Anthozoa
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Anthozoans are probably the most famous cnidarians: they include the corals that build great reefs in tropical waters, as well as sea anemones, sea fans, and sea pens. They also have a long and diverse fossil record, extending back at least 550 million years. The oldest anthozoans are probably some of the polyp-like and sea pen-like fossils from the Vendian (late Precambrian). A few tens of millions of years later, in the Cambrian period, the first mineralized coral-like organisms appeared. True corals of the kind living today did not appear until the middle Triassic, at about the same time that the first dinosaurs were evolving.
Fossil Record
The origins of the Anthozoa lie in the Precambrian, but concrete evidence is sketchy. A number of the Vendian, or latest Precambrian, soft-bodied "medusoids" are now thought to represent benthic polyp-like organisms. Some of the frond-like fossils of the time could represent colonial anthozoans similar to living "sea pens," Eoporpita is one such Vendian fossil that could be a single anthozoan polyp, and Charnia is a frond-like fossil that in the past has been linked with "sea pens" or soft corals. However, this issue has not been resolved to everyone's satisfaction, although some well-preserved frond-like fossils of the genus Charniodiscus from Australia may show spicules and individual polyps. In any case, there is molecular evidence to suggest that the Anthozoa are the earliest branch of the phylum Cnidaria.
Cambrian-age localities with soft-bodied organisms preserved also include some soft-bodied sea anemones and "sea pens," such as Mackenzia and Thaumaptilon from the Burgess Shale and Xianguangia from the Chengjiang biota of China. A few of the Cambrian "small shelly fossils" are spicules — nearly microscopic, mineralized, needle-like pieces — that appear similar to the spicules of living octocorals.
A few mineralized corals and coral-like organisms also appeared as early as the Lower Cambrian. A minor group of corals, the Cothoniida, is known from the Middle Cambrian. However, it was not until the Ordovician that mineralized corals became important parts of marine ecosystems. By the middle of the Ordovician, several lineages of corals had become distinct: the Tabulata, the Rugosa, and the smaller Heliolitida. The best-known rugose corals are the "horn corals," solitary polyps, usually with a conical or horn-like shape, that are abundant at many Paleozoic localities (such as the large Middle Devonian fossil Heliophyllum pictured at the top of this page). However, all three groups of corals could and did form massive reefs. Heliolitids went extinct in the late Devonian, at the time of a mass extinction event that also impacted the Rugosa and, to a lesser extent, the Tabulata. However, these two groups soon recovered and flourished until the end of the Permian.
Both tabulate and rugose corals disappeared in the Permo-Triassic mass extinction about 245 million years ago. In the middle Triassic, a new clade of corals appeared, the Scleractinia. The Scleractinia do not appear to be close relatives of either the Tabulata or the Rugosa, and probably evolved from sea anemone-like ancestors that have not been preserved as fossils. Like earlier corals, the scleractinians soon evolved massive colonial species that formed great reefs. The image above shows an early scleractinian reef, now a massive limestone layer preserved in Dunlap Canyon, Nevada. Colonial scleractinians have continued to dominate most tropical reef habitats since the Triassic, while other scleractinians live as solitary polyps in a wide range of habitats.
The fossil record of octocorals is sparse. As mentioned, there are some Cambrian fossils that indicate their presence. There are also a few Paleozoic fossil "sea pens" and "sea fans". However, most of the known fossil octocorals are Cretaceous and Cenozoic in age. Fossil octocorals may be more common than is usually suspected, but probably go unrecognized when found.
Life History and Ecology
Anthozoans, unlike other cnidarians, completely lack a medusa stage; they live exclusively as polyps. While anthozoans retain their nematocysts, or stinging cells, and may feed on large prey or particulate food, a number of anthozoans supplement their diet by growing symbiotic algae in their tissues. Hermatypic, or reef-building, scleractinian corals in particular owe their success to the fact that most have symbiotic dinoflagellate algae in the genus Symbiodinium living inside their tissues. Coral polyps also use their tentacles and nematocysts to feed, but Symbiodinium may actually produce most of a coral's food. For this reason, reef-building corals are exclusively shallow-water dwellers; without light they cannot survive (although solitary corals, octocorals, and anemones may inhabit much cooler and deeper waters). Therefore, one result of global warming and rising sea levels will be the death of coral reefs.
Systematics
Anthozoans are divided into three main subclasses. The Zoantharia includes the "true" corals and most sea anemones, and it is by far the best represented in the fossil record. On the cladogram pictured above, the Zoantharia is shown subdivided into constituent clades: the Scleractinia (living true corals); Zoanthiniaria, Corallimorpharia, and Actinaria (collectively known as sea anemones); and Rugosa, Heliolitida, and Tabulata (extinct groups of corals).
A second subclass, the Octocorallia, includes the sea pens, soft "corals", and sea fans. This clade has a less well-documented fossil record than the corals, but many of its members form mineralized, microscopic spicules.
The third subclass, the Ceriantipatharia, is very rare in the fossil record; it includes certain burrowing "sea anemones" (Ceriantharia) and precious black "corals" (Antipatharia). Some zoologists prefer to regard these two as separate groups and do not believe that they form a monophyletic group.
More on Morphology
Anthozoans completely lack a medusa stage; they are polyps throughout their life cycles. An anthozoan individual has a sac-like body divided by radial partitions known as septa; these septa can easily be seen in corals, and their arrangement is an important character for classification.
To the right is a diagram of the septal arrangement in two types of corals. On the left is a scleractinian (extant stony coral) showing hexagonal symmetry. The extinct rugose corals of the Paleozoic seas had a very different arrangement, diagrammed on the right; they were once called tetracorals (four-sided corals), but the arrangement is actually bilateral. Compare the above diagram with the corals shown below, Tubastraea, on the left, and Lithostrotionella, on the right.
