Amsassia(calcareous alga) from the Lower Ordovician (Tremadocian) of western Newfoundland, and the biologic affinity and geologic history of the genus

Abstract

Modular coral-like fossils from Lower Ordovician (Tremadocian) thrombolitic mounds in the St. George Group of western Newfoundland were initially identified as Lichenaria and thought to include the earliest tabulate corals. They are here assigned to Amsassia terranovensis n. sp. and Amsassia? sp. A from the Watts Bight Formation, and A. diversa n. sp. and Amsassia? sp. B from the overlying Boat Harbour Formation. Amsassia terranovensis n. sp. and A. argentina from the Argentine Precordillera are the earliest representatives of the genus. Amsassia is considered to be a calcareous alga, possibly representing an extinct group of green algae. The genus originated and began to disperse in the Tremadocian, during the onset of the Great Ordovician Biodiversification Event, on the southern margin of Laurentia and the Cuyania Terrane. It inhabited small, shallow-marine reefal mounds constructed in association with microbes. The paleogeographic range of Amsassia expanded in the Middle Ordovician (Darriwilian) to include the Sino-Korean Block, as well as Laurentia, and its environmental range expanded to include non-reefal, open- and restricted-marine settings. Amsassia attained its greatest diversity and paleogeographic extent in the Late Ordovician (Sandbian–Katian), during the culmination of the Great Ordovician Biodiversification Event. Its range included the South China Block, Tarim Block, Kazakhstan, and Siberia, as well as the Sino-Korean Block and Laurentia, and its affinity for small microbial mounds continued during that time. In the latest Ordovician (Hirnantian), the diversity of Amsassia was reduced, its distribution was restricted to non-reefal environments in South China, and it finally disappeared during the end-Ordovician mass extinction. UUID: http://zoobank.org/ef0abb69-10a6-46de-8c78-d6ec7de185fe

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

“…Coral-like fossils assigned to Amsassia occur in microbial patch reefs in the Watts Bight Formation and the middle member of the Boat Harbour Formation, in rocks of Early Ordovician (Tremadocian) Skullrockian and Stairsian ages, respectively (Elias et al, 2021; Lee et al, 2021; Figs. 1.2, 2).…”

“…Modular coral-like fossils from Lower Ordovician (Tremadocian) strata within the St. George Group in western Newfoundland, Canada—initially identified as the tabulate coral Lichenaria Winchell and Schuchert, 1895 (Pratt and James, 1982, 1989a, b)—recently have been assigned to Amsassia Sokolov and Mironova, 1959 (Elias et al, 2021; Lee et al, 2021). Amsassia terranovensis Lee, Elias, and Pratt, 2021, occurs in the Watts Bight Formation at Green Head, and A . diversa Lee, Elias, and Pratt, 2021, appears in the middle member of the overlying Boat Harbour Formation at Eddies Cove West and Boat Head (Figs.…”

“…Amsassia terranovensis Lee, Elias, and Pratt, 2021, occurs in the Watts Bight Formation at Green Head, and A . diversa Lee, Elias, and Pratt, 2021, appears in the middle member of the overlying Boat Harbour Formation at Eddies Cove West and Boat Head (Figs. 1, 2).…”

“…1), which are present from Port au Port Peninsula to the northern tip of Great Northern Peninsula. Samples of Amsassia described by Lee et al (2021) were collected at Green Head, Eddies Cove West, and Boat Head; samples of Reptamsassia n. gen. for the present study were collected at Isthmus Bay (for detailed map see Knight and James, 1987, fig. 3d).

Figure 2.Generalized stratigraphic section of the St. George Group in western Newfoundland (based on Knight et al, 2008, fig.

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“…Regarding biologic affinity, the Ordovician genus Amsassia has been excluded from corals and calcareous sponges, as well as cyanobacteria, and referred to calcareous algae (Sun et al, 2014; Lee et al, 2016, 2018, 2021; Elias et al, 2021). Its characteristic morphologic features are different from those of tetradiids, and suggest that Amsassia may have been a green alga (Lee et al, 2021). The morphology of Reptamsassia n. gen. is consistent with that interpretation.…”