Fauna and Flora

FAUNA AND FLORA Corals are marine coelenterates attached to the seafloor (Fig. 63). They began to build extensive carbonate reefs in the Ordovician, build island chains and transform continental coastlines. Reef-building reefs created the foundations for spectacular underwater buildings that today cover about 750,000 square miles and house about a quarter of all marine species. Corals diverged into two basic lineages before they developed calcified skeletons, suggesting that they may have developed reef-building capabilities twice in geologic history. Many corals declined by the end of the Paleozoic and were replaced by sponges and algae as the seas they inhabited receded. Bryozoans (Fig. 64), often called moss animals, resemble corals on a smaller scale. However, they are more closely related to brachiopods. They consist of microscopic individuals that live in small colonies up to a few inches long, giving the seafloor a moss-like appearance. Bryozoans are retractable creatures, encased in calcareous vase structures where they retreat for safety when threatened. Living species occupy the sea at various depths, with certain rare members adapting to life in fresh water. Free-moving singlelarval bryozoans form new colonies by fixing solids and grow into many individuals by budding, which is the production of growth results. The polyp has a circle of ciliated tentacles, forming a kind of web around the mouth and used to filter microscopic food that floats. The rhythmic tentacles beat back and forth, producing a water current that helps capture digested food in a U-shaped intestine. Fossil bryozoans are common in Paleozoic formations, especially in the American Midwest and the Rocky Mountains. Bryozoan species are identified by their complex skeletal structure, which helps describe a specific geological period. Bryozoans are very abundant, ranging from the Ordovician to the present. Their fossils are very useful for making rock correlations. Due to their small size, bryozoans are ideal microfossils for dating oil well cuttings. Bryozoan fossils are commonly found in sedimentary rocks, especially when covering bed surfaces. They resemble modern descendants, with several larger groups possibly contributing to Paleozoic reef building, resulting in extensive limestone formations. Its fossils are most abundant in limestone and less abundant in shale and sandstone. Often, the fine lines of bryozoans can be seen inlaid with fossilized shells of aquatic animals, rocks, and other hard objects. Of great importance to geologists are ostracods, or clam shrimp, whose fossils are useful for connecting rocks from the Ordovician onwards. Starfish are also common and leave fossils in Ordovician rocks from the central and eastern United States. Their skeletons consist of tiny silicate or calcite plates that are not rigidly joined and therefore usually disintegrate when animals die, making whole starfish fossils rare. Sea cucumbers have large tube feet modified into tentacles and skeletons consisting of isolated plates that are sometimes found as fossils. As the Ordovician drew to a close, a mass extinction wiped out about 100 families of marine animals 440 million years ago. The climate became cold, and glaciation reached its peak. Ice sheets radiate out from the center in North Africa, which drifts directly above the South Pole. Most of the victims are tropical species that are sensitive to environmental fluctuations. Among those that became extinct were many species of trilobites. Before extinction, trilobites accounted for about two-thirds of all species but only one-third after. One dominant group of trilobites called the Ibex fauna vanished at the end of the Ordovician mass extinction, possibly due to widespread glaciation. They were replaced by the Whiterock fauna, which multiplied their genera and sailed through extinction practically unscathed. Graptolit (Figure 65) is a cupped colony of organisms resembling a conglomeration of floating stems and leaves. Certain groups became extinct by the end of the Ordovician. Graptolite is thought to have gone into complete extinction at the end of the Carboniferous about 300 million years ago. However, the discovery of living pterorbranchs, likely modern counterparts of graptolites, suggests these may be living fossils. Toward the end of the Ordovician 450 million years ago, atmospheric oxygen concentrations produced sufficient levels of ozone in the upper stratosphere to protect Earth from the Sun's deadly untraviolet rays. So, for the first time, plants began to come ashore to fill the soil. When early plants first left the oceans and lakes to settle on dry land, they were greeted by the harsh environment. Ultraviolet radiation, desert conditions, and nutritional deficiencies make life difficult. The first thing that welcomes land plants is soil bacteria that stir sediment into viscous brown mounds. The presence of these bacteria helps speed up the weathering process, without which hot bare rocks will cover most of the landscape and land plants will have little success getting roots. Cyanobacteria, formerly called blue-green algae, may have prepared the soil for land invasion as early as 3 billion years ago. Ancient cyanobacteria, resistant to high levels of ultraviolet radiation, first lived in shallow tidal ponds, from where they eventually colonized the continents. They may have improved the terrestrial climate for extraaquatic life by pulling atmospheric carbon dioxide, thus countering the greenhouse effect, which prevents thermal energy from escaping the planet. Soil bacteria help resist erosion by binding sediment grains together and absorbing rainwater. Bacteria also provided nutrients to early land plants. Plant fossils from the Ordovician period appear to consist almost entirely of algae similar to present-day algal mats found on beaches and pond bottoms (Figure 66). Some marine algae live in the intertidal zone and can only survive for a short time outside the ocean due to the risk of dehydration. Even after developing protective measures to help organisms survive outside of water for longer periods, they still depend on the ocean for reproduction. Mosses, which are symbiotic relationships between algae and fungi in which they live on each other, may be taking the first tentative step onto dry land. Following mosses are lichens and liverworts. Fungi also had a symbiotic relationship with plant roots when they first evolved, helping vegetation with nutrient absorption and receiving carbohydrates in return. Bacteria also play an important role in nitrogen fixation, abundant soil gas and essential nutrients for plant life. In the nitrogen cycle, denitrifying bacteria convert dissolved nitrates back into elemental nitrogen. Otherwise, all the nitrogen in the atmosphere would have disappeared long ago.