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PHYS 0106 (Spring, 2001) Earth History Timeline |
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| Phanerozoic Eon: Paleozoic Era: Devonian Period | |||
Timeline Links Phanerozoic Neoproterozoic Mesoproterozoic Paleoproterozoic Archean Other Links Historical Geology Page Physical Geology Page Mike's Home Page |
Writer/Researcher: Bob Mauriello Editor: Cindy Kennedy Web Page Builder: Bob Mauriello The Devonian Period was known as the "the age of fish" because they were so abundant. Scientists believe that fish showed up during the Silurian Period. Throughout the Silurian and until the end of the Devonian, some fish evolved into Tetrapods and others evolved into the fish we have today. Tetrapods were the first four-legged land animals. The earliest fish were the Agnathans or jawless fish. The Agnathans had round mouthparts that were used for sucking and filter feeding. These fish were heavily scaled in order to protect themselves from other animals at that time. There are two Agnathans that are still around today: the lamprey and the hagfish. From these fish came the jawed fish. Jaws only evolved once rather than many times in different species. Jawd came from the gill arches that supported ther gills. These early jaws were very complicated. Even more complicated than the human jaw because humans onlty have one moving part while these fish had twenty-four moving parts. Jaws allowed fish to hunt for their food, which allowed them to become more agile, reduce their armor, and decrease muscle density. These jawed fish contained two groups:Acanthodians and Placoderms. Acanthodians were small very stramlined fish with big eyes. They lived around 450 million years ago. Placoderms on the other hand were small during the Silurian but increased in size during the Devonian, when they dominated. They could grow to be 35 feet in length. Placoderms were heavily armored in the front and scaly in the rear. When the Placoderms no longer dominated the seas, Chondritchthyes took over. Some of these were equally as scary. This group included sharks, skates, and rays. These evolved between 400-450 million years ago. Sometimes this group is referred to as the cartilaginous fish because they have cartilage and calcified cartilage instead of true bone. There are two types of jaws associated with these fish; sharks have a jaw used for biting and crushing, whils skates and rays use their jaws for bottom feeding and filtering.
During the Mid-Devonian, the Osteichtyes became abundant because the number of Placoderms and Chordritchthyes decreased. These fish are the bony fish that make up most of the species of fish that we have today. All bony fish share a common organ, the swim bladder. This allows the fish to float anywhere in the water. There are two types of bony fish: 1) Ray-finned fish that live in freshwater or saltwater, and 2) Lobe-finned or Sacrcopterygii (also includes Tetrapods) that were thought to have been extict until one was discovered in the 1930's. Lungfish, a type of lobe-finned fish, are believed to have evolved into Tetrapods. There is still much controversy over the evolution of fish. Fish have a very complex evolutionary history that produced some extict species like Placoderms, and evolved into all land vertebrates of today. |
