300 million years apart: The extreme case of macromorphological skeletal convergence between deltocyathids and a turbinolid coral (Anthozoa, Scleractinia)

By integrating morphological and molecular analysis, the family Deltocyathidae was recently erected to accommodate Deltocyathus species previously ascribed to the family Caryophylliidae. However, Deltocyathus magnificus was, based only on molecular data, recovered within the family Turbinoliidae, phylogenetically distant from Deltocyathidae. To elucidate the enigmatic evolutionary history of this species, the phylogenetic relationships using both nuclear ultraconserved and exon loci and complete mitochondrial genomes, and skeletal microstructural features of representatives of both families were investigated. Both nuclear and mitochondrial phylogenomic reconstructions confirmed the position of Del. magnificus within turbinolids. Furthermore, a novel mitochondrial gene order was uncovered for Deltocyathidae species. This gene rearrangement was not present in either Turbinoliidae or in Del. magnificus, which both have the scleractinian canonical gene order, further indicating the taxonomic utility of mitochondrial gene order. Thus, Del. magnificus is formally moved to the family Turbinoliidae and accommodated in a new genus (Dennantotrochus gen. nov. Kitahara, Vaga & Stolarski). Surprisingly, turbinolids and deltocyathids do not differ in microstructural organization of the skeleton, which consists of densely packed, individualized rapid accretion deposits and thickening deposits composed of fibers perpendicular to skeleton surface. This result suggests that both groups although evolutionary divergent, may still share conservative biomineralization mechanisms.