Microstructural disparity between basal micrabaciids and other Scleractinia: new evidence from Neogene Stephanophyllia

Abstract

Recent molecular analyses based on mitochondrial and nuclear markers place the Micrabaciidae in the basal clade of scleractinian corals. The molecular distinctiveness of micrabaciids is supported by a set of unique morphological characters, among which the microstructure of thickening deposits is the most characteristic one. In all extant and well-preserved Mesozoic micrabaciids (extinct Micrabacia, and still living Letepsammia, Rhombopsammia, Stephanophyllia, Leptopenus), thickening deposits consist of irregular meshwork of small chip-like bundles of fibres. Here, we document Neogene (Miocene and Pliocene) forms identified as Stephanophyllia whose thickening deposits consist of long and thin parallel fibres that, instead of bundles (like in majority of Scleractinia), form layers of thatch-like structures that thicken the septa. This microstructural pattern distinguishes Neogene Stephanophyllia from all examined so far micrabaciids and suggests that mechanisms of biologically controlled mineralization within this clade were more diverse. Nonetheless, the group as a whole is still clearly separated microstructurally from other scleractinians. Despite their basal position in scleractinian phylogeny, the fossil record of Micrabaciidae starts only in the Lower Cretaceous. No Palaeozoic, Triassic or Jurassic forms that could be considered ancestral to micrabaciids and would share some microstructural or morphological (e.g. septal insertion pattern) characters have yet been found. Possible explanations of such morphological disparity of micrabaciids from other scleractinians are either sudden emergence by skeletonization of long evolved, soft-bodied group of basal hexacorallians or migration of their skeletonized, deep-water ancestors to shallow-waters.