To the previous page of the Jurassic Reef Park?

Today and during the Jurassic Period

An incredible wealth of organisms lives in the reef. All have important tasks and duties. Putting it schematically, reef organisms may be classified into reef builders, reef destroyers and reef dwellers. This is valid both for modern and Ancient reefs, hence also for Jurassic reefs. I will show you but a small selection of these organisms.

Corals are the most important reef builders of all modern and many Ancient reefs. Particularly important for reef construction are the stony corals, which are corals with a calcareous outer skeleton (Figs. 4, 5).

Fig. 4: The staghorn coral Acropora is one of the most successful corals of modern reefs (Image: small 6kb, large 30kb) Fig. 5: The pore coral Porites is another important reef builder and also often dominates major parts of reefs (Image: small 8kb, large 46kb

Some corals grow up to 25 cm per year. Such intense production of calcium carbonate (which is limestone) is only possible with the help of certain algae (known as Zooxanthellae). These algae live in the soft tissues of the coral (Fig. 6). Due to their photosynthetic lifestyle they produce corals and carbohydrates which are, to a large part, used by the coral animal. On the other hand, the algae gain protection and can use phosphates and carbon dioxide produced by the coral. Phosphates inhibit the growth of calcareous coral skeletons but are a must for algal growth, although they are very rare in the nutrient deserts of the tropical seas. Only by this interdependent relation are reef corals able to grow rapidly despite the overall lack of nutrients. Such a mutual dependence of organisms to each other's advantage is termed 'symbiosis'. It is due to this relation that that most reef building corals grow in the shallow, light-penetrated water. This is the only way how the symbiotic algae can maintain their photosynthetic activity. Corals of slightly deeper waters mostly exhibit a plate shape, in order to develop a large upper surface directed to the faint light (Figs. 7, 8).

Abb.6 Fig. 6: Photosymbiotic algae ('Zooxanthellae'; small dots) within the tentacle of a coral (Image: 10 kb)
Abb.7 Abb.8
Fig. 7: The coral Agaricia forms lettuce-like heads in the very shallow water, and is hence known as lettuce coral (Image: small 10kb, large18kb) Fig. 8: In somewhat deeper water (depending on turbidity, from several meters of depth downwards to about 25 meters) the same coral forms flat, upward-facing 'leaves' in order to catch more light for its symbiotic algae. This growth form is known as disc coral (Image: small 7kb, large 41kb).

Similarly, many Jurassic corals (Figs. 9, 10) must have already had such algal symbionts. This can be particularly deduced by similar changes in growth form of some coral species towards deeper waters (Fig. 11).

Abb.9 Fig. 9 (left): Thecosmilia, a typical branching coral of Jurassic coral reefs (Image 7kb)

Fig. 10 (right): Latiphyllia, another Jurassic coral (Image: small 3kb, large 10kb)

Abb11 Fig. 11: In very shallow water, the Jurassic coral Microsolena agariciformis developed a nodular growth form (right), in deeper water it grew plate-like (cf. Fig. 45); the columnar growth form (left) is an adaptation to cope up with the deposition of mud and sand (i.e. with a high sedimentation rate). (Image: small 7kb, large 33kb)

Many other reef organisms developed similar symbiotic relationships.

Many other organisms bore into, and continuously fragment, corals and other organisms with a calcareous skeleton. We know boring mussels, boring worms, boring and rasping sea-urchins and snails, and many reef fishes (for example parrotfishes) which bite off parts of corals in order to reach the soft body of corals and boring microalgae living within the coral skeleton. These reef 'destroyers' fulfill an important role within the reef and, in a healthy reef, will never cause the death of a reef, on the contrary: Sea-urchins and snails rasp off soft algae and by doing so keep the skeletal surfaces of organisms clean. Larvae of stone corals and other organisms will only settle on such cleaned surfaces. Boring sea-urchins and mussels create a pronounced small-scale relief within the reef where other organisms can settle or coral larvae can develop without the danger of being plucked off by reef fishes. Parrotfishes discharge enormous amounts of calcareous mud which is washed into reef lagoons and helps keeping them shallow so that many light-dependant organisms such as calcareous algae can live there and contribute to the growth of a reef-lagoon-system (which is also known as carbonate platform).

Abb18 Fig. 18 (left): The boulder coral Montastrea is attacked by the orange boring sponge Cliona. Tube-shaped soft sponges are visible in foreground (Image: small 9kb, large 20kb)

Fig. 19 (right): Remnant of a Jurassic coral which was heavily bored by a boring sponge; polished rock slab (Image: small 7kb, large 43kb)

Abb20 Fig. 20: The gorgeous parrotfish gnaws at, and bites off corals with its sharp teeth. By doing so, it keeps soft algae short which otherwise would be quite dangerous for a reef. During night, some species of the parrotfishes coat themselves with a self-produced, mucous 'morning-gown'. This buffers their smell so that no enemies are attracted (Image: small 5kb, large 17kb)

Within a reef, organisms which bind and fix sediment particles are of paramount importance. Constant wave agitation, storms and boring reef organisms are constantly producing loose material (mostly skeletal debris). Most reef organisms need a firm, stable floor to settle and cannot start growing on a motile unstabilized sand bottom. Moreover, fine muddy material could pollute the tentacles of corals, and larger sand grains would grind the organisms like emery paper. Organisms such as encrusting sponges, unicellular encrusting foraminifers and, above all, encrusting algae and calcifying microbial mats stabilize the loose carbonate grains and thus cement the reef body. This is why in the course of time, that is during hundreds and thousands of years, a reef may form an elevation and can grow up to the sea level. The most important encrusting organisms of modern reefs are the light-dependant calcareous red algae (known also as coralline algae) which are able to encrust loose material even in extremely agitated waters. In shaded and dark areas of the reef, that is particularly in reef cavities and caves, microbial mats harden the reefs from inside: tropical warm waters are supersaturated with dissolved calcium carbonate and the mucus produced by these microbes acts as a catalyst and triggers the rapid calcification of these microbial mats.

Abb21 Fig. 21 (left): Encrusting calcareous coralline red algae are very important binding and cementing organisms within modern reefs. (Image: small 7kb, large 25kb)

Fig. 22 (right): Greenish microbial films within a modern reef cave. Red algae cannot grow here due to lack of illumination (Image: small 7kb, large 29 kb)


Coralline red algae are only important since about 100 million years (that is since Late Cretaceous times). Before this, calcifying microbial mats had to ensure the stabilization of reefs. This is why reefs developing before the Late Cretaceous (hence including the reefs of the Jurassic) are frequently characterized by a high amount of calcified microbial mats (known as microbial crusts). We will see below that such crusts could even form entire reef bodies.

Abb23 Fig. 23 (left): Polished microbial crust limestone. The darker microbial crusts form small cavities. (Image: small 3kb, large 16kb).

Fig. 24 (right): Microscopic view of a Jurassic microbial rock. The darker calcareous microbial crusts clearly form a framework. Small caverns are visible as well. (Image: small 6kb, large 20kb)



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Last changes 18. Nov. 98 by Reinhold Leinfelder

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