Of greater importance are the coenothecalians, a group of extant reef-builders that includes the Blue Coral (Heliopora coerulea). This species of anthozoan is the only octocoral that produces a massive carbonate skeleton.
But the greatest reef-builders of all are the true "stony corals", those that secrete a CaCO3 cup or theca which serves as the basic building block of the reef. The cup may be divided by vertical plates or walls called septa, and may display horizontal laminations or "growth lines" called tabulae. It is these skeletal features that we use to classify the various stony corals.
Three major orders of stony coral are recognized in the fossil record, two of them are extinct and one is still living. The tabulate corals have distinctive tabulae but either lack septa completely or have weak, poorly-developed septa. This large order of corals includes the "honeycomb" corals, the "chain" corals, and the chaetetids. The rugose corals have well-developed septa that occur in multiples of four. As growth proceeds, the new septa appear within the theca four at a time. Rugose corals are therefore called "tetracorals". The scleractinian corals also have well-developed septa, but in this case, they occur in multiples of six. This gives them a beautiful hexameral symmetry which is a hallmark of the "hexacorals", as they are commonly referred to. All three groups are famous for their reef-building feats.
The tabulate corals were the first true stony corals. Indeed, the oldest known anthozoan fossil is a tabulate coral called Lichenaria. This simple coral occurs in early Ordovician limestones in Minnesota. The massive thallocorals or "honeycomb" corals first appeared during the Ordovician and include some of the most prolific reef-builders in Earth's history. The classic "honeycomb" coral, Favosites, was a major constructor of reefs during the Silurian Period. But it was not alone. Other "honeycomb" corals like the distinctive "chain" coral Halysites and the famous "organ pipe" coral Syringopora were also important reef-builders during this period of time.
The chaetetids were a group of tabulate corals whose skeletons were comprised of thin-walled, closely-spaced, closed tubes connected with many tabulae, but having few or no septa. Unlike the majority of the tabulate corals, the chaetetids thrived during the latter part of the Paleozoic Era. They are very common in Carboniferous limestones and represent the last major gasp of tabulate coral evolution and reef-building.
The rugose corals gradually replaced the tabulate corals as the dominant reef-builders during the Devonian Period. Known as the "horn corals", this distinctive group of corals consists of both solitary and colonial reef-building species, many of which display the classic horn-shaped, conical skeleton that gives them their name. Prime examples of the solitary, horn-shaped corals include Homalophyllum, Heterophrentis, Naos, Lophophyllidium, and Heliophyllum. The colonial species of rugose corals are some of the greatest reef-builders of all time. Three of the most important examples of these massive compound corals are Hexagonaria, Pachyphyllum, and Lithostrotionella.
Both the tabulate corals and rugose corals had their heyday during the Paleozoic Era. In the following era of Earth history, another group of amazingly successful corals will make its appearance and come to dominate the oceans of our planet. These scleractinians, as they are called, are a complex and diverse group of corals that includes a number of important reef-building families. Examples of these include the most primitive scleractinian family of corals, the massive colonial Astrocoeniida, the minor reef-builders known as the Dendrophylliida, and the two most important families of reef-builders, the Faviida and the Fungiida. The scleractinian corals appeared during the Triassic Period, shortly after the demise of the tabulate and rugose corals. They are still with us today. Their presence is a testament to their adaptability, their efficiency as predators, and their mutualistic, symbiotic arrangement with photosynthetic algae.
The unlikeliness of a pelecypod family constructing reefs hundreds of miles long and out-competing the scleractinian corals while doing it is at best extreme, but a group of clams known as the rudistids were major reef-builders at one time in Earth's history.
The rudistids are a unique group of marine heterodont bivalves that strongly resemble corals in their lifestyle and morphology. The heterodont pelecypods are a large and diverse group of mollusks that include the cockle shells (like Dinocardium robustum), the clams, the magnificant giant clams (like Tridacna gigas), the trough shells, tellins, and even the shipworms and rock borers. It is this group that gave rise to the reef-building rudistids.
Their reign as reef-builders was short. They arose during the Jurassic Period, expanded, and then peaked during the Cretaceous. Their ability to construct reefs was reflected by the changes that took place in their basic bivalve morphology. For instead of having two valves (or shells) of similar size and shape, the rudistids have only one. The other valve has evolved into a large, robust, roughly conical structure that is attached to the substrate and which served as the clam's living space. The smaller, true valve is reduced in size and forms a "lid" for the whole structure. The lid was sometimes flat and simple, sometimes low and conical in form, occasionally spiral, and in rare cases stellate in form. Superficially, the rudistids look very much like large horn corals. In species like Durania cornupastoris and Hippurites gosaviensis, the resemblance is striking.
The molluscan phylum has produced one important reef-building order in the rudistid clams. It has also produced a number of reef-contributors whose CaCO3 shells and hard parts help to bind and "glue" together the framework structure of the reef. Tube-shell snails construct hollow, tube-like structures out of calcium carbonate which are then cemented to the substrate. When the substrate is hard coral rock, these gastropods become reef-contributors in every sense of the word.
Although they are not reef-builders, the bryozoans are important contributors to the framework structure of reefs. The bryozoans or "moss animals" as they are sometimes called, are colonial creatures that construct their skeletons out of CaCO3. Bryozoan colonies may be upright and branching or may form a crust on the surface of the reef or on bedrock. The encrusting colonies in particular resemble fine lacework and are sometimes called "lace corals", but bryozoans are not corals. They are more closely related to brachiopods.
Modern-day bryozoans are extremely efficient in colonizing virtually any surface in the marine environment. Their carbonate skeletons appear as encrustations on the seabed and even on floating objects. Today, they are minor contributors to the reef systems of the world.
It was a much different story during the early part of the Paleozoic Era. The Ordovician seas of our planet witnessed the rise of the trepostome bryozoans, the most important group of bryozoans in the fossil record. This of course, means that they were prodigious secretors of hard parts! The trepostomes constructed massive stony colonies and were significant contributors to Ordovician reef systems. These "stony bryozoans" dominated the Ordovician, but began to fade by the following period in Earth's history. Indeed, only about half of the trepostomes made it through to the Silurian Period. A number of important genera have been recognized in the fossil record. Hallopora and Prasopora head the list.
The Kingdom Protista has a long and venerable history of reef interaction, in some cases acting as an important contributor to the reefs, in other cases providing the bulk of the reef-building effort. Near the end of the Proterozoic Eon, a number of algal groups joined the cyanobacteria in producing organosedimentary structures in the world's oceans. The green algae, brown algae, and red algae all make their appearance during this time in Earth's history and all probably contributed in some way to the stromatolitic reefs of that day.
But the red algae, in particular, were fated for greater things. Members of the Phylum Rhodophyta, the carbonate-secreting red coralline algae have always been significant reef-contributors, but today they rival the stony corals as the main constructors of reefs. Species like Porolithon gardineri produce the bulk of the limestone reef rock in some reef systems of the world. This is the case in Hawaiian waters today.