Ordovician Time

The Ordovician period started at a major extinction event called the Cambrian-Ordovician extinction events some time about 488.3 ± 1.7 million years ago (Mya) and lasted for about 44.6 million years. It ended with another major extinction event about 443.7 ± 1.5 Ma (ICS, 2004) that wiped out 60% of marine genera. Melott et al. (2006) suggested a ten-second gamma ray burst could have destroyed the ozone layer and exposed terrestrial and marine surface-dwelling life to deadly radiation, but most scientists agree that extinction events are complex with multiple causes.

The dates given are recent radiometric dates and vary slightly from those used in other sources. This second period of the Paleozoic era created abundant fossils and in some regions, major petroleum and gas reservoirs.

Though less famous than the Cambrian explosion, the Ordovician featured an adaptive radiation that was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna. The trilobite, inarticulate brachiopod, archaeocyathid, and eocrinoid faunas of the Cambrian were succeeded by those which would dominate for the rest of the Paleozoic, such as articulate brachiopods, cephalopods, and crinoids; 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. The first bryozoa appeared in the Ordovician as did the first coral reefs. Solitary corals date back to at least the Cambrian. Molluscs, which had also appeared during the Cambrian or the Ediacaran, became common and varied, especially bivalves, gastropods, and nautiloid cephalopods. It was long thought that the first true vertebrates (fish - Ostracoderms) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian. The very first jawed fish appeared in the Late Ordovician epoch. Now-extinct marine animals called graptolites thrived in the oceans. Some cystoids and crinoids appeared.

During the Middle Ordovician there was a large increase in the intensity and diversity of bioeroding organisms. This is known as the Ordovician Bioerosion Revolution. It is marked by a sudden abundance of hard substrate trace fossils such as Trypanites, Palaeosabella and Petroxestes.

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 and Nautiloid cephalopods while other trilobites such as Aeglina prisca evolved to become swimming forms. Some trilobites even developed shovel-like snouts for ploughing through muddy sea bottoms. Another unusual clade of trilobites known as the Trinucleids developed a broad pitted margin around their head shields.

Other trilobites such as (Asaphus kowalewski) evolved long eyestalks to assist in detecting predators while some trilobite eyes by contrast took the opposing evolutionary direction and disappeared completely.