The Silurian was the period from the end of the Ordovician, about 443.4 million years ago (mya) to the beginning of the Devonian period, about 419 mya. It is the third period of the Paleozoic era and the Phanerozoic eon and the shortest period in the Paleozoic.
As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The beginning of the Silurian is set at a major extinction when 60% of marine species were wiped out, the End-Ordovician extinction event.
First terrestrial biota Edit
The Silurian was the first period that produced fossils on land that are macroscopic. That means you can see them without using a microscope. These fossils are moss forests along lakes and streams, and millipedes and scorpions on land later in the period. The fossil record of Eurypterids (sea scorpions) reached its greatest extent in the middle Silurian, about 430 million years ago.
The first fossil records of vascular plants, appeared in the second half of the Silurian period. The earliest known representatives of this group are the Cooksonia (mostly from the northern hemisphere) and Baragwanathia (from Australia). A primitive Silurian land plant with xylem and phloem but no differentiation in root, stem or leaf, was much-branched Psilophyton. This plant reproduced by spores and respired through stomata on every surface, and probably photosynthesized in every tissue exposed to light.
Some evidence suggests the presence of primitive predatory arachnids and myriapods in later Silurian rocks. Predatory invertebrates would indicate that simple food webs were in place which included non-predatory prey animals.
Related pages Edit
- Andrew J. Jeram, Paul A. Selden and Dianne Edwards 1990. Land animals in the Silurian: arachnids and myriapods from Shropshire, England. Science 658-61.
- Anna K. Behrensmeyer, John D. Damuth et al. 1992. Terrestrial ecosystems through time. University of Chicago Press.