Ordovician Period
The Ordovician is a geological period that spans 41.6 million years, from the end of the Cambrian Period to the start of the Silurian Period. It was named after the Welsh tribe of the Ordovices and was defined by Charles Lapworth in 1879 to resolve a dispute between geologists who were placing the same rock beds in North Wales in the Cambrian and Silurian systems. The period was marked by significant changes in life on Earth, including the first appearance of land plants and the establishment of fish with jaws. The Ordovician also saw the Great Ordovician Biodiversification Event, which increased the diversity of life on Earth. Additionally, the period was marked by a significant increase in meteorite strikes, with about 100 times as many strikes per year compared to today.
World Map
The Ordovician period was marked by significant geological activity, with the southern continents assembled into Gondwana, a massive landmass that stretched from the equator to the South Pole. The Panthalassic Ocean, centered in the northern hemisphere, covered over half the globe, with smaller continents such as Laurentia, Siberia, and Baltica separated from Gondwana by a significant distance of ocean. Avalonia, a small continent, rifted from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean. Towards the end of the period, Avalonia collided with Baltica, reflecting the complex and dynamic nature of the Earth's crust.
The Ordovician period was also marked by extensive tectonism and volcanism, with mountain-building occurring along active continental margins. The Taconic orogeny, which began in Cambrian times, continued into the Ordovician period, with the collision of volcanic island arcs and the formation of the Appalachian Mountains. The period was characterized by episodic switching from extension to compression, with accretion of new crust limited to the Iapetus margin of Laurentia.
Tectonic activity was also widespread along the northwest margin of Gondwana, with the accretion of island arcs to eastern Australia and subduction in Argentina. In addition, there was significant back-arc rifting in these regions. The interior of Gondwana was tectonically quiet until the Triassic period.
One notable event during this period was the ash fall of the Millburg/Big Bentonite bed, which was the largest in the last 590 million years. Remarkably, this event had a relatively small impact on life. Towards the end of the period, Gondwana began to drift across the South Pole, contributing to the Hibernian glaciation and an associated extinction event. Overall, the Ordovician period was marked by significant geological activity, reflecting the complex and dynamic nature of the Earth's crust.
Climate
The Early Ordovician period was marked by a very hot climate, with intense greenhouse conditions and sea surface temperatures comparable to those during the Early Eocene Climatic Optimum. As the period progressed, the climate cooled, leading to a more temperate climate in the Middle Ordovician. This cooling trend eventually gave way to the Early Palaeozoic Ice Age, which may have begun as early as the Darriwilian or Floian stage.
In addition to the changing climate, sea levels played a significant role in the Early Ordovician. During this period, sea levels rose continuously throughout the Early Ordovician, with some local regressions. This led to the formation of many shelf deposits under hundreds of meters of water. However, as the climate cooled, sea levels began to fall steadily over about 3 million years leading up to the Hirnantian glaciation. Despite this trend, there were brief periods of warming, such as the Boda Event, which led to the deposition of bioherms and radiation of fauna across Europe.
The geography of the Earth during the Ordovician period was also characterized by extensive oceans and shallow seas. Gondwana was largely covered in shallow seas during this period, while the Panthalassic Ocean covered much of the Northern Hemisphere. Other minor oceans included Proto-Tethys, Paleo-Tethys, Khanty Ocean, Iapetus Ocean, and the Rheic Ocean. The shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts.
Flora
Green algae were abundant in the Late Cambrian period and continued to thrive during the Ordovician period. It's likely that terrestrial plants evolved from these green algae, and the first plants to colonize land likely appeared in the middle to late Ordovician period, around 450-440 million years ago. These early plants were likely tiny, non-vascular forms that resembled liverworts.
The colonization of land was likely limited to shorelines, where plants could have established themselves in the early days. Fossil spores found in Ordovician sedimentary rock are similar to those of bryophytes, which are non-vascular plants. This suggests that the early land plants were similar to modern-day bryophytes, such as mosses and liverworts.
One type of fungus, arbuscular mycorrhiza fungi (Glomerales), may have played a crucial role in facilitating the colonization of land by plants. Through a process called mycorrhizal symbiosis, these fungi form relationships with plant cells, making mineral nutrients available to them. Fossilized fungal hyphae and spores from the Ordovician period have been found, dating back around 460 million years ago. This suggests that these fungi may have been present on land during the early days of plant colonization, helping plants to thrive and establish themselves in their new environment.
Fauna
The Ordovician period, which lasted from 485 to 443 million years ago, saw a remarkable explosion of life on Earth, known as the Ordovician radiation. This event was characterized by the rapid diversification of marine faunas, with many new groups of organisms emerging. The fauna was dominated by tiered communities of suspension feeders, with short food chains. Many of the organisms that emerged during this period have persisted until the present day.
One of the most notable features of the Ordovician fauna was the miniaturization of many species, which became much smaller than their Cambrian counterparts. Filter-feeding organisms, such as brachiopods and crinoids, also became more diverse and widespread. Trilobites, which were abundant in the Cambrian, continued to thrive and evolve, with many new species developing bizarre spines and nodules to defend against predators.
The Ordovician period also saw the emergence of new groups of organisms, such as articulate brachiopods, cephalopods, and graptolites. Reef-forming corals appeared for the first time, and molluscs, including bivalves, gastropods, and nautiloid cephalopods, became common and varied. Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments.
The Ordovician period also saw the development of more complex ecosystems, with a greater diversity of organisms and more complex food chains. The introduction of bioeroding organisms, such as sponges and worms, marked a significant shift in the way that organisms interacted with their environment. The Ordovician period was a time of great change and diversification on Earth's oceans, and it laid the foundation for the evolution of many of the marine species that we see today.
Etymology
The Ordovician period was named by Charles Lapworth in 1879 to resolve a dispute between geologists Adam Sedgwick and Roderick Murchison, who were disagreeing on the classification of rock layers in North Wales. Lapworth recognized that the fossil finds in the disputed areas were distinct from those of the Cambrian and Silurian periods, and therefore created a new system. The Ordovician period received international recognition 40 years after Lapworth's death, when it was officially adopted as a period of the Paleozoic Era by the International Geological Congress in 1960.
Stratigraphy
The Ordovician period was named by British geologist Charles Lapworth in 1879, after an ancient Celtic tribe known for their resistance to Roman domination. The period is characterized by the presence of graptolites, extinct planktonic organisms, which are often used to correlate Ordovician strata. The boundaries between the Ordovician and surrounding periods, Cambrian and Silurian, were determined using radiometric dating and are marked by the appearance of specific graptolite species. The type locality for the boundary between the Ordovician and Silurian is Dobs Linn, near Moffat, Scotland, where black graptolite-bearing shales are exposed.
The Ordovician period is divided into several epochs and series, each with a type location in Britain where characteristic faunas can be found. The period is characterized by the presence of lime and other carbonate rocks that accumulated in shallow subtidal and intertidal environments, as well as quartzites and dark, organic-rich mudstones that contain graptolites and iron sulfide seams. These rocks are commonly found in areas such as China, Western Australia, Argentina, the United States, and Canada. The Ordovician period is part of the Paleozoic Era, which includes the Cambrian and Silurian periods.
Sources
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