GEOLOGY

Detailed explanation-1: -The more likely cause is that the Earth cooled, particularly the oceans where most of the organisms lived during the Ordovician (Remember there were no land plants and no evidence of land organisms yet).

Detailed explanation-2: -The Ordovician is best known for its diverse marine invertebrates, including graptolites, trilobites, brachiopods, and the conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, primitive fish, cephalopods, corals, crinoids, and gastropods.

Detailed explanation-3: -Around 443 million years ago, 85% of all species on Earth went extinct in the Ordovician-Silurian extinction. The extinction was a most likely a result of global cooling and reduced sea levels, which dramatically impacted the many marine species living in warm, shallow coastal waters.