Corals

“Coral” is a rather colloquial term that is used to refer to organic structures made of calcic carbonate by cnidarians, an extensive group of organisms. Cnidarians usually have two basic body forms: sessile (fixed to the substratum) polyps and swimming medusae. The polyps of some species build around them carbonate skeletons that form the rigid structures known as corals, that are very frequent in the fossil record.
Cnidarians known as “corals” usually lack the medusae form and live their whole life as sedentary animals. Some species live on their own, although more frequently form colonies where individual organisms contribute to the formation of the calcareous skeleton.

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Sketch of the vital cycle of a cnidarian: medusae form (left) and polyp (right).

Generaly, corals live in shallow, marine waters, although some species are adapted to relatively deep habitats. Corals are the main contributor to current reefs and have played an important role in the construction of reefs in past geological eras.
Corals are preserved in most of the fossil record, which reflect the important morphological and adaptative changes that this group has experienced throughout time.

Rugose corals
Rugose corals constitute one of the most important Palaeozoic group of corals. The correct identification of these animals requires a detailed exam of the internal structure of the skeleton, which are divided by walls of calcium carbonate. Rugose corals developed both colonial and solitary habits. Colonies could have a wide variety of forms and sizes, whereas solitary forms usually had a conic morphology.

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Colonial and solitary rugosa corals (left and right)


Tabulate corals
Los corales tabulados acompañaron a los corales rugosos durante el Paleozoico y en la actualidad se encuentran también extinguidos. Los esqueletos de los corales tabulados están formados por numerosos cálices prismáticos adosados unos a otros y comunicados por pares de poros.

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Sección de coral tabulado (izquierda) y detalle de las coralitas (derecha)


Scleractinian corals
La extinción de los corales rugososo y tabulados a finales del Paleozoico precedió la aparición de este grupo, que incluye a los corales modernos. La principal característica de los corales escleractinios es la división de la coralita mediante tabiques radiales según un modelo de simetría hexámera (es decir, un modelo estructural que se repite seis veces alrededor de un eje de simetría).

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Colony of scleractinian coral (left) and detail of the internal structure (right)