The Silurian Period

Pamela J. W. Gore
Georgia Perimeter College

Silurian 438-408 my

Paleogeography

High sea levels worldwide following Late Ord. low sea level
- partial melting of Ord. glaciers?

Ordovician-Silurian Deposits

When the Tippecanoe Sea flooded North America, it deposited the famous St. Peter Sandstone, an unusually pure, well-sorted, well-rounded quartz sandstone. The sandstone is overlain by extensive limestone deposits, locally replaced by dolomite.

In the eastern U.S., the limestones are overlain by and interbedded with shales along the periphery of the Queenston delta or clastic wedge. The Niagara Falls area is a classic locality where these rocks are exposed.


Niagara Falls from the Canadian side. The Middle Silurian Lockport Dolomite forms the resistant ledge at the top of the falls. The Rochester Shale forms the slopes below. The Upper Ordovician Queenston formation is at the bottom of the falls.

Sea levels dropped again in Late Sil. in much of world

In Georgia:

Life

  1. Invasion of the land by vascular plants
    (plants with water-conducting tissues, as opposed to non-vascular plants like mosses)
    psilophytes - small Middle Silurian plants with horizontal stalks just below the surface of the ground, with vertical stems bearing spore sacs.

    Psilophyton, a psilophyte.
    This specimen is 370 my and is from
    the Devonian of Quebec, Canada.
    Smithsonian Institution,
    Museum of Natural History
    Washington, D.C.

    Colonization by plants builds up the food web to later allow colonization by animals.

  2. Renewed marine adaptive radiation following Ord mass extinction. Niches refilled.

  3. Large tabulate-stromatoporoid reefs common (5-10 m high)

  4. Predators
    1. Invertebrate predators: eurypterids (sea scorpions), some up to 5 feet long

      Eurypterus remipes
      Early Silurian (425 mya)
      Fiddler's Green Formation, New York
      Length of longest animal is 11 inches
      Denver Museum of Natural History

    2. Vertebrate predators: first JAWED fishes

    Jaws probably evolved from gill supports (example of alteration of an existing structure for a new function)

  5. Fishes were diverse in both freshwater and marine envs.
    1. Ostracoderms (jawless) bony skin, heavily armored
      Ostracoderm
      Denver Museum of Natural History

    2. acanthodians (first jaws, paired fins, scales instead of bony plates. Spiney fishes.

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This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
October 1995
Modified November 12, 1997
Modified July 17, 1999
Modified June 2000