Rugosa (unidentified)
Anthozoa is sometimes considered a subphylum, with its major consituents making up the classes. These being Class Ceriantharia, Hexacorallia (including Scleractinia and Rugosa), Octocorallia, and Tabulata. These will all be included in this collection, but ordered by their type above the usual ordering by level of taxonomic precision and alphabetically.
Cnidaria is a phylum of simple aquatic animals, best known for their radial symmetry, nematocysts (stinging cells), and a body plan organised around a central cavity. The phylum includes organisms like jellyfish, sea anemones, hydras, and corals. Most cnidarians have two basic body forms: the free-swimming medusa (as seen in jellyfish) and the sessile polyp (typical of corals and sea anemones). Cnidarians exhibit a diploblastic structure, meaning they possess two primary cell layers, the ectoderm and endoderm, with a gelatinous layer called the mesoglea in between. While many cnidarians are carnivorous, using their stinging cells to capture prey, some, particularly corals, have developed symbiotic relationships with photosynthetic organisms like zooxanthellae, which assist in nutrient production.
Within this diverse phylum, Class Anthozoa includes organisms that exist exclusively in the polyp form and lack a medusa stage. Anthozoans are primarily sessile, attached to the substrate, and include groups like corals and sea anemones. Among anthozoans, corals are the most significant from a geological and palaeontological perspective due to their capacity to build massive reef structures over geological time. Coral polyps typically secrete calcium carbonate to form exoskeletons, which fossilise readily, making them important indicators in the fossil record. Anthozoa is further divided into orders such as Hexacorallia, which includes the modern reef-building corals, and Octocorallia, which comprises soft corals and sea fans.
The fossil record of corals is particularly rich, with three major types standing out: tabulate corals, rugose corals, and scleractinian corals, each representing different eras of coral dominance in Earth's history.
Tabulate corals were dominant during the Palaeozoic era, especially from the Ordovician to the Permian. These corals are characterised by their colonial nature and the presence of horizontal internal divisions known as tabulae. Unlike later corals, tabulate corals lacked septa (vertical internal walls) and often formed large, tightly packed colonies. They contributed significantly to reef ecosystems in shallow tropical seas during the Silurian and Devonian periods. However, they became extinct at the end of the Permian, during the Permian-Triassic mass extinction. Their decline mirrored broader ecological upheavals that affected much of marine life at that time.
Rugose corals, also known as horn corals, coexisted with tabulate corals and appeared in the Ordovician, flourishing through the Devonian and into the Carboniferous. These corals could be either solitary or colonial, and their most distinctive feature is the presence of septal divisions within the coral skeleton, radiating from a central point. The solitary forms often resembled a horn in shape, giving them their common name. Rugose corals also contributed to Palaeozoic reef systems and are frequently found as fossils in limestone formations from these periods. Like the tabulate corals, rugose corals were wiped out during the Permian-Triassic extinction, marking the end of their dominance in marine ecosystems.
Following the extinction of tabulate and rugose corals, the Scleractinian corals (modern corals) emerged in the Triassic and have been the primary reef-builders ever since. Scleractinian corals also possess calcareous skeletons but differ from their predecessors in their skeletal microstructure, which is composed of aragonite rather than calcite. These corals are notable for their ability to form both solitary and colonial structures, with colonial forms building the vast coral reefs seen in modern oceans. Reef-building scleractinians rely heavily on symbiotic zooxanthellae, which enable them to thrive in nutrient-poor, sunlit waters by performing photosynthesis. Scleractinians became the dominant corals from the Jurassic onwards, and they continue to dominate coral reef ecosystems today, making them critical components of modern marine biodiversity.
