Multiple collisions of the Earth's plates give rise to a supercontinent called Gondwana. It incorporates South America, Africa, Antarctica, and Western Australia as well as peninsular India and parts of Arabia. The global climate is generally mild. Marine metazoans with mineralized skeletons, such as sponges, bryozoans, corals, brachiopods, molluscs, arthropods, and echinoderms, flourish. One group of arthropods, the trilobites, are particularly dominant in the seas.
Plant life is limited to marine algae.
The end of the Cambrian Period is marked by a huge decline in trilobite numbers, possibly due to a glacial incursion. Ordovician Period 490-443 million years ago North America, Europe, and Africa merge. Shallow seas cover much of North America at the beginning of the period, then recede, leaving a thick layer of limestone. Later in the period, the seas recover North America, depositing quartz, sandstones, and more limestone. Marine invertebrates are still the dominant form of life on Earth.