Coral And The Like
The most familiar living forms of this group, sometimes now called the cnidariaCnidaria, are the flower-like sea anemones, free swimming jellyfish, and rock-like corals. Most exist in salt water with only a few species living in fresh water seas and lakes. Members of this phylum can have one or both of two different life forms called, polyp and medusa forms. The forms which are polyp types are tubular or cup-shaped and have central mouths surrounded by tentacles. Most attach themselves to the sea bottom and secrete hard calcareous skeletons. The umbrella shaped medusa form is displayed by modern and fossil jellyfish. Having no hard parts, jellyfish are very rare as fossils and only under exceptional conditions are impressions of them preserved - mostly in Paleozoic strata of the Grand Canyon.
The phylum is divided into three major groups: the Hydrozoa; the Scyphozoa and the Anthozoa. The stony anthozoans (the horn and tubular corals) are the only important Arizona fossil corals.
Horn Coral: The best-known rugose corals are the "horn corals." Horn Coral are solitary polyps, usually with a conical or horn-like shape, that are abundant at many Paleozoic localities. I've seen mass accumulations in some rocks that were a hundred feet thick. Because horn coral are often replaced by silica (quartz) they resist weathering by the acids in rainwater that gradually dissolve the surrounding limestone matrix. Sometimes slabs or horn coral can be spectacular, either making good exhibit pieces or doorstops. Horn coral are one of my favorites, because they are just so easy to find in many Paleozoic outcrops.
The anthozoans (which include the soft sea anemones) are important because of their abundance, variety and long geologic range from the Ordovician to the present. Anthozoans are strictly marine, grow to large size, and secrete much more complex skeletons than the hydrozoans. The organism is short and rests upon its secreted base which grows by periodical secretion of calcium carbonate (CaCO2) into a long cone as in solitary horn corals or into united colonial networks. Of the six thousand or so living Anthozoans, most prefer warm, shallow water. This indicates that many of Arizona's sea deposits were tropical, where coral reefs flourished, as they do today.
Unlike fossil sponges, most corals are preserved as ridged colonies easily preserved in many forms of marine sediments. Weathered specimens are easily collected from talus slopes, or whole specimens can easily be removed from rock ledges with hammer and chisel.
As far as possible, an attempt should be made to take the time needed to remove a specimen whole, and if a specimen is found broken a collector should gather as many fragments as possible and fit them together, or catalog them as a unit.
The most important-measurable characteristics of corals occur within the interior of the specimen, so even fragmentary specimens are important. Acid treatments can be used to expose such features. Such specimens preserved on natural rock bedding planes are easily broken and no attempt should be made by the collector in the field to remove excess matrix.
Most other coelenterates, such as the medusa of hydrozoa (jellyfish) and sea anemones, are rare as fossils and only preserved in the finest of clastic sediments such as fine-grained shale or sandstone. If present in a deposit, such specimens can be collected with a hammer and a wide, thin chisel. Then residual carbonized films or the cavity of a natural mold of a specimen produce a weak zone within the sedimentary rock which will allow the rock to nearly always cleave in such a way as to leave the fossil exposed.

Fossil Jellyfish

Living Jellyfish
The use of acid in the preparation of fossils can be dangerous. Use any acids or other chemicals only after reading directions.
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