The Ordovician period (500 to 440 million years ago) comes after the Cambrian in the early Paleozoic era. The period is named for a Celtic tribe named the Ordovices who once lived in the area of Wales (in Britain) where the rocks were first studied. Ordovician limestones are over 6.4 kilometers (4 miles) thick in places and are found on all continents except Antarctica. The uniformity and thickness of the bed indicates a long period of warm and stable climate that allows them to develop.
In fact, the Ordovician period was as remarkable for the diversity of its species as the Cambrian period was for the appearance of most major phyla . A burst of evolutionary creativity in shape, size, and function tripled the number of marine species that appeared. Specialization became the dominant theme of life, with new forms filling every possible niche .
The appearance of highly efficient predators such as the nautiloids and the lobster-size sea scorpions forced the marine community to evolve protective strategies or disappear. Various species responded by developing larger size, thicker shells, or more elaborate defenses. A proliferation in the shapes of the shells of bivalve mollusks allowed them to burrow deeply into sand or mud. Other mollusks learned to swim freely by rapidly clapping their valves together. And still others developed intricate teeth-and-socket arrangements that allowed them to close so tightly that they were almost impossible to open.
Exploring the oceans of the Ordovician world would have been quite similar to exploring the oceans of today. Sea urchins, starfish, and sea lilies lived in profusion among the rocks. The first great coral reefs appeared and gave shelter to crustaceans of all kinds. Sea mats, sea snails, and sea cucumbers abounded in the tide pools. A huge diversity of bivalve mollusks made their slow way across the muddy ocean floor, leaving their tracks and burrows in the fossil record .
|Era||Period||Epoch||Million Before Years Present|
The very first primitive fishes appeared, slow and heavily armored, without fins or heads with brains. These agnathans (jawless fishes) were the first animals to have a notochord (flexible rod spine), a precursor of a true spinal chord. These chordates were the ancestor of all animals with backbones.
While almost all animals of the Ordovician were marine, another remarkable occurrence is recorded in the rocks of northwest England. There, arthropods (animals with jointed legs) that lived in shallow, freshwater pools left the first tracks in fossilized mud. Scientists speculate that evaporation of their pools forced these centipede-like creatures to adapt to terrestrial conditions. From this point on, the arthropods, a group that includes insects, spiders, and crabs, ruled the land for 40 million years.
The massive Ordivician limestone ends abruptly with a jumble of glacial till, indicating an ice age that so disrupted Earth's climate that more than half of all species became extinct. This first great extinction wiped out huge numbers of trilobites , with their precise and sensitive eyes, brachiopods , crinoids , and other marine invertebrates . The life-forms that survived the cataclysmic end of the Ordovician contributed to the genetic makeup of the animal kingdom to the present.
see also Geological Time Scale.
Fortey, Richard. Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Viking Press, 1998.
Gould, Stephen Jay, ed. The Book of Life. New York: W. W. Norton & Company, 1993.
Lambert, David. The Field Guide to Prehistoric Life. New York: Facts on File, 1985.
In geologic time , the Ordovician Period, the second period of the Paleozoic Era , covers the time roughly 505 million years ago (mya) until 438 mya. The name Ordovician derives from that of the Ordovices, an ancient British tribe.
The Ordovician Period spans three epochs. The Lower Ordovician Epoch is the most ancient, followed in sequence by the Middle Ordovician Epoch, and the Upper Ordovician Epoch. The Ordovician Period is divided chronologically (from the most ancient to the most recent) into the Tremadocian, Arenigian, Llanvirnian, Llandeilian, Caradocian and Ashgillian stages.
Much of the continental crust that exists now had already been formed by the time of the Ordovician Period and the forces driving plate tectonics actively shaped the fusing continental landmasses. Near the margins of the continental landmasses, extensive orogeny (mountain building) allowed the development of mountain chains .
The fossil record provides evidence to support the demarcation of the preceding Cambrian Period from the Ordovician Period. Drastic changes of sea levels resulted in massive extinctions among marine organisms. In accord with a mass extinction, many fossils dated to the Cambrian Period are not found in Ordovician Period formations.
The fossil record establishes that vertebrates existed during the Ordovician Period. As with the Cambrian Period, the Ordovician Period ended with a mass extinction of nearly a third of all species. This mass extinction, approximately 438 mya, marked the end of the Ordovician Period and the start of the Silurian Period .
Although there is no evidence of an occurrence equivalent to the K-T event , it is possible that an impact from a large meteorite may have been responsible for the mass extinction marking the end of the Cambrian Period and start of the Ordovician Period. Impact craters dating to the Ordovician Period have been identified in Australia .
See also Archean; Cenozoic Era; Cretaceous Period; Dating methods; Devonian Period; Eocene Epoch; Evolution, evidence of; Fossils and fossilization; Historical geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Miocene Epoch; Mississippian Period; Oligocene Epoch; Paleocene Epoch; Pennsylvanian Period; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Proterozoic Era; Quaternary Period; Tertiary Period; Triassic Period