Skeletal ontogeny in basal scleractinian micrabaciid corals

The skeletal ontogeny of the Micrabaciidae, one of two modern basal scleractinian lineages, is herein reconstructed based on serial micro-computed tomography sections and scanning electron micrographs. Similar to other scleractinians, skeletal growth of micrabaciids starts from the simultaneous formation of six primary septa. New septa of consecutive cycles arise between septa of the preceding cycles from unique wedge-shaped invaginations of the wall. The invagination of wall and formation of septa are accompanied by development of costae alternating in position with septa. During corallite growth, deepening invagination of the wall results in elevation of septa above the level of a horizontal base. The corallite wall is regularly perforated thus invaginated regions consist of pillars inclined downwardly and outwardly from the lower septal margins. Shortly after formation of septa (S2 and higher cycles) their upper margins bend and fuse with the neighboring members of a previous cycle, resulting in a unique septal pattern, formerly misinterpreted as "septal bifurcation." Septa as in other Scleractinia are hexamerally arranged in cycles. However, starting from the quaternaries, septa within single cycles do not appear simultaneously but are inserted in pairs and successively flank the members of a preceding cycle, invariably starting from those in the outermost parts of the septal system. In each pair, the septum adjacent to older septa arises first (e.g., the quinaries between S2 and S4 before quinaries between S3 and S4). Unique features of micrabaciid skeletal ontogeny are congruent with their basal position in scleractinian phylogeny, which was previously supported by microstructural and molecular data.