The Devonian Period

• Sea levels were high worldwide.
  Extensive sedimentary record of Devonian period.
  Reefs, carbonates, evaporites suggest warm climate.

• Acadian Orogeny.
  Iapetus Ocean (Proto-Atlantic) closed.
  Laurentia and Baltica collided, forming a mountain belt.

Life

1. Invertebrates
   • Trilobites

     Trilobites Phacops rana milleri, Early Devonian Silica Shale, Ohio Denver Musum of Natural History

   • Brachiopods

     Brachiopods Denver Musum of Nature and Science

2. Age of Fishes

   ostracoderms ("bony skin") - no jaws

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   Placoderms ("platey skin"). This is the skull of Bothriolepis canadensis, a bony-headed fish. The body was probably soft and decayed rapidly.

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   Another placoderm Dunkleosteus (30 ft long; bony skull shown here is about 1 meter high). Note protrusions of jaw that functioned as teeth, and note the eye sockets protected by a bony ring. Top predator. Extinct at the end of the Devonian

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   • acanthodians (spiny fishes)

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   • Cartilaginous fish, sharks and rays were present and may have been the last major group of fish to evolve (cartiledge)

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   • Bony fish
     There are two groups of bony fish
     1. Ray-finned fish
        began their evolution in Devonian lakes and streams (freshwater) and then spread to the sea. They are the dominant fishes of the modern world.

     2. Lobe-finned fish
        Lobe-finned fish have muscular fins with articulating bones. There are two groups of lobe finned fish.
        1. The lungfish
           Lungfish live today in freshwater.

        2. The crossopterygians
           This is an important group of lobe-finned fish because it gave rise to the amphibians during the Devonian.

   Eusthenopteron foordi, Late Devonian (365 m.y.) Escuminac Formation, Quebec, Canada. It is structurally similar to amphibians and is considered to be transitional to the amphibians.

   Denver Museum of Natural History Museum of Natural History, Smithsonian Institution

   Another group of lobe-finned crossopterygian fish invaded the sea and gave rise to the coelacanths. The coelacanths are considered to be living fossils because they were long-believed to be extinct, but one was caught in 1938 near Madagascar. More have been caught since.

   Coelacanth. Note shape of tail.

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   Fishes were the only vertebrates on Earth until the Late Devonian.

   Adaptive radiation of fish during the Devonian.

   Many of the armored fish became extinct at the end of the Devonian.

   Lobe-finned fishes and lung-fishes also declined at the end of the Devonian.
   Out-competed by amphibians??

3. Swimming predators were increasing. Many new types.
   • fish
   • ammonoids
   • eurypterids (Silurian and Devonian)
     Eurypterid
     Museum of Natural History, Smithsonian Institution

   Predators may have contributed to the decline in trilobite diversity.

4. Invasion of the land by animals
   • wingless insects and scorpions
   • amphibians (in Late Devonian)

     Amphibian Saurerpeton obtusum

     An ancestral salamander, Amphibamus lyelli.

     Icthyostega is intermediate between lobe-finned fishes and amphibians. Four legs. Skull structure resembles lobe-finned fish. Also had fish-like tail.

5. First trees
   Trees stand tall against the pull of gravity by the Late Devonian. (Some more than 7 m tall). Requires the development of good system for water circulation.

   Lycopods (scale trees and club mosses) appeared. Moisture loving plants. Reproduced by means of spores. Requires moist habitat. Continued and thrived in Carboniferous.

   
   Lepidodendron
   Note leaf scars on the trunk.
   (Trees grew to 30 m tall; 90 ft).
   The large lycopod trees became extinct by the end of the Paleozoic.

   Plant roots begin to stabilize the soil against erosion.

6. First Seeds.
   Seed-bearing plants appeared. Non-flowering plants. Gymnosperms.
   Seed-bearing plants no longer require moist habitats.
   Expansion of plants into drier areas.

7. Mass extinction just before the end of the Devonian.
   • tabulate-stromatoporoid reefs disappear
   • many fish extinct
   • many extinctions among floating and swimming animals
   • many freshwater forms extinct

   Tropical taxa were most severely affected. WHY?

   • global cooling?
   • drying?
   • glaciation?

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Photographs were taken in the Denver Museum of Natural History.

This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
October 1995
Modified November 12, 1996
Modified November 10, 1997
Modified July 17, 1999
Image added March 21, 2003
Updated November 11, 2003