SPONGES AND CORALS
SPONGES (Phylum Porifera)
Sponges are simple animals that generally have poorly articulated skeletons of more or less isolated small elements called "spicules". Usually they fall apart upon death, and only the isolated spicules, if anything, remain. Sometimes, however, a sponge may be quickly covered with sediment and preserved as a mold, as happened to the unidentified specimen from Late Eocene rocks near Leesburg, GA shown in the first photograph. This is a typical form for sponges. Small pores in the wall (which give the phylum its name) allow the animal to pull water into a central opening, and the water exits through the large hole in the top. The scale is 1 cm. (Photo by B. Carter)
Another group of sponges that were very common during the Paleozoic include the "stromatoporoids". Don't confuse this word with "stromatolite", which refers to fossil Monera. Here again the "stroma" part of the name refers to the laminar structure of the fossils, but these have more complicated internal and external structure than do stromatolites.
CORALS (Phylum Cnidaria, Class Anthozoa)
Sea anemones, which are commonly kept in marine aquaria, are large and typical members of this Phylum and Class. The body shape, a central digestive cavity with a mouth opening into it and a circle of stinging tentacles surrounding the mouth, is the same as for the corals. The difference is that the anemones do not have skeletons and the corals do. There are three major coral Orders that are of interest to geologists. Two are restricted to Paleozoic rocks and the third is both Mesozoic and Cenozoic. So identifying a coral fossil even very generally allows a fairly good idea of the age of the rocks it came from.
"Honeycomb corals" (Order Tabulata) derive their name from the horizontal partitions ("tabulae") in the skeleton. Other coral orders use vertical partitions ("septa") instead, so the presence of tabulae and absence of septa make for easy recognition of tabulate corals. Tabulates are always colonial, and the openings for each individual in the colony is a millimeter or two across (roughly the size of a BB). The openings can be jammed close beside each other or more widely spaced, but there is no skeletal material between them in the latter case. They lived only during the Paleozoic. The first photograph below (Favosites niagarensis from late Silurian rocks at Lobelville, TN) shows a top view of the colony with closely spaced, non-septate openings, the second photo (Halysites gracilis from Late Ordovician rocks near El Paso, TX) shows a side view of a broken colony with the tabulae visible. The scale in both photos is 1 cm. (Both photos by B. Carter.)
"Horn corals" (Order Rugosa) are given that common name because the solitary types have skeletons that taper toward the opening to create a shape like a horn. As with the Tabulata, rugose corals lived only during the Paleozoic. The photograph below (Streptelasma rusticum from Upper Ordovician rocks near Camden, OH) shows the typical horn form of the solitary types. The septa are easily visible around the edges of the openings of the specimens, but are obscured near the centers by sediment. Colonial forms of rugose corals do occur but are not usually encountered in introductory level courses. For our purposes solitary corals belong to this Order. Scale bar in photograph is 1 cm. (Photo by B. Carter)
Modern corals -- Order Scleractinia are the only types of corals with skeletons still around, having held that distinction since the early Mesozoic. Many are solitary, but the only ones we will see in the introductory course are colonial. The individual tubes have septa, making it easy to distinguish them from tabulates. In addition, when the individuals are not crowded together on the colony surface (the more usual state) there is obvious skeletal material between them. The photograph below is of a Recent scleractinian (Astrangaea danae) from Jacksonville, FL. Three different colonies have encrusted an ark shell. This species is unusual in living so far north -- most corals are tropical. The scale is 1 cm. (Photo by B. Carter)
