The role of the substrate in coralligenous development: examples from the present and the past.

Coralligenous is the most monumental bioconstruction made by calcareous red algae between 10 and 140 m of water depth in the Mediterranean Sea. Marine bionomists consider the nature of C substrate to be a key factor in distinguishing the C d'horizon inferieur de la roche littorale (substrate = rocky cli), always on an antecedent hard substrate, from the C de plateau. Originally, the latter was indicated as a biogenic framework developed from the coalescence of rhodoliths. Then it was more generally conceived as forming a solid substrate settled on an originally mobile substrate. Therefore, the nature of the substrate is fundamental for such genetic denition of the C de plateau. However, since controlling the nature of the substrate of C is very challenging, some authors suggested that in some cases the C de plateau frameworks could have grown on sub-horizontal rocky outcrops. Moreover, the substrate geometry controls the present-day categorized geomorphological expression assumed by such bioconstruction: 1) banks {at frameworks with thickness ranging from 0.5 to 4 m mainly built over more or less horizontal substrata, and 2) rims { structures on submarine vertical clis or surrounding the opening of submarine caves, generally located in shallower waters than banks. Within this approach, the key factor to distinguish the two morphotypes is basically the geometry of the substrate, sub-horizontal versus sub-vertical. Nevertheless, under the term "coralligenous bank" several C morphotypes were merged: tabular banks, discrete reliefs and hybrid banks. We collected and compared several examples of coralligenous outcrops from present day Mediterranean Sea (Apulian Platform, Egadi islands, Pontine islands) and from the stratigraphic record (Calabria marine terraces, Pleistocene) in order to investigate the nature of the substrate of C and the resulting C morphotypes. On present-day examples, we conducted a systematic analysis on the acoustic data (digital terrain model, seismic proles) coupled with ROV video analysis to describe substrate and correlated C morphotypes. We investigated the same aspects in several fossil examples, where the present-day outcrop exposition allowed the direct visualization of the substrate and how C developed on it. We collected variable combinations C morphotypes/substrate. In some cases, we recorded the C de plateau in its original, genetic denition. In most cases, we identied a suite of other variable substrates, beyond rhodoliths, on which C grew with distinct morphotypes. We conclude that C morphotypes on sub-horizontal substrate do not show a univocal correlation with the type and composition of their substrate. We address the need of a discussion about the meaning of "suitable substrate" for coralligenous development and about the correct use of C de plateau denition.