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The dry conditions did not affect the evolution of insects, although they were restricted to being close to sources of water. Reptiles continued to thrive and at this time the sail-backed pelycosaurs developed. Mammals are probably descended from these reptiles. The first mammal-like reptiles were the therapsids. Unlike the reptiles, they were small.
At the end of the Permian Period there was the most severe of all the mass extinctions that had occurred during life on earth. Only four out of every hundred species survived. At this time, the rugose corals became extinct, as did nearly all the brachiopods; molluscs were almost wiped out, as were ammonoids. Trilobites were destroyed completely. Three-quarters of all amphibians and four-fifths of all reptiles became extinct, and many plants also disappeared.
The Mesozoic Era The Triassic Period
(Above) Jurassic echinoid Cidaris.
190 million to 225 million years ago
Mass extinction of tabulate corals, ammonoids, amphibians, reptiles greatly reduced in numbers. Gastropods, bivalves, and ammonoids become more common. First scleractinian corals, dinosaurs, marine reptiles and mammals
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The Permian extinction is such a dramatic boundary that it led the early geologists to call the Permian Period the end of the Palaeozoic Era, and the Triassic Period the beginning of a completely new era of life – the Mesozoic Era.
Although all groups were almost wiped out, some recovered more quickly than others. One of the most successful groups was the ammonoids. They were of little importance in the Palaeozoic Era, but during the Triassic Period they developed into
400 genera. The corals developed into modern forms and recovered, molluscs also began to recover. Brachiopods survived but remained few in number.
On land, conifers and seed plants developed. The Petrified Forest of Arizona has trees of this age,
but it was the reptiles that came to dominate in the
(Below) Jurassic ammonite: Dactylioceras.