The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods respectively.
Lapworth recognized that fossils he discovered were different from those of either the Cambrian or the Silurian periods, thus realizing that they should be placed in a period of their own.
Lapworth’s Ordovician Period started 488 million years ago at a major extinction event called the Cambrian–Ordovician extinction that lasted for about 44.6 million years.
Then the Ordovician–Silurian extinction event began about 443.4 million years ago that wiped out 60% of marine life.
This Ordovician-Silurian extinction event occurred when melting glaciers caused the sea level to rise.
Our planet’s life forms rebounded as well due to the sustained re-flooding of the continental shelves and allowed an increase in biodiversity.
Microscopic life (first in the form of trilobites) began to appear in oceans as oxygen levels increased.
Fossils found in shale begin to explain the formations of life.
This Ordovician period of the Paleozoic era created abundant fossils and in some regions, major petroleum and gas reservoirs.
Life continued to flourish during the Ordovician as it had in the Cambrian, even though the end of the Cambrian period was marked by the Cambrian-Ordovician extinction.
Invertebrates, namely mollusks and arthropods, dominated the oceans.
What makes vertebrates unique?
- vertebrates have a central nerve cord that is hollow and runs along the back. The nerve cord is, indeed, enclosed by the spinal vertebrate.
- All vertebrates belong to the phylum Chordata (notochord). In all other phyla, the nerve cord, if it exists, is solid, not hollow, and runs along the abdomen, rather then the back.
- All vertebrates have throats that are perforated by gill slits through which water can be passed. From that water, food can be filtered out and oxygen can be absorbed. These are not present in other phyla. To be sure, in land vertebrates like humans such gill slits do not exist, but if we follow the embryonic development of such vertebrates, we find that at an early stage gill slits begin to develop, but that they wither away. This is true even in the human embryo. There are many such traces of more primitive stages in embryonic development – the human embryo has the beginnings of a tail for a time, as an example. Such things are among the many strong lines of evidence in favor of biological evolution to some extent.
- All vertebrates have, at some time during their embryonic development, an internal stiffing rod of a tough, light, flexible, gelatinous substance that runs down the back. This is what is called a notochord (greek for back string). In all vertebrates this is replaced by vertebrae before the embryonic development is completed, but is always there at first.
Fish, the world’s first true vertebrates, began to evolve, and those with jaws may have first appeared late in the Ordovician period.
Life had yet to diversify on land.
488 million years ago the continent Laurentia (present-day North America), Siberia, and Baltica (present-day northern Europe), was still an independent continent.
Near the beginning of the Ordovician, around 480 million years ago, the microcontinent of Avalonia – a landmass that would become the northeastern United States, Nova Scotia, and parts of current Great Britain, Iberia and the Maghreb – broke free from Gondwana and began its drift towards Laurentia.
Baltica began to move towards Laurentia later in the Ordovician period, causing the Iapetus Ocean to shrink between them.
Baltica, Laurentia, and Avalonia all came together by the end of the Ordovician period to form a minor supercontinent called Euramerica or Laurussia, closing the Iapetus Ocean.
The collision also resulted in the formation of the northern Appalachian Mountains, making them one of the oldest mountain ranges in the world.
Also during the Ordovician, southern continents were collected into a single continent called Gondwana.
At the beginning of the Ordovician period Gondwana was located in equatorial latitudes. As the period progressed, Gondwana drifted towards the South Pole and became glaciated.
The Rheic Ocean between Gondwana and Avalonia was formed as a result.
In the beginning of the Late Ordovician, from 460 to 450 million years ago, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide into the atmosphere, turning the planet into a hothouse.
As with North America and Europe, Gondwana was largely covered with shallow seas during the Ordovician.
Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts.
For most of the later Ordovician period, life continued to flourish, but at or near the end of the period there were additional mass-extinction events.
These mass-extinction events seriously affected planktonic forms like conodonts, graptolites, and some groups of trilobites which completely died out.
The end of the Ordovician period was one of the coldest times in the last 600 million years of earth history.
Though less famous than the Cambrian explosion, the Ordovician featured the Ordovician radiation, that was no less remarkable.
Marine life increased fourfold, resulting in 12% of all known marine fauna.
Another change in marine fauna was the strong increase in filter feeding organisms.
Articulate brachiopods, in particular, largely replaced trilobites in shelf communities. Their success epitomizes the greatly increased diversity of carbonate shell-secreting organisms in the Ordovician compared to the Cambrian.
In North America and Europe, the Ordovician was a time of shallow continental seas rich in life. Trilobites and brachiopods in particular were rich and diverse.
Although solitary corals date back to at least the Cambrian, reef-forming corals appeared in the early Ordovician, corresponding to an increase in the stability of carbonate and thus a new abundance of calcifying animals.
Now-extinct marine animals called graptolites thrived in these oceans, even as new cystoids and crinoids appeared.
It was long thought that the first true vertebrates (fish) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian. However, the very first gnathostome (jawed fish) appeared in the Late Ordovician.
Trilobites in the Ordovician were very different than their predecessors in the Cambrian. Many trilobites developed bizarre spines and nodules to defend against predators such as primitive sharks while other trilobites evolved to become swimming forms.
Some trilobites even developed shovel-like snouts for ploughing through muddy sea bottoms. Other trilobites evolved long eyestalks to assist in detecting predators while other trilobites lost thier eyes and disappeared completely.
The first evidence of green land plants and fungi also appeared.
Terrestrial plants probably evolved from green algae, first appearing as tiny non-vascular forms resembling liverworts.
The green algae were similar to today’s sea moss.
The Ordovician period came to a close in a series of extinction events that, taken together, comprised the second largest of the five major extinction events in Earth’s history.
The extinctions occurred approximately 445–443 million years ago and mark the boundary between the Ordovician and the following Silurian Period.
At that time all complex multicellular organisms lived in the sea, and about 49% of genera of fauna disappeared forever; brachiopods and bryozoans were greatly reduced, along with many trilobite, conodont and graptolite families.
The most commonly accepted theory is that these events were triggered by an Ice age.
Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.
At the end of this short Ice age, melting glaciers caused the sea level to rise and stabilize once more.