Evolutionary History of Corals

Origins and Early History

The history of corals extends far back into geological time. Their origins lie in the Cambrian, about 500 million years ago. However, they were not the earliest reef builders—this title, depending on the definition, belongs to stromatolites if one considers them as reefs. In a narrower sense, the Archaeocyathids were the earliest reef builders. Archaeocyaths are a sponge-like, reef-building group of animals that went extinct by the end of the early Cambrian.

The oldest corals belong to the class Tabulata and Rugosa, which flourished during the Paleozoic. These early corals did not form reefs but lived mostly solitarily or in small colonies. In the Ordovician and especially in the Silurian, corals faced strong competition from bryozoans and stromatoporoids. Starting from the Devonian, corals became increasingly important in fossil marine ecosystems.

The Mesozoic Era and the Revolution of Reefs

During the Mesozoic, especially the Jurassic and Cretaceous periods (about 201 to 66 million years ago), corals underwent significant evolution. The modern corals, belonging to the class Scleractinia, emerged during this time. They replaced the Rugosa and Tabulata corals, which went extinct at the end of the Permian, and began to form the impressive reefs we know today. These Scleractinia corals have a robust calcium carbonate skeleton that helped them build large reef structures.

The Cenozoic Era and Modern Corals

After the extinction of the dinosaurs at the end of the Cretaceous, about 66 million years ago, Scleractinia corals continued their dominance in the seas. During the Cenozoic, corals evolved into one of the most important reef builders and played a crucial role in marine ecosystems. Today, corals are primarily found in tropical and subtropical oceans, forming complex structures that host countless marine species.

The Phylogeny of Corals

Corals belong to the phylum Cnidaria, which also includes jellyfish and sea anemones. Within the Cnidaria, corals are part of the class Anthozoa. Anthozoans are divided into two main subclasses: Hexacorallia, which exhibit six-rayed symmetry (including most reef-building corals), and Octocorallia, which have eight-rayed symmetry (including soft corals and gorgonians).

Hexacorallia includes the order Scleractinia (stony corals), which are the primary reef builders. Octocorallia includes orders like Alcyonacea (soft corals) and Gorgonacea (horn corals).

Anatomy of Corals

Corals consist of small polyps that live colonially. Each polyp has a cylindrical body with a central mouth surrounded by tentacles equipped with cnidocytes for hunting and defense. These tentacles capture small planktonic organisms and food particles. Coral polyps have a symbiotic relationship with zooxanthellae, microscopic algae that live in their tissues and provide energy through photosynthesis.

Fossil Corals for sale

Corals are often very well preserved and also ofen abundant as fossils. Due to their immense diversity of forms, fossilized corals are a popular and often very decorative field of study and collection.