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Ordovician Period (485-443 million years ago) was a time of renewed animal diversification, especially the evolution of heavily skeletonized animals in the world's oceans.

The number of genera recorded by fossils increased fivefold, suggesting that species diversity might have increased by an order of magnitude (most genera contain multiple species). The Ordovician radiation established a marine ecosystem that persisted for more than 200 million years.
Land plants evolved during the Ordovician Period



At the end of the Permian Period, 252 million years ago, environmental catastrophe eliminated most genera in the oceans. Paleozoic coral-like cnidarians become extinct, as did the trilobites. Brachiopods survived as a group, but most species disappeared. As noted in Chapter 23, the trigger for this devastation was massive volcanism that unleashed global warming, ocean acidification, and oxygen loss from subsurface oceans.

Bivalves and gastropods diversified; new groups of arthropods radiated, including the ancestors of the crabs and shrimps we see today; and surviving cnidarians evolved a new capacity to make skeletons of calcium carbonate

Flies, beetles, bees, wasps, butterflies, and moths radiated later, beginning in the early Mesozoic Era. Their rise in diversity parallels that of the flowering plants and reflects coevolution of flowering plants and the insects that both pollinate and feed on them.

Dominating terrestrial ecosystems until the end of the Cretaceous Period, 66 million years ago, when another mass extinction, caused by a catastrophic asteroid impact, eliminated nearly all dinosaur species