Early-Middle Ordovician Alcyonacean (octocoral) sclerites from the Argentine Precordillera

VOLDMAN, G.G.; MANGO, M.J.; CARRERA, M.G.; NESTELL, G.P.

Simposio; 2020 Virtual Annual Meeting of IGCP 653; 2020

Alcyonacean octocoral sclerites present a highly incomplete stratigraphic distribution, probably reflecting biases in the fossil record related to the difficulties of finding and extracting these tiny calcareous fossils. Its oldest established record is the form genus Atractosella Hinde, present in the Llandovery-Wenlock (Silurian) of Gotland (Sweden), England, Scotland, and from an erratic boulder in Germany. In the present contribution, we analyse the occurrence of silicified Early and Middle Ordovician alcyonacean sclerites in the San Juan Formation of the Argentine Precordillera. The sclerite assemblage consists of spindle morphotypes, ranging from slightly fusiform elongated, rod-like forms to oval or strongly ellipsoidal. They are ornamented with small regular granules or tubercles, and some sclerites are linearly connected by their tips. The recovered sclerites occur in different stratigraphic levels and localities of the Argentine Precordillera and therefore could represent associations of different species or genera. Notwithstanding that, they provide clear evidence on the Floian (Early Ordovician) occurrence of Alcyonacean sclerites in the fossil record. The variable morphologic characteristics of these fused and unfused Alcyonacean sclerites are present in the suborders Alcyoniina (soft corals) and the Scleraxonia (horny octocorals). Although any of these groups assignment is still speculative at this state of knowledge, it may imply that these basal forms could be elements of a possible Alcyoniina-Scleraxonia stem-group.