Article 21: Perhaps the most groundbreaking occurrence of the Ordovician is the colonization of the land. The end of the Ordovician is a ‘Snowball Earth’ period, the first mass extinction in the history of our planet.
The Ordovician Period of the Earth’s history lasts 40 million years, beginning 485 and ending 445 million years ago.
The Ordovician Period starts at a major extinction event, called the Cambrian–Ordovician extinction event, which occurred approximately 488 million years ago. This early extinction event eliminates many brachiopods and conodonts (resembling eels) and severely reduces the number of trilobite species. It is preceded by the less-documented (but probably worse) End-Botomian extinction event, around 517 million years ago, and the Dresbachian extinction event, about 502 million years ago. The Cambrian-Ordovician extinction event ends the Cambrian Period and leads into the Ordovician Period. The Ordovician itself ends with the Ordovician-Silurian extinction event.
During the Ordovician the Earth experiences a mild climate. The weather is warm and the atmosphere contains a lot of moisture. Sea levels are high. The Ordovician is best known for its diverse marine invertebrates, including graptolites, trilobites, brachiopods and the conodonts, early vertebrates.
Conodont, armed to the teeth.Credit: Parsons – deviantart.com
Conodonts have a long wormlike body, numerous small teeth, and a pair of eyes; they are now believed to be the earliest vertebrates.
The Ordovician is also marked by the appearance of the oldest complete vertebrate fossils. These are jawless, armored fish.
The major global patterns of life undergo tremendous change during the Ordovician. Shallow seas covering much of Gondwana become breeding grounds for new forms of trilobites.
Perhaps the most groundbreaking occurrence of the Ordovician is the colonization of the land. Remains of early terrestrial arthropods are known from this time, as are microfossils of the cells, cuticle (epidermis) and spores of early land plants.
During the Ordovician, the area north of the tropics is almost entirely ocean, the Panthalassic Ocean. Most of the world’s land – southern Europe, Africa, South America, Antarctica, and Australia – is collected together in the super-continent Gondwana.
Throughout the Ordovician, Gondwana shifts towards the South Pole and much of it is submerged under water. When Gondwana finally settles on the South Pole during the Upper Ordovician, massive glaciers form, causing shallow seas to drain and sea levels to drop. This likely causes the mass extinctions that characterize the end of the Ordovician, in which 60% of all marine invertebrate genera and 25% of all families are extinct.
This ‘Snowball Earth’ period is considered the first mass extinction and the second deadliest extinction in the history of life on our planet Earth.
Ordovician-Silurian boundary on Hovedøya, Norway, showing gray Ordovician sandstone and brown Silurian mudstone. The layers have been overturned by the Caledonian orogeny, a mountain building era recorded in the northern parts of Ireland and Britain, the Scandinavian Mountains, Svalbard (Spitsbergen), Eastern Greenland, and parts of North-Central Europe.
Thanks to the University of California, Berkeley, Museum of Paleontology, to Christopher R. Scotese, and to Ivar B. Ramberg’s ‘The Making of a Land; Geology of Norway’