Alcyonacea awakens: Palaeobiology and palaeoecology of Palaeozoic octocorals known from their sclerites

Abstract

Most Alcyonacea (subclass Octocorallia) have skeletons made up of small calcareous pieces called sclerites that are difficult to both preserve and recognize, which explains the scarce and irregular fossil record of the group. In this work, we study 56 geological samples bearing Syringoalcyon, a genus constituted by Alcyonacea represented by sclerites encrusting the outer walls and stolons of a syringoporoid coral. These samples come from 18 Silurian to mostly Carboniferous localities in Canada, USA, Mexico, Morocco, Algeria, Spain, Iran, and China. A morphological and numerical study was carried out to determine the main morphological and palaeobiological features of the sclerites and the Alcyonacea itself and to establish a new genus for these sclerites. The study facilitated distinguishing four sclerite morphotypes (spindle, elongate spindle, club, and arrowhead). Live sclerites can be attached through biotic or abiotic linkages to biotic substrate patches by insertion or cementation. Spindle sclerites arranged in vertical fences are the most common model, but other arrangements such as stacking or short horizontal fences have been observed even in a single specimen. The morphotypes, dimensions of the sclerites, substrates, and types of arrangement observed are well known in Recent Alcyonacea. According to the arrangements of the sclerites, the alcyonacean morphology was different depending on the shape and dimensions of the substrate. This Syringoporidae–Alcyonacea association is a good example of ecological engineering, but several data suggest that the relationship between these two corals was closer than hard substrate colonization.