A modern scleractinian coral with a two-component calcite-aragonite skeleton

Area of Science:

• Marine Biology
• Paleontology
• Geochemistry

Background:

• Scleractinian corals are believed to exclusively form aragonite skeletons, a conserved trait throughout their 240-million-year fossil record.
• Ocean chemistry fluctuations have not previously been observed to alter coral skeletal mineralogy.
• The Cretaceous coral *Coelosmilia* is the only known exception, with a presumed calcitic skeleton.

Purpose of the Study:

• To investigate the skeletal composition of the modern asymbiotic scleractinian coral *Paraconotrochus antarcticus*.
• To explore the evolutionary implications of novel skeletal structures in corals.
• To understand the significance of the Southern Ocean as a refuge for ancient marine traits.

Main Methods:

• Skeletal analysis of *Paraconotrochus antarcticus*.
• Microstructural examination of coral skeletons.
• Phylogenetic analysis to determine evolutionary relationships.

Main Results:

• The modern coral *Paraconotrochus antarcticus* forms a bimineralic skeleton composed of high-Mg calcite (inner) and aragonite (outer).
• *P. antarcticus* and *Coelosmilia* share a unique microstructure, suggesting a close phylogenetic link.
• This capability for bimineralic structures in scleractinians dates back at least 100 million years.

Conclusions:

• Scleractinian corals possess a more diverse biomineralization capability than previously understood.
• The Southern Ocean harbors extant species with ancient evolutionary traits, like bimineralic skeletons.
• The early divergence of *P. antarcticus* within the Vacatina clade occurred in the Mesozoic, highlighting Southern Ocean's role in preserving ancient lineages.