Vertebrates: Fish Vertebrates are highly mobile, complex organisms with internal skeletons. The oldest vertebrates are fish, and the earliest fish are the jawless agnathans, which had heavy head skeletons, but only a cartilaginous internal skeleton. Closely related are the placoderms, the first jawed fish, also with heavy head shields, and the cartilaginous chondrichthyians, including modern sharks and rays. The most derived fish (osteichthyians) have bony skeletons and include all modern bony fish as well as the early ancestors of land-dwelling vertebrates.  The first fish appeared in freshwater environments but rapidly spread from lakes and rivers to lagoons and then to open seas. By the silurian, fish occupied all water depths from lakes and streams to the deepest oceans, so expect to find fossils in nearly any environment.
 With the exception of placoderms, all major groups of fish are still alive, although older groups are reduced in number and diversity. Lampreys and hagfish are all that remain of the agnathans but these have lost their head shields. Sharks and rays have tooth-like bones in their scales and their numerous generations of teeth are common fossils, unfortunately their cartilaginous skeletons do not fossilize well. 
Bony fish are now the most numerous. The lone surviving member of the group of bony fish that gave rise to land-dwelling animals, the coelcanth, was discovered in the 1930s in deep water off the African coast. Most bony fish respire with gills, but some forms have lungs and can breathe air.  As abundant as fish are, they are relatively rare fossils. The bony teeth of sharks fossilize well and are common in some environments. Otherwise, fish are so easily broken up after death that they make poor candidates for fossilization. In some minor dark shales, fish bones can be very common. Since most fish are active swimmers their fossils are likely to be spread through a variety of rocks. Look for them in shales, especially marine black shales or shales deposited in freshwater lakes. Fossil bones may be abundant in lake shales, where they are preserved on bedding planes. Fossil Fish fecal material, coprolites, may be locally common in Permian shales of brackish or fresh water.  Bones may also be concentrated in sediments on lake bottoms, where they have been protected from decay by lack of oxygen. Whole fish are more common in lake shales. In rare instances, in certain limestones, whole fish are preserved, uncrushed, in three dimensions.
FROM A FRAGMENT, Such as this shark tooth, a whole animal can be reconstructed. Many fossil sharks are known only from their teeth.
See Also: Ammonites, Trilobites or Wooly Mammoth Reproduced from "The Nature Company Guide to Rocks and Fossils" by Arthur B. Busbey III, Robert R. Coenraads, David Roots, and Paul Willis, Consultant Editors David Roots and Paul Willis, published by Time Life Books available at (800) 227-1114 |
