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The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 million years ago (Ma) to the start of the Silurian Period 443.8 Mya.
The Phanerozoic is the current and the latest of the four geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and plant life has proliferated, diversified and colonized various niches on the Earth's surface, beginning with the Cambrian period when animals first developed hard shells that can be clearly preserved in the fossil record. The time before the Phanerozoic, collectively called the Precambrian, is now divided into the Hadean, Archaean and Proterozoic eons.
Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. Recent analyses have favored the idea that the living pterobranch Rhabdopleura represents an extant graptolite which diverged from the rest of the group in the Cambrian. Fossil graptolites and Rhabdopleura share a colony structure of interconnected zooids housed in organic tubes (theca) which have a basic structure of stacked half-rings (fuselli). Most extinct graptolites belong to two major orders: the bush-like sessile Dendroidea and the planktonic, free-floating Graptoloidea. These orders most likely evolved from encrusting pterobranchs similar to Rhabdopleura. Due to their widespread abundance, planktonic lifestyle, and well-traced evolutionary trends, graptoloids in particular are useful index fossils for the Ordovician and Silurian periods.
Chitinozoa are a group of flask-shaped, organic walled marine microfossils produced by an as yet unknown organism. Common from the Ordovician to Devonian periods, the millimetre-scale organisms are abundant in almost all types of marine sediment across the globe. This wide distribution, and their rapid pace of evolution, makes them valuable biostratigraphic markers.
The Hirnantian is the final internationally recognized stage of the Ordovician Period of the Paleozoic Era. It was of short duration, lasting about 1.4 million years, from 445.2 to 443.8 Ma. The early part of the Hirnantian was characterized by cold temperatures, major glaciation, and a severe drop in sea level. In the latter part of the Hirnantian, temperatures rose, the glaciers melted, and sea level returned to the same or to a slightly higher level than it had been prior to the glaciation.
The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species Undulograptus austrodentatus around 467.3 million years ago. It lasted for about 8.9 million years until the beginning of the Sandbian around 458.4 million years ago. This stage of the Ordovician was marked by the beginning of the Andean-Saharan glaciation.
Promissum is an extinct genus of conodonts, primitive chordates, that lived during the Upper Ordovician period.
In the geological timescale, the Llandovery Epoch occurred at the beginning of the Silurian Period. The Llandoverian Epoch follows the massive Ordovician-Silurian extinction events, which led to a large decrease in biodiversity and an opening up of ecosystems.
Dendrograptus is an extinct genus of graptolites from the Lower Ordovician.
Didymograptus is an extinct genus of graptolites with four rows of cups. They lived during the Middle Ordovician, to Late Ordovician.
Tetragraptus is an extinct genus of graptolites from the Ordovician period.
Constellaria is an extinct genus of bryozoan from the Middle Ordovician to Early Silurian from North America, Asia and Europe. These branching coral-like bryozoans formed bushy colonies 10-15 mm across on the seabed. The fairly thick branches were erect, often compressed in one direction, and covered with distinctive tiny, star-shaped mounds called maculae or monticules. Feeding zoids were located along the rays of the stars. The maculae probably formed "chimneys" for the expulsion of exhalant feeding currents from the surface of a colony, after water had been filtered to obtain food for the organisms.
Conocardium is an extinct genus of Rostroconchian mollusk. Its shell-mouth grew to be 2–3 inches (51–76 mm) across. It fed on tiny plants and animals in the water. Fossils have been found all over the world from Ordovician to Permian formations.
Isograptus is an extinct genus of graptolites from the Ordovician.
Goniograptus is an extinct genus of graptolites.
Paleontology in Maine refers to paleontological research occurring within or conducted by people from the U.S. state of Maine. The fossil record of Maine is very sparse. Maine came into existence during the Ordovician as other ancient land masses accreted onto North America. At the time Maine was covered by a sea inhabited by a menagerie of invertebrates which included graptolites. During the Devonian, geologic uplift raised Maine above sea level. Early land plants flourished in the terrestrial environments. There is a gap in the local rock record spanning the remainder of the Paleozoic, the Mesozoic, and the Tertiary period of the Cenozoic era. During the Ice Age, Maine was varyingly covered by glaciers or seawater. The Devonian Pertica plant, Pertica quadrifaria, is the Maine state fossil.
Paleontology in Missouri refers to paleontological research occurring within or conducted by people from the U.S. state of Missouri. The geologic column of Missouri spans all of geologic history from the Precambrian to present with the exception of the Permian, Triassic, and Jurassic. Brachiopods are probably the most common fossils in Missouri.
Paleontology in Minnesota refers to paleontological research occurring within or conducted by people from the U.S. state of Minnesota. The geologic record of Minnesota spans from Precambrian to recent with the exceptions of major gaps including the Silurian period, the interval from the Middle to Upper Devonian to the Cretaceous, and the Cenozoic. During the Precambrian, Minnesota was covered by an ocean where local bacteria ended up forming banded iron formations and stromatolites. During the early part of the Paleozoic era southern Minnesota was covered by a shallow tropical sea that would come to be home to creatures like brachiopods, bryozoans, massive cephalopods, corals, crinoids, graptolites, and trilobites. The sea withdrew from the state during the Silurian, but returned during the Devonian. However, the rest of the Paleozoic is missing from the local rock record. The Triassic is also missing from the local rock record and Jurassic deposits, while present, lack fossils. Another sea entered the state during the Cretaceous period, this one inhabited by creatures like ammonites and sawfish. Duckbilled dinosaurs roamed the land. The Paleogene and Neogene periods of the ensuing Cenozoic era are also missing from the local rock record, but during the Ice Age evidence points to glacial activity in the state. Woolly mammoths, mastodons, and musk oxen inhabited Minnesota at the time. Local Native Americans interpreted such remains as the bones of the water monster Unktehi. They also told myths about thunder birds that may have been based on Ice Age bird fossils. By the early 19th century, the state's fossil had already attracted the attention of formally trained scientists. Early research included the Cretaceous plant discoveries made by Leo Lesquereux.
The geological history of North America comprises the history of geological occurrences and emergence of life in North America during the interval of time spanning from the formation of the Earth through to the emergence of humanity and the start of prehistory. At the start of the Paleozoic Era, what is now "North" America was actually in the Southern Hemisphere. Marine life flourished in the country's many seas, although terrestrial life had not yet evolved. During the latter part of the Paleozoic, seas were largely replaced by swamps home to amphibians and early reptiles. When the continents had assembled into Pangaea, drier conditions prevailed. The evolutionary precursors to mammals dominated the country until a mass extinction event ended their reign.
Variabiloconus is an extinct genus of conodonts.
References
- ↑ R. A. Cooper & R. A. Fortey (1982). "The Ordovician graptolites of Spitsbergen". Bulletin of the British Museum (Natural History), Geology. 36: 1–171.
- ↑ Don Lessem & Jan Sovak (1999). Dinosaurs to Dodos: An Encyclopedia of Extinct Animals. Scholastic. ISBN 0-590-31684-2.
- ↑ Dichograptus at Fossilworks.org
