The Ordovician Period

The Ordovician is the second geological time period of the Palaeozoic era, spanning from 485.4 to 443.4 Ma. The Ordovician is named after the Celtic tribe the Ordovices. This period was the stage for a number of revolutions in the world's flora and fauna.

Ordovician oceanscape. Image credit: dustdevil on DeviantArt

For most of the Ordovician, the climate was warm and wet. This caused the sea levels to rise 600 metres above todays levels, this created new habitats such as inland seas and freshwater areas.

The fauna still dominated the planet's oceans. After the Cambrian extinction, coiled cephalopods, called a Nautilus, exploited the empty niche and became successful and effective predators. A straight cephalopod also evolved to become one of the larger predators of it's time, this was the Orthocone. Trilobites and cnidarians also continued to thrive.

Fossil fish became more abundant in the Ordovician. The jawless mouths of these fish are found positioned on the ventral surface of the head. This suggests that they sucked up their food from the sea floor rather than being active and swift predators. We also find the early evolution of armour plating in fish in the Ordovician, the fish depicted above have bony armour plates on their heads. These fish are the ancestors of lampreys and hagfish that we have today.

Crinoids also find their origins in the Ordovician. They pinnules filtered the Ordovician waters for plankton. To read more about Ordovician crinoids, see my post 'My Collection #1' where I discuss a crinoid fossil that I have.

Life also began to make advances on land as well. Hard bodied arthropods; Eurypterids, also known as sea scorpions. could survive on land for short periods of time. This was due to the ability to diffuse gases across their exoskeleton. The living fossil Horseshoe crabs are also believed to have ventured onto land to spawn as they still do today.

Horseshoe crabs spawning, scenes like this would have
been common during the Ordovician. Image credit:
capeandislands.org

The very first terrestrial plants are seen in the Ordovician. They likely evolved from green algae. They appeared as tiny non vascular plants, that resemble Liverworts. Evidence for these plants comes from not only their fossils, but the fossils of their spores that have been identified in Upper Ordovician sediments. This shows that the plants were immediately exploiting the land, using reproductive methods that allowed for rapid expansion across the barren landscape.

The Ordovician was closed with the second largest mass extinction in the Earth's history. It is believed that this event took place between 447 and 444 Ma.

Chronostratigraphical Timescale of the
Ordovician. Image credit:
keyword-suggestions.com

A massive 49% of marine fauna was pushed to extinction, while other phyla saw individual numbers fall dramatically. This is commonly attributed to an ice age. Research by Page et al states that temperate climates did not return until the end of the Silurian. The ice age's trigger is disputed, the more popular hypothesis is that as Gondwana drifted over the South Pole, ice caps formed over where Africa would have been location on the supercontinent. This locked up vast amounts of the Earth's water, sea levels consequently dropped, destroying shallow sea habitats. The falling temperatures also pushed tropical species to extinction. An alternative hypothesis that was put forward by Melott et al in 2004 suggested that a gamma ray burst of a mere 10 seconds destoryed the ozone layer. This exposed the life on Earth to great amounts of radiation. This radiation is believed to have triggered a sharp fall in global temperatures, trigging an ice age.