Silurian period

With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe. The high sea levels of the Silurian and the relatively flat land (with few significant mountain belts) resulted in a number of island chains, and thus a rich diversity of environmental settings.

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 southern continents remained united during this period. 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. The continents of Avalonia, Baltica, and Laurentia 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 two phases of the Tethys— the Proto-Tethys and Paleo-Tethys— the Rheic Ocean, a seaway of the Iapetus Ocean (now in between Avalonia and Laurentia), and the newly formed Ural Ocean.

The Silurian period enjoyed relatively stable and warm temperatures, in contrast with the extreme glaciations of the Ordovician before it, and the extreme heat of the ensuing Devonian. Sea levels rose from their Hirnantian low throughout the first half of the Silurian; they subsequently fell throughout the rest of the period, although smaller scale patterns are superimposed on this general trend; fifteen high-stands can be identified, and the highest Silurian sea level was probably around 140 m higher than the lowest level reached.

During this period, the Earth entered a long, warm greenhouse phase, and warm shallow seas covered much of the equatorial land masses. Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian. 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. Later in the Silurian, the climate cooled slightly, but in the Silurian-Devonian boundary, the climate became warmer.

The climate and carbon cycle appears to be rather unsettled during the Silurian, which has a higher concentration of isotopic excursions than any other period. The Ireviken event, Mulde event and Lau event each represent isotopic excursions following a minor mass extinction and associated with rapid sea-level change, in addition to the larger extinction at the end of the Silurian. Each one leaves a similar signature in the geological record, both geochemically and biologically; pelagic (free-swimming) organisms were particularly hard hit, as were brachiopods, corals and trilobites, and extinctions rarely occur in a rapid series of fast bursts.

The Silurian was the first period to see macrofossils of extensive terrestrial biota, in the form of moss forests along lakes and streams. However, the land fauna did not have a major impact on the Earth until it diversified in the Devonian.

The first fossil records of vascular plants, that is, land plants with tissues that carry food, appeared in the second half of the Silurian period. The earliest known representatives of this group are Cooksonia (mostly from the northern hemisphere) and Baragwanathia (from Australia). Most of the sediments containing Cooksonia are marine in nature. Preferred habitats were likely along rivers and streams. Baragwanathia, appears to be almost as old dating to the Early Ludlow (420 million years) and has branching stems and needle-like leaves of 10-20 cm. The plant shows a high degree of development in relation to its age. As mentioned, fossils of this plant are only found in Australia.

The much-branched Psilophyton was a primitive Silurian land plant with xylem and phloem but no differentiation in root, stem or leaf. It reproduced by spores, had stomata on every surface, and probably photosynthesized in every tissue exposed to light. Rhyniophyta and primitive lycopods were other land plants that first appear during this period. Neither mosses nor the earliest vascular plants had deep roots. Silurian rocks often have a brownish tint, possibly a result of extensive erosion of the early soils.

The first bony fish, the Osteichthyes, appeared, represented by the Acanthodians covered with bony scales; fish reached considerable diversity and developed movable jaws, adapted from the supports of the front two or three gill arches. A diverse fauna of Eurypterids (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. Leeches also made their appearance during the Silurian Period. Brachiopods, bryozoa, molluscs, hederelloids, tentaculitoids, crinoids and trilobites were abundant and diverse.

Reef abundance was patchy; sometimes fossils are frequent but at other points are virtually absent from the rock record.

The earliest known terrestrial animals appear during the Mid Silurian, including the millipede Pneumodesmus. Some evidence also suggests the presence of predatory trigonotarbid arachnoids and myriapods in Late Silurian facies. Predatory invertebrates would indicate that simple food webs were in place that included non-predatory prey animals. Extrapolating back from Early Devonian biota, Andrew Jeram et al. in 1990 suggested a food web based on as yet undiscovered detritivores and grazers on micro-organisms.