Silurian

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Of or belonging to the geologic time, system of rocks, or sedimentary deposits of the third period of the Paleozoic Era, characterized by the development of jawed fishes, early invertebrate land animals, and land plants. See Table at geologic time.
2. n. The Silurian Period or its system of deposits.

Century Dictionary and Cyclopedia

1. Of or belonging to the Silures, a people of ancient Britain, or their country.
2. In geology, of or pertaining to the Silurian. See II.
3. n. A name given by Murchison, in 1835, to a series of rocks the order of succession of which was first worked out by him in that part of England and Wales which was formerly inhabited by the Silures. The various groups of fossiliferous rocks included in the Silurian had, previous to Murchison's labors, been classed together as one assemblage, and called by the Germans grauwacke, sometimes Anglicized into graywacke (which see), also the Transition series or Transition limestone. In England and Germany these lower rocks have been greatly disturbed and metamorphosed, and have also been frequently invaded by eruptive masses; hence it was not until after considerable progress had been made toward a knowledge of the sequence of the higher fossiliferons groups that the lower (now designated as Silurian and Devonian) began to be studied with success. Almost contemporaneously with the working out of the order of succession of these lower rocks by Murchison in Great Britain, groups of strata of the same geological age, but lying for the most part in almost entirely undisturbed position, began to be investigated on and near the Atlantic coast of the United States, especially in New York, by the Geological Survey of that State, and a little later in Bohemia by Joachim Barrande. Murchison, Barrande, and James Hall, paleontologist of the New York Survey, are all agreed as to the adoption of the name Silurian, and in regard to the essential unity of the series or system thus designated. The Silurian is the lowest of the four great subdivisions of the Paleozoic, namely Silurian, Devonian, Carboniferous, and Permian. When undisturbed and unmetamorphosed, the Silurian is usually found to be replete with the remains of organic forms, of which by far the larger part is marine. The Silurian is divided into an Upper and a Lower Silurian, and each of these again is subdivided into groups and subgroups varying in nomenclature in various countries. The line between the Upper and Lower Silurian is drawn in Great Britain at the top of the May Hill sandstone or Upper Llandovery group; in New York, at the top of the Hudson River or Cincinnati group. The almost entire absence of vertebrates and of land-plants, and the paucity of plant-life in general, are the most striking features of Silurian life. The most prominent forms of the animal kingdom were the graptolites, trilobites, and brachiopods, and of these the first-mentioned are the most characteristic of all, since they range through nearly the whole Silurian, and disappear in the Devonian; while the trilobites, which begin at the same time with the graptolites, continue through the Devonian, and end only with the Carboniferous. As the line between the Silurian and Devonian is commonly drawn in England—namely, so as to include in the former the Ludlow group—the first vertebrates, in the form of a low type of fishes, appear near the top of the Upper Silurian; traces of land-animals (scorpions) have also been found in the Upper Silurian of Sweden and Scotland; and in France, in the Lower (?) Silurian, traces of insect life. A scorpion has also been found in the United States, at Waterville, New York, in the Waterlime group, or near the middle of the Upper Silurian. Mr. Whitfield, by whom the specimen was described, inclines to the opinion that the species, for which he instituted a new genus (Proscorpius), was aquatic and not air-breathing, and that it forms a link between the true aquatic forms like Eurypterus and Pterygotus and the true air-breathing scorpions of subsequent periods. He intimates that the same is likely to be true of the Swedish and Scottish Silurian scorpions. The traces of land-plants in the Silurian are rare, and for the most part of doubtful identification. Algæ, on the other hand, are of somewhat frequent occurrence. As the line between Silurian and Devonian is drawn in the United States—namely, between the Oriskany sandstone and the Cauda-galli grit—there are neither land-animals nor fishes in the Silurian; and the evidence of the existence of land plants lower than the Devonian is for the most part of a very doubtful character. The Silurian rocks are widely spread over the globe, with everywhere essentially the same types of animal life. This part of the series is of importance in the United States, especially in the northeastern Atlantic States and in parts of the Mississippi valley.

Wiktionary

1. adj. geology Of a geologic period within the Paleozoic era; comprises the Llandovery, Wenlock, Ludlow and Pridoli epochs from about 439 to 409 million years ago.
2. adj. archaeology Of or related to the Silures, a pre-Roman British tribe.
3. n. geology The Silurian period.

GNU Webster's 1913

1. adj. (Geol.) Of or pertaining to the country of the ancient Silures; -- a term applied to the earliest of the Paleozoic eras, and also to the strata of the era, because most plainly developed in that country.
2. n. The Silurian age.

WordNet 3.0

1. n. from 425 million to 405 million years ago; first air-breathing animals