A lowermost Ordovician tabulate-like coralomorph from the Precordillera of western Argentina: a main component of a reef-framework consortium

Abstract

Abstract.-Although putative corals of uncertain affinities occur in the early Cambrian, the earliest definite tabulate corals have not been described prior to the Early Ordovician in North America. This paper reports a new finding of a tabulate-like coralomorph forming part of biostratigraphically well-constrained reef mounds in the latest Cambrian-Early Ordovician La Silla Formation in the Argentine Precordillera. The oldest record of the coralomorph genus Amsassia is reported and a new species, A. argentina, is erected. The discovery of this genus in the lowermost Ordovician modifies the previously proposed paleogeographic distribution and patterns of origination and migration routes of this coral-like organism. Amsassia argentina n. sp. constitutes a main framework builder together with a complex microbial consortium. This oldest occurrence of Amsassia as a reef builder represents a new record of a skeletal organism in the gap of metazoan reef constructors after the demise of archaeocyaths in the late early Cambrian.

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

“…Temporal and paleogeographic ranges.-The earliest known representatives of Amsassia are A. terranovensis n. sp. from western Newfoundland and A. argentina from the Argentine Precordillera (Carrera et al, 2017;Elias et al, 2021). Both appeared in the early Tremadocian, just after the beginning of the Ordovician Period.…”

“…Cerro La Silla, Argentine Precordillera.-Amsassia argentina occurs ∼95 m above the base of the La Silla Formation, within the Cordylodus angulatus Conodont Zone, in a 2-m-thick unit, apparently representing a low-relief carbonate mound (Carrera et al, 2017). The unit consists of matrix-rich boundstone with peloidal mudstone and grainstone.…”

“…In a recent analysis, however, it was determined that Tremadocian material from Newfoundland and elsewhere in North America, which had long been attributed to Lichenaria, actually belongs to the Ordovician genus Amsassia Sokolov andMironova, 1959 (Elias et al, 2021). The only other Tremadocian record of Amsassia is A. argentina Carrera, Astini, and Gomez, 2017, from the La Silla Formation in the Argentine Precordillera (Carrera et al, 2017), which is approximately coeval with the lowermost occurrence of the genus in the St. George Group (Elias et al, 2021). Amsassia is more widely distributed in the Middle and especially Upper Ordovician, occurring in North America and numerous places in Asia (e.g., M. Lee et al, 2014Sun et al, 2014).…”

New insights on Lower Ordovician (Floian) reefs from the Argentine Precordillera: Biostratigraphic, sedimentologic and paleogeographic implications

“…After that, the Upper Cambrian thrombolithic microbialites were recorded in the La Silla Formation (Keller, 1999;Cañas, 2002). The most recent study in the La Silla Formation describes a reef mound, where the Amsassia argentina Carrera et al (coral-like organism) constitutes the main framework builder organism together with a complex microbial consortium, and these organisms belongs to the latest Cambrian-Early Ordovician (Carrera et al, 2017).…”

Ordovician sponges from the Lenoir Limestone, Tennessee: new evidence for a differential sponge distribution along the margins of Laurentia

“…Lower Ordovician bioherms usually built by microbes, sponges, and calathids occur in a broad belt along the southern and western margins of the North American continent, constituting the Laurentian sponge-algal facies (Alberstadt and Repetsky, 1989). Sponges associated with reef structures are recorded world-wide and are restricted to tropical or subtropical areas; with main occurrences in China and Korea (Rigby et al, 1995; Wang et al, 2012; Choh et al, 2013; Adachi et al, 2015; Li et al, 2015 and references therein), Siberia (Webby, 1984), Baltica (Kröger et al, 2017), and the Precordillera basin of Western Argentina (Cañas and Carrera, 1993; Carrera et al, 2017).…”