Club mosses and other primitive land plants are abundant in this period. Some arthropods begin to live on the land.
The first great mass extinction event took place about 434 million years ago, when according to the fossil record, 60% of all genera of both terrestrial and marine life worldwide were exterminated.
The Silurian system was first identified by Sir Roderick Murchison, who was examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s. He named the sequences for a Celtic tribe of Wales, the Silures, extending the convention his friend Adam Sedgwick had established for the Cambrian. In 1835 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales, which was the germ of the modern geological time scale. As it was first identified, the "Silurian" series when traced farther afield quickly came to overlap Sedgwick's "Cambrian" sequence, however, provoking furious disagreements that ended the friendship. Charles Lapworth eventually resolved the conflict by defining a new Ordovician system including the contended beds.
The French geologist Joachim Barrande, building on Murchison's work, used the term Silurian in a more comprehensive sense than was justified by subsequent knowledge. He divided the Silurian rocks of Bohemia into eight stages. His interpretation was questioned in 1854 by Edward Forbes, and the later stages of Barrande, F, G and H, have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the oldest fossils.
Scorpions are thought to be the oldest arachnids, appearing around 430 million years ago. Some marine species were huge, with one recently discovered fossil specimen estimated to be some 8.2 feet (2.5 meters) long. Scientists think this species, Jaekelopterus rhenaniae, was likely too large to have emerged from the water. But its smaller cousin, Brontoscorpio, which measured about three feet (one meter) in length, likely made short forays ashore when molting its shell. In time, fully terrestrial arachnids would emerge.
Plants began to take root on land about 430 million years ago. The transition from water presented plants with a difficult evolutionary challenge. Their solution was a vascular system of tubelike tissues for transporting water and nutrients on dry land. Vascular plants also developed rigid stems for standing upright. The earliest species were confined to marshy ground, growing only to ankle height. One of the first known fossil examples was Cooksonia, which had leafless, branching stems with rounded spore sacs at their tips.
The Silurian Period of time is usually broken into early (Llandovery and Wenlock) and late (Ludlow and Pridoli) subdivisions (epochs). Nevertheless, some schemes use an early (Llandovery), middle (Wenlock) and late (Ludlow and Pridoli) breakdown. These faunal stages are characterized by their index fossils, new species of colonial marine Graptolites that appeared in each. Epochs of time correspond to series of rocks (as periods of time correspond to systems of rocks), which are referred to as belonging to the lower, middle, or upper part of the rock column, analogous to early, middle, or late Silurian time. The epochs and stages from youngest to oldest are:
- Pridoli Epoch - no stages defined (late Silurian)
- Ludlow Epoch divided into
- Ludfordian (late Ludlow - late Silurian)
- Gorstian (early Ludlow - late Silurian)
- Wenlock Epoch divided into
- Homerian (late Wenlock - early or middle Silurian)
- Sheinwoodian (early Wenlock - early or middle Silurian)
- Llandovery Epoch divided into
- Telychian (late Llandovery - early Silurian)
- Aeronian (mid Llandovery - early Silurian)
- Rhuddanian (early Llandovery - early Silurian)
In North America a different suite of regional stages is used:
- Cayugan (Late Silurian - Ludlow)
- Lockportian (Middle Silurian - Wenlock)
- Tonawandan (Middle Silurian - Wenlock)
- Ontarian (Early Silurian - Llandovery)
- Alexandrian (Early Silurian - Llandovery)
During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late Ordovician glaciation. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity. Other cratons and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as Euramerica.
When the proto-Europe collided with North America, the collision folded coastal sediments that had been accumulating since the Cambrian off the east coast of North America and the west coast of Europe. This event is the Caledonian orogeny, a spate of mountain building that stretched from New York State through conjoined Europe and Greenland to Norway. At the end of the Silurian, sea levels dropped again, leaving telltale basins of evaporites in a basin extending from Michigan to West Virginia, and the new mountain ranges were rapidly eroded. The Teays River, flowing into the shallow mid-continental sea, eroded Ordovician strata, leaving traces in the Silurian strata of northern Ohio and Indiana.
The vast ocean of Panthalassa covered most of the northern hemisphere. Other minor oceans include, Proto-Tethys, Paleo-Tethys, Rheic Ocean, a seaway of Iapetus Ocean (now in between Avalonia and Laurentia), and newly formed Ural Ocean.
During this period, the Earth entered a long warm greenhouse phase, and warm shallow seas covered much of the equatorial land masses. The period witnessed a relative stabilization of the Earth's general climate, ending the previous pattern of erratic climatic fluctuations. Layers of broken shells (called coquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces.
Silurian high sea levels and warm shallow continental seas provided a hospitable environment for marine life of all kinds. Silurian beds are oil and gas producers in some areas. Extensive beds of Silurian hematite -- an iron ore -- in eastern North America were important to the early American colonial economy.
Coral reefs made their first appearance during this time, built by extinct tabulate and rugose corals. The first bony fish, the Osteichthyes appeared, represented by the Acanthodians covered with bony scales; fishes reached considerable diversity and developed movable jaws, adapted from the supports of the front two or three gill arches. A diverse fauna of Eurypterus (Sea Scorpions) -- some of them several meters in length -- prowled the shallow Silurian seas of North America; many of their fossils have been found in New York State. Brachiopods, bryozoa, molluscs, and trilobites were abundant and diverse.
Myriapods became the first proper terrestrial animals. The terrestrial ecosystems included the first multicellular terrestrial animals that have been identified, relatives of modern spiders and millipedes whose fossils were discovered in the 1990s.
Bony and cartilaginous fish had a common ancestor, they split apart around 420 million years ago.
The first fossil records of vascular plants, that is, land plants with tissues that carry food, appeared in 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, reproducing by spores and breathing through stomata on every surface, and probably photosynthesizing in every tissue exposed to light. Rhyniophyta and primitive lycopods were other land plants that first appear during this period.
The first land fungi probably appeared by this time.
The first face
Entelognathus primordialis is one of the earliest, and certainly the most primitive, fossil fish that has the same jawbones as modern bony fishes and land vertebrates including ourselves. A 419-million-year-old fish fossil some consider the earliest fossil of a face.
- The Columbia History of the World, Harper & Row, 1972