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 Five

The Silurian

The Silurian period (440 to 410 million years ago) saw a return to the moderate climates that had existed before the Cambrian and Ordovician glaciations. Eukaryotic life colonized terrestrial environments. Fish underwent an adaptive radiation with the jawed fish appearing and spreading into freshwater.

Corals, which had appeared possibly as early as the late Proterozoic, diversified into a number of groups during the Silurian. Tabulate corals and rugose corals were major components of the new, larger reefs built during the Silurian through Permian. Rugose corals included the horn corals, while tabulate corals were colonial. Both the rugose and tabulate corals went extinct at the close of the Permian period.

Crinoids, a group of organisms in the phylum Echinodermata, had been present since the Ordovician (and possibly the Cambrian). With the flooding of continents during the Silurian, crinoids underwent an adaptive radiation and produced a number of interesting forms. The crinoid fossil can consist of three body parts, usually separated by sedimentological processes after the organism died: the basal holdfast, stalk, and head. Crinoid stems were so common in some areas of the U.S. Midwest that native peoples used them as a form of currency. Crinoids were significant elements of the marine fauna until they nearly went extinct (down to a single genus) at the close of the Permian period.

This rare specimen of a Silurian crinoid, Icthyocrinus sp., from the Rochester Shale of New York illustrates the three regions of the body that can be often found as isolated fossils. The specimen of Caryocrinites ornatus shows details of the stalk and head region.
Used with permission from

Stromatoporoids, once thought to be coral, but actually sponges, continued their significant role in Silurian reefs. Some stromatoporoid specimens are over 16 feet in diameter. Stromatoporoids finally went extinct at the end of the Cretaceous extinction.

One arthropod group became more diverse during the Silurian and Devonian times, the eurypterids, or sea scorpions. The oldest fossil eurypterids are from the Ordovician, but the group increased in species number and size during the Silurian-Devonian before its eventual extinction at the close of the Permian. Eurypterids were chelicerates, the group of arthropods that includes the spiders and scorpions. Eurypterids were among the major swimming predators of the Silurian-Devonian seas. Eurypterids are so common in the Silurian rocks of New York that they are the state fossil.

Eurypterus remipes, from the Fiddlers Green Formation in New York.
Used with permission from

Fish continued to diversify, with the oldest jawed fish, the placoderms, joining the ostracoderms and acanthodian fish. The great explosion of these fish groups would occur during the Devonian period. During the Silurian, however, fish established themselves in both marine and freshwater environments.

Read the section below and choose a section to elaborate on in writing. How do living things today deal with that issue you have chosen?

Perhaps the most significant advance of life during the Silurian was the colonization of the land, first by plants and insects, and later (during the Devonian) by certain "fish" and their offshoots, the amphibians. The Silurian land was populated by early land plants as well as a variety of insects. Both plants and animals had a number of challenges when they moved from the water to land.

  1. Drying out. Once removed from water and exposed to air, organisms must deal with the need to conserve water. A number of approaches have developed, such as the development of waterproof skin (in animals), living in very moist environments (amphibians, bryophytes), and production of a waterproof surface (the cuticle in plants, cork layers and bark in woody trees).

  2. Gas exchange. Organisms that live in water are often able to exchange carbon dioxide and oxygen gases through their surfaces. These exchange surfaces are moist, thin layers across which diffusion can occur. Organismal response to the challenge of drying out tends to make these surfaces thicker, waterproof, and to retard gas exchange. Consequently, another method of gas exchange must be modified or developed. Many fish already had gills and swim bladders, so when some of them began moving between ponds, the swim bladder (a gas retention structure helping buoyancy in the fish) began to act as a gas exchange surface, ultimately evolving into the terrestrial lung. Many arthropods had gills or other internal respiratory surfaces that were modified to facilitate gas exchange on land. Plants are thought to share common ancestry with algae. The plant solution to gas exchange is a new structure, the guard cells that flank openings (stomata) in the above ground parts of the plant. By opening these guard cells the plant is able to allow gas exchange by diffusion through the open stomata.

  3. Support. Organisms living in water are supported by the dense liquid they live in. Once on land, the organisms had to deal with the less dense air, which could not support their weight. Adaptations to this include animal skeletons and specialized plant cells/tissues that support the plant.

  4. Conduction. Single celled organisms only have to move materials in, out, and within their cells. A multicellular creature must do this at each cell in the body, plus move material in, out, and within the organism. Adaptations to this include the circulatory systems of animals, and the specialized conducting tissues xylem and phloem in plants. Some multicellular algae and bryophytes also have specialized conducting cells.

  5. Reproduction. Organisms in water can release their gametes into the water, where the gametes will swim by flagella until they encounter each other and fertilization happens. On land, such a scenario is not possible. Land animals have had to develop specialized reproductive systems involving fertilization when they return to water (amphibians), or internal fertilization and an amniotic egg (reptiles, birds, and mammals). Insects developed similar mechanisms. Plants have also had to deal with this, either by living in moist environments like the ferns and bryophytes do, or by developing specialized delivery systems like pollen tubes to get the sperm cells to the egg.

Now go on to the next part. Next

© 2007 Aventa Learning. All rights reserved.