The Ordovician period was named by by Lapworth in 1879 after an ancient Celtic tribe (i.e., the Ordovices) that once inhabited the region in Wales where rock strata of this period occur. This period was originally established by Lapworth to resolve the Murchison-Sedgwick conflict over their overlapping claims for their Silurian and Cambrian systems. This period witnessed the origin and rapid evolution of many new types of invertebrate animals which replaced their Cambrian predecessors. The end of the Ordovician is marked by an extinction event which eliminated 60% of all marine invertebrate genera and 25% of all families. One theory for the cause of the extinction holds that when Gondwana finally settled on the South Pole during the Late Ordovician, the resulting glaciers drained the shallow seas causing sea levels to drop.
Tectonics and Paleoclimate
During the Ordovician, Southern Europe, Africa, South America, Antarctica and Australia remained joined together into the super continent of Gondwanaland which had moved down to the South Pole and was widely glaciated. North America straddled the equator. Western and Central Europe were separate from the rest of Eurasia and were in the southern tropics. Scotland and England are united into a single landmass. North America is engaged in a slow collision with the micro continent of Baltica which forms the core of what is later to become Europe. The glaciation causes global temperatures to drop as the period progresses and the world enters an ice age although conditions remain mild and equitable in the tropics. There were widespread shallow, warm seas throughout the Ordovician, especially during the middle epoch.
During the Ordovician, creeping lichens and bryophytes move onto land for the first time in Earth’s history.
The Ordovician was an age of evolutionary experimentation and adaptive radiation in which new organisms evolved to replace those that died out at the end of the Cambrian and spread out to populate an unprecedented amount of sea ecosystems. The widespread shallow, warm continental seas were the perfect environment for many groups of organisms. Micro-organisms such as colonial blue-green algae (i.e., stromatolites) are widespread, foraminifera (marine amoebas which build tiny shells) evolve for the first time, acritarchs become more common, and stromatoporoids which are thought to be sponge-like organisms appear for the first time.
There is a dramatic shift from crawling mud-grubber animals that were common during the Cambrian to filter feeding organisms in response to what many believe was a significant increase in the amount of micro-plankton in the seas. The corals (e.g., rugose (solitary) and tabulate (colonial) forms), bivalve mollusks, and the planktonic graptolites, and bivalve mollusks make their appearance in the oceans. The lophophorates, Bryozoa, and articulate brachiopods appear in large numbers. Small and rare in the late Cambrian, the nautiloids evolve quickly along many different lines, most of which appear for the first time during the early or middle part of the Ordovician. These relatively intelligent, carnivorous mollusks replaced the Cambrian Anomalocarids as the dominant life form and top predator of the world's ocean. Up until the conclusion of the Ordovician, the biggest mollusks (e.g., the endocerids) had shells that were up to 10 meters in length making them the largest animals that had ever lived.
Ordovician trilobites were abundant and varied in form including those with bizarre spines and nodules, those that moved via pelagic nektonic swimming, those with huge eyes, those with no eyes, those with shovel-like snouts for plowing through mud, those with fused body segments, and those with broad pitted margins around the head shield. The Ordovician was the high point of the graptolites which were colonial hemichordates that possessed an anatomical structure suggestive of a partial spinal cord. These creatures are considered to be vertebrate cousins rather than vertebrate ancestors.
The vertebrate ostracoderms remain rare although several different groups of Pteraspidomorphi evolve. As did their cousins, the eel-like conodonts that were a major and probably predatory component, in the marine food-chain.
Meteorite Impacts on Earth
I included a list of meteorite impacts relevant to this time period as a point of reference since many of the explanations for mass extinctions throughout Earth’s history include meteorite impact(s) as a possible cause. The meteorite impact information below was obtained from the ‘Earth Impact Database’ maintained by the Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada ( www.passc.net/EarthImpactDatabase).