THE SILURIAN PERIOD

Paleogeography

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

In Georgia:

• red SH & SS of Sequatchie Fm
• red to gray SS & SH of Red Mountain Fm (iron ores in ALA)
• end of Middle Ord clastic wedge molasse deposition

Life

1. Invasion of the land by vascular plants
   (plants with water-conducting tissues, as opposed to non-vascular plants like mosses)
   
   • waxy outer coating to prevent water loss
   • pores for gas exchange
   • repro. structures that could function on land
   • complex water circulation system
   psilophytes - small Middle Silurian plants with horizonatal stalks just below the surface of the ground, with vertical stems bearing spore sacs.

   Psilophyte 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
DeKalb College, Clarkston, GA
October 1995.
Last modified November 12, 1997