominant groups at the familial level that occurred in the Early Mesozoic; (3) diversity patterns within the heterotrophic endolithic guild show similar timing of onset between macroborers and embedders in the Ordovician, but differences in substrate availability may provide an ecological explanation for the decline of host-specific bioclaustrations in the Devonian relative to macroboring diversity.