Reef Builder Study Materials

REEF BUILDER Silurian marine invertebrates (Figure 80) are intermediaries in evolution between the Ordovician and Devonian lines. The Silurian was the "dark age" of paleontology in the sense that little is known about the soft bodies of animals inhabiting the oceans. Softbodied marine animals that lived in the Silurian about 430 million years ago included a number of strange worms and arthropods. Among the strange animals were a halfinch-long hairy worm and a small shrimp-like creature with a tentacle head, a segmented body, and a triangular tail. More mysterious is a half-inch-long creature that may be an unknown relative of a group of tiny, fat-legged worms called lobopods. A wide variety of worm-like creatures began in the Cambrian and seem to have evolved into higher animal life forms. Coral reef formation during the Silurian is widespread, indicating the presence of warm shallow seas with little seasonal temperature variation. Corals began to build extensive coral reefs in the Ordovician, forming barrier islands and island chains. They also built atolls on top of extinct submerged volcanoes. When a volcano recedes under the sea, the rate of coral growth corresponds to the rate of decline of the volcano. This keeps corals at a constant shallow depth for photosynthesis. Coral reefs are among the oldest ecosystems on Earth and important land builders. They formed a chain of islands and changed the coastline of the continent. The main structural feature of coral reefs is coral fortifications, which reach almost to the surface of the water. It consists of a large round coral head and various branching corals. The front reef is the sea from the top of the reef, where the reef covers almost the entire seabed. In deeper waters, many corals grow on flat, thin sheets to maximize their lightgathering area. In other parts of the reef, corals form large supports separated by narrow, sandy channels composed of calcareous debris from dead corals, calcareous algae, and other organisms that live on the coral. The channel resembles a narrow, winding canyon with vertical walls of dense corals. They dissipate wave energy and allow the free flow of sediment, which prevents corals from choking on debris. Below the front reef is a coral terrace, followed by a sandy slope with an isolated coral top, then another terrace, and finally a vertical drop close to a dark ravine. Coral polyps (Fig. 81) are soft-bodied animals crowned by a ring of tentacles surrounding the mouthlike opening. The tentacles are tipped with poisonous stings to attack prey swimming nearby. Polyps live in skeletal cups or tubes called theca consisting of calcium carbonate. Polyps extend their tentacles to feed at night and withdraw to their thecae during the day or at low tide to keep them from drying out in the sun. Because algae inside polyp bodies need sunlight for photosynthesis, corals are restricted to warm, shallow water that is generally less than 100 feet tall. A wide variety of corals are well represented in the fossil record and resemble many of their modern counterparts. Tabulated corals, which became extinct at the end of the Paleozoic, consist of densely spherical or polygonal thecae, some with pores covering the walls of the thecae. Rugose, or horn coral, named for its horn-like shape, is very abundant in the Silurian.They became the main reef builders of the late Paleozoic, eventually becoming extinct at the beginning of the Triassic.Hexacrally, with thecae separated by six sepia, or walls, ranges from the Triassic to the present day and is the main reef builder of the Mesozoic and Cenozoic eras. Tetracorals (Fig. 82), which are arranged in groups of four, are an extinct group of reef-building corals. Rudis (Fig.83) were molluscs that replaced corals as the dominant reef-builders in the Cretaceous and became extinct by the end of the period. In the geological past, large coral structures turned into some of the largest limestone deposits on Earth. Corals build barrier reefs and atolls. They also played a major role in changing the face of the planet. Coral reefs contain abundant organic matter. Many ancient reefs consist mostly of carbonate mud with skeletal remains of various other species literally "floating" in sediments, producing some of the finest fossil specimens. Tropical plant and animal communities thrive on reefs because of corals' ability to build large wave-resistant structures. Unfortunately, these are the same species that experienced several episodes of extinction due to their narrow range of living conditions. Extinctions are most severe affecting organisms that are anchored to the seafloor or unable to migrate out of the region due to physical and biological barriers. Despite the excellent climate and extreme success of trilobites in the lower Paleozoic, they began to decline rapidly in the Silurian. Many species succumb to extinction by the end of the period. Echinoderms with fivefold symmetry and bilateral and exoskeletons consisting of many calcite plates are among the most prolific animals in the Silurian sea. The most successful Silurian echinoderms are crinoids, commonly called sea lilies because they resemble flowers on stalks anchored on the ocean floor. Some crinoids are also planktonic or free-swimming types (Fig. 84). They became the dominant echinoderms of the middle and upper Paleozoic, with many species still present. Long stalks of crinoids grow upwards to lengths of 10 feet or more in good condition. They consist of perhaps 100 or more calcite discs called columns and are anchored to the ocean floor by root-like appendages. A cup called a petal perches on top of the stalk and houses the digestive and reproductive organs. The animal filters food particles from passing water currents with five furry arms extending from the petals, giving the crinoid a flower-like appearance. The extinct Paleozoic crinoids and their blastoid relatives, whose petals resemble rosebuds, make excellent fossils, especially the trunk, which on weathered limestone outcrops often looks like long strands of beads. Echinoids are a class of echinoderms that includes sea urchins, sea urchins, and sand dollars. They have an exoskeleton consisting of limy plates that are characteristically spiny, rounded, or radially symmetrical. Some of the more advanced shapes are elongated and bilaterally symmetrical about the central point. Sea urchins live mostly among rocks inlaid with algae, where they feed. Unfortunately, such an environment is not conducive to fossilization. Likewise, familiar sand dollars that sometimes wash up on beaches are rare in the fossil record.