from 248 to 65 million years ago

Introduction: At the beginning of the Mesozoic Era all the continents moved together to form a huge supercontinent, Pangaea. This meant, for example, that animals of the Mesozoic living in what is today east Africa could stroll over to Brazil without getting their feet wet! As time went on, Pangaea split up and continents went their own way, severing links among previously connected land masses. Although during most of the Paleozoic Era the land was barren, by the beginning of the Mesozoic there were forests and swamps; and plants dominated the land. Animals on land and in the ocean, however, were struggling to recover from the great Permian extinction which marks the end of the Paleozoic Era. The mammal-like reptiles (therapsids) that flourished in the Permian Period, had been decimated in the extinction, yet a few species held on until the middle of the Mesozoic when their kind left the Earth forever. But other animals recovered their numbers and/or evolved into a great variety of different forms, claiming niches left vacant by extinct species.

Life: In the ocean, many new kinds of fishes appeared as well as swimming reptiles; and mollusks became very prominent. It was during this time of rapid species radiation that the first mammals appeared. They were small and hid in the shadows of the great dinosaurs which already dominated the land. Dinosaurs emerged during the early Mesozoic and were very diversified by the end of the Mesozoic:   they ranged from small, foraging species to enormous herbivores and carnivores.

Plants also came into their own. By the end of the Mesozoic, flowering plants arrived and quickly overran the dominating conifers as rulers of the land flora. This shift was not only aesthetic (by our human sensibilities!) but it was a major step in evolution. The reproductive strategy of conifers is to produce as much pollen as possible, depend on the wind to blow it in all directions, and hope for the best. The newly evolved flowering plants, however, depended primarily on insects and other animal vectors to disperse their pollen for reproduction. This method was less random and the plants did not have to put so much energy into making huge amounts of pollen. Thus the co-evolution of species became an important evolutionary pressure.

The first birds, evolved from dinosaurs, appeared at the end of the Jurassic Period. Scaly reptile skin transformed into feathers. This modification was entirely different from the flying reptiles of the time, the Pterosaurs, whose stretched skin between their fingers gave them flight.

Death: Sixty-five million years ago (the end of the Cretaceous Period), 183 million years after the Permian extinction and the beginning of the Mesozoic Era, a cataclysmic event occurred which resulted in the extinction of the dominant dinosaurs as well as many species of plants and animals on land and in the ocean. The evidence to date points to an impact with a comet or asteroid about 10 km in diameter, just off the coast of what is today the Yucatan peninsula in Mexico. It left a crater 200 km across. The incoming object (speed between 100,000 and 250,000 km/hr) tore a superheated path through the air, and the violent impact that followed caused supersonic shock waves that reached as far as Canada within minutes. The shock waves and superheated air started fires from Mexico to Canada. The resultant ash would have darkened the sun for at least a year and possibly more. Which consequence of the impact was most responsible for the extinction of the dinosaurs is not yet clear, but that the two events coincided is now certain.