SILURAN PERIOD: 444 to 416 million years ago

DEVONIAN PERIOD: 416 to 359 million years ago



During the Siluran Period, seas retreated from the interior of North America. Glaciers existed at the south pole and global climate was probably similar to that of modern times (Seyfert, 1979). By the Devonian Period, terrestrial landmasses consisted of two continents: Gondwana in the south and a northern landmass composed of Ancestral North America, Ancestral Europe, Ancestral Siberia, and Ancestral China. These two landmasses were separated by a narrow Tethys seaway which was closing as the two continents neared each other. The Tethys remained open in the expanse which separated Europe and Asia from India, Australia, and Antarctica. Small glaciers existed in Gondwana (Stokes, 1982; Seyfert, 1979).


Inverebrates continued to evolve into new lineages, such as the sea scorpions which were dominant predators.

Primitive jawed fish evolved (perhaps as early as the Ordovician) and produced early sharks and bony fish in addition to several extinct groups.



By the Mid-Paleozoic, early plants had diversified and had even produced the first trees. No flowering plants or gymnosperms yet existed. Diverse arthropods adapted to terrestrial environments including the first insects, which were wingless. A group of fish became increasingly adapted to terrestrial environments until they evolved into the first amphibians.



At the end of the Devonian, 50% of the marine genera and 75% species became extinct during a period of 4 million years. Warm-water invertebrates the most severely affected. Trilobites, tabulate corals, rugose corals, ammonoids, graptolites, stromatoporoid sponges, conodonts, acritarchs, and brachiopods virtually disappeared. Many groups of agnathans and placoderms became extinct. Global cooling seems to have been a primary cause and there may also have been some meteorite impacts during this time.