Biology : Semester II


IntroductionSection 1 | Section 2 | Section 3 | Section 4 | Section 5

  Section Three:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10 | Part 11 | Part 12 | Part 13 | Part 14

Biology : The Time of Ancient Life : Part Eight

Marine Vertebrates

The various fish groups that had appeared during the Silurian period (or in a few cases possibly even earlier) continued into the Devonian. The Devonian has been called the "Age of the Fishes" because of the tremendous diversity of fish groups that evolved during this period of geologic time.

Small, jawless, and finless ostracoderms were the earliest vertebrates. They were filter feeders, but probably were also able to move water through their gills by muscular action. Ostracoderms have been found as fossils from the Cambrian through Devonian periods, when the group finally went extinct. Although extant jawless fishes lack protection, many early jawless fishes had large defensive head shields.

The first jawed fish were the Placoderms, a now-extinct group from the Devonian. Placoderms were armored with heavy plates and had strong jaws and paired pectoral and pelvic fins. Paired fins allow fish to balance and to maneuver well in water, which facilitate both predation and escape.

Cast of the placoderm, Bothriolepis.

The evolution of jaws is an example of evolutionary modification of existing structures to perform new functions. Jaws are modified gill arches, and allowed the exploitation of new roles in the habitats: predators with powerful jaws. There are two classes of jawed fish: the cartilaginous fish and the bony fish.

The class Chondrichthyes contains the skates, rays, and sharks. They have jaws, lots of teeth, paired fins, and a cartilage endoskeleton. Cartilaginous fish first appeared during the Devonian and expanded in diversity during the Carboniferous and Permian before nearly disappearing during the great extinction that occurred near the end of the Permian. Some fragmentary evidence suggests cartilaginous fish were present during the Ordovician, although the best skeletal evidence is from the Devonian. Despite scanty skeletal presence (cartilage not preserving nearly so well as bone), shark teeth are common fossils in some deposits.

The crossopterygian fish (represented by the marine extant deep-living coelacanth and extinct freshwater forms) are regarded as ancestors of early amphibians. Extinct crossopterygians had strong fins, lungs, and a streamlined body capable of swimming as well as traveling short distances out of water.

Comparison of the skeletons of a crossopterygian lobe-finned fish and an early amphibian.

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