The Silurian period only lasted 30 or so million years, but this period of geologic history witnessed at least three major innovations in prehistoric life: the appearance of the first land plants, the subsequent colonization of dry land by the first terrestrial invertebrates, and the evolution of jawed fish, a huge evolutionary adaptation over previous marine vertebrates. The Silurian was the third period of the Paleozoic Era (542-250 million years ago), preceded the Cambrian and Ordovician periods and succeeded by the Devonian, Carboniferous and Permian periods.
Climate and Geography
Experts disagree about the climate of the Silurian period; global sea and air temperatures may have exceeded 110 or 120 degrees Fahrenheit, or they might have been more moderate ("only" 80 or 90 degrees). During the first half of the Silurian, much of the earth's continents were covered by glaciers (a holdover from the end of the preceding Ordovician period), with climatic conditions moderating by the start of the ensuing Devonian. The giant supercontinent of Gondwana (which was destined to break apart hundreds of millions of years later into Antarctica, Australia, Africa and South America) gradually drifted into the far southern hemisphere, while the smaller continent of Laurentia (the future North America) straddled the equator.
Marine Life During the Silurian Period
Invertebrates. The Silurian period followed the first major global extinction on earth, at the end of the Ordovician, during which 75 percent of sea-dwelling genera went extinct. Within a few million years, though, most forms of life had pretty much recovered, especially arthropods, cephalopods, and the tiny organisms known as graptolites. One major development was the spread of reef ecosystems, which thrived on the borders of the earth's evolving continents and hosted a wide diversity of corals, crinoids, and other tiny, community-dwelling animals. Giant sea scorpions - such as the three-foot-long Eurypterus - were also prominent during the Silurian, and were by far the biggest arthropods of their day.
Vertebrates. The big news for vertebrate animals during the Silurian period was the evolution of jawed fish like Birkenia and Andreolepis, which represented a major improvement over their predecessors of the Ordovician period (such as Astraspis and Arandaspis). The evolution of jaws, and their accompanying teeth, allowed the prehistoric fish of the Silurian period to pursue a wider variety of prey, as well as to defend themselves against predators, and was a major engine of subsequent vertebrate evolution as the prey of these fish evolved various defenses (like greater speed). The Silurian also marked the appearance of the first identified lobe-finned fish, Psarepolis, which was ancestral to the pioneering tetrapods of the ensuing Devonian period.
Plant Life During the Silurian Period
The Silurian is the first period for which we have conclusive evidence of terrestrial plants - tiny, fossilized spores from obscure genera like Cooksonia and Baragwanathia. These early plants were no more than a few inches high, and thus possessed only rudimentary internal water-transport mechanisms, a technique that took tens of millions of years of subsequent evolutionary history to develop. Some botanists speculate that these Silurian plants actually evolved from freshwater algae (which would have collected on the surfaces of small puddles and lakes) rather than ocean-dwelling predecessors.
Terrestrial Life During the Silurian Period
As a general rule, wherever you find terrestrial plants, you'll also find some kinds of animals. Paleontologists have found direct fossil evidence of the first land-dwelling millipedes and scorpions of the Silurian period, and other, comparably primitive terrestrial arthropods were almost certainly present as well. However, large land-dwelling animals were a development for the future, as vertebrates gradually learned how to colonize dry land.
Next: the Devonian Period