FOSSIL RECORD OF EMBEDDING ORGANISMS IN BIOGENIC SUBSTRATES: EXAMPLES FROM QUÉBEC AND WYOMING

Embedments are cavities formed by the deflected growth of a hard skeleton-producing host organism around an invading organism (endosymbiont). Embedments are distinguished from borings in that they are not formed by excavation of the substrate. Cavities formed by the embedding process provide a resistant shelter for the endosymbiont with little metabolic cost on its part, and in the fossil record they preserve close-proximity relationships that existed between the living host and endosymbiont. While the nature of the host-endosymbiont relationship may be unclear (e.g., mutualism, commensalism or parasitism), embedments may provide a record of often poorly preserved taxa (e.g., soft-bodied worms) that have adapted to a life confined to a particular host.

Common in modern oceans, the embedding life habit was firmly established by Ordovician time, especially within producers of laminar calcareous skeletons. Tabulate corals and stromatoporoids from the Ordovician-Silurian carbonate ramp succession preserved on Anticosti Island (Québec) contain at least 3 distinct types of embedment, each demonstrating differing modes of embedment initiation, morphology, and host-specificity.

The earliest embedding organisms may have exploited stromatolites. Embedments observed in modern and Tertiary lacustrine stromatolites (e.g., in the Eocene Green River Formation of Wyoming) resemble digitate or thrombolitic growth forms of ancient stromatolites in longitudinal view. Embedments in stromatolites may have been a very early mechanism for metazoans to acquire hard protective shelters.