Reptamsassia n. gen. (Amsassiaceae n. fam.; calcareous algae) from the Lower Ordovician (Floian) of western Newfoundland, and the earliest symbiotic intergrowth of modular species

Modular coral-like fossils occur in thrombolitic reefal beds at two stratigraphic levels within the Lower Ordovician (Floian) Barbace Cove Member of the Boat Harbour Formation, in the St. George Group of western Newfoundland. They are here assigned to Reptamsassia n. gen.; R. divergens n. gen. n. sp. is present at both levels, whereas a comparatively small-module species, R. minuta n. gen. n. sp., is confined to the upper level. Reptamsassia n. gen. resembles the Ordovician genus Amsassia in its phacelocerioid structure, back-to-back walls of adjoining modules, module increase by longitudinal fission involving infoldings of the wall, tabula-like structures that are continuous with the vertical module wall, and calices with concave-up bottoms. The new genus is differentiated by its encrusting habit, modules with highly variable growth directions and shapes throughout skeletal growth, and modules that may separate slightly or diverge from one another following fission. Together, Amsassia and Reptamsassia n. gen. are considered to represent a distinct group of calcareous algae, the Amsassiaceae n. fam., which possibly belongs to the green algae. The Early Ordovician origination of Amsassia followed by Reptamsassia n. gen. contributed to the beginning of the rise in diversity on a global scale and in reefal settings during the Great Ordovician Biodiversification Event. Reptamsassia minuta n. gen. n. sp. was an obligate symbiont that colonized living areas on its host, R. divergens n. gen. n. sp., with isolated modules of R. divergens n. gen. n. sp. able to persist in the resulting intergrowth with R. minuta n. gen. n. sp. This is the earliest known symbiotic intergrowth of macroscopic modular species, exemplifying the development of ecologic specialization and ecosystem complexity in Early Ordovician reefs.