Early development and coloniality in Oligophylloides from the Devonian of Morocco—Are Heterocorallia Palaeozoic octocorals?

Abstract

Heterocorals represent an enigmatic group of Palaeozoic corals, known from relatively short time intervals in the Devonian and Carboniferous periods. The major differences between Heterocorallia and other Palaeozoic corals are the lack of an external theca (epitheca), lack of calices and the presence of dichotomously dividing septa-like structures. Heterocoral skeleton was presumably externally covered by the soft tissue and each branch of their skeleton has, until now, been regarded as a corallite–a skeleton of a single polyp. We investigated upper Famennian Oligophylloides from Morocco, focussing on branching processes, wall structure, previously poorly known initial growth stages and the growing tip, described here for the first time. We demonstrate that Oligophylloides shows a unique colony development not known in any group of anthozoans possessing a septate-like architecture and suggest that the previously postulated homology between true septa in hexa- and rugose corals on one hand, and Oligophylloides on the other, must be rejected. Based on the skeleton structure and branching patterns, we postulate, contrary to former ideas, that the stem and branches of heterocorals represent the skeleton of a multi-polyp colonial coral, similar to many extant octocorals. We found numerous potential homologies with octocoral skeletons (notably the Keratoisidinae within the Isididae) and, as a result, we propose the inclusion of the order Heterocorallia within the subclass Octocorallia. This suggestion requires, however, further research on the other taxa of heterocorals. We also propose some changes to the morphological terminology for the Heterocorallia.

3D morphology of post mortem acrothoracican borings in Famennian heterocorals – a morphological diversification or a continuum ichnospecies?

“…These corals belong to the problematic order of Palaeozoic corals, known as Heterocorallia Schindewolf, 1941. The origin and affinities of this group have for a long time been debatable [see discussions in Fedorowski (1991), Oliver (1966), Scrutton (1997)], but recent studies (Berkowski et al, 2021) included heterocorals in the subclass Octocorallia. Heterocorals did not have calices nor an external theca (epitheca; Wrzołek, 1981;Fedorowski, 1991;Weyer, 1997Weyer, , 2016Chwieduk, 2001) and these characters distinguish them from Palaeozoic rugose and tabulate corals.…”

“…Heterocorals did not have calices nor an external theca (epitheca; Wrzołek, 1981;Fedorowski, 1991;Weyer, 1997Weyer, , 2016Chwieduk, 2001) and these characters distinguish them from Palaeozoic rugose and tabulate corals. The entire skeleton was presumably covered by soft tissue with numerous polyps (Berkowski et al, 2021). Most heterocorals, and especially Oligophylloides, inhabited environments interpreted as oxygen-depleted and relatively deep-water (mesophotic and aphotic; e.g., Weyer, 2016;Dworczak et al, 2020).…”

“…The wall is built of lamellar, low-Mg calcite crystals, which create a dense and compact microstructure (Chwieduk, 2001;Dworczak et al, 2022). Colonies of Oligophylloides formed fan-shaped skeletons with relatively long and thin branches and were attached to the substrate by a massive holdfast (Weyer, 2016;Berkowski et al, 2021). Hence, very often, only fragments (mostly branches) of colony are found in the fossil record.…”

“…The detailed investigations of apparently well-preserved specimens of the heterocoral O. maroccanus demonstrating acrothoracican borings provide new evidence of post mortem encrustation by acrothoracican barnacles. It is commonly accepted that the skeleton of the heterocoral corallum was entirely enveloped by a thick layer of soft tissue, presumably with armoured polyps, which performed the function of armour against the larvae of boring and epibiontic organisms (Weyer, 2016;Dworczak et al, 2020;Berkowski et al, 2021). However, heterocorals could roll up their soft tissue, which resulted in exposure of the proximal part of the skeleton (Weyer, 2016;Dworczak et al, 2020;Berkowski et al, 2021).…”

“…It is commonly accepted that the skeleton of the heterocoral corallum was entirely enveloped by a thick layer of soft tissue, presumably with armoured polyps, which performed the function of armour against the larvae of boring and epibiontic organisms (Weyer, 2016;Dworczak et al, 2020;Berkowski et al, 2021). However, heterocorals could roll up their soft tissue, which resulted in exposure of the proximal part of the skeleton (Weyer, 2016;Dworczak et al, 2020;Berkowski et al, 2021). The exposed skeleton could be easily colonised by intruders.…”