The earliest Helicosalpinx from Canada and the global expansion of commensalism in Late Ordovician sarcinulid corals (Tabulata)

Spiral embedment cavities that formed around metazoan endosymbionts are preserved in the septa and intercorallite spaces of Columnopora and Calapoecia corals from Manitoulin Island, Ontario. These are the oldest described Helicosalpinx asturiana, and this report extends the range of these trace fossils from the Richmondian (Ashgill, Upper Ordovician) to the Givetian (upper Middle Devonian). The sinistrally coiled traces show regular morphology, suggesting a physiological basis for their shape. Coral growth parameters are not affected by the presence of Helicosalpinx, suggesting that the endosymbiont was not parasitic and not in direct competition for resources with the host. H. asturiana is interpreted as trace fossil evidence of commensalism.Two additional endosymbiotic traces occur in Late Ordovician Columnopora. The particular association of straight, cylindrical Chaetosalpinx with Columnopora is widespread during the Richmondian. A dependent association is suggested to have originated between the endosymbiont and Columnopora prior to the Richmondian expansion and to have continued into the Hirnantian (latest Ordovician). The association between this endosymbiont and host is the earliest known temporally and globally significant inter-metazoan symbiosis. Although host corals, Columnopora and Calapoecia, did not survive the end-Ordovician mass extinction events, both Chaetosalpinx and Helicosalpinx occur in other host corals during the Silurian and Devonian.