LIFE IN A LIVING HARD SUBSTRATE: THE RISE AND FALL OF SKELETAL ENDOSYMBIONTS DURING THE PALEOZOIC

Bioclaustrations are cavities produced by the interaction of one organism (an endosymbiont) that lived within the growing skeleton of a live host organism. These trace fossils provide rare, direct evidence of past symbioses, and they are first recognized in the skeletal remains of marine animals from the Late Ordovician (Caradoc). Bioclaustrations have a wide geographic distribution and occur in a variety of marine invertebrate hosts, including tabulate and rugose corals, stromatoporoid and chaetetid sponges, bryozoans, brachiopods, and crinoids. Ten bioclaustration ichnogenera currently are recognized, and they occur repeatedly in particular host taxa, suggesting a strong host preference among many Paleozoic endosymbionts.

A comprehensive compilation of Paleozoic bioclaustrations reveals large-scale diversity trends in marine invertebrate endosymbiosis for the Early to Middle Paleozoic. There is a marked rise in bioclaustration diversity and abundance during the Caradoc, with a continued increase in ichnotaxa throughout the Silurian and Middle Devonian (Givetian), followed by a dramatic post-Givetian decline in the diversity of bioclaustrations. The collapse in bioclaustration diversity during the Late Devonian accompanies the extinction of many preferred host corals and calcareous sponges, such as favositid tabulates, which were in decline prior to the Frasnian–Famennian mass extinction.

The rapid evolution of endosymbiotic relationships during the Late Ordovician is concurrent with the diversification of bioeroders in hard substrates (i.e., the Ordovician Bioerosion Revolution). Based on comparisons with modern marine analogues, the endosymbiotic associations that developed during the Late Ordovician likely added to the growing ecological complexity in marine ecosystems by providing new strategies of infaunalization for the purposes of niche partitioning and protection from predation.