<P> The era began in the wake of the Permian--Triassic extinction event, the largest well - documented mass extinction in Earth's history, and ended with the Cretaceous--Paleogene extinction event, another mass extinction which is known for having killed off non-avian dinosaurs, as well as other plant and animal species . The Mesozoic was a time of significant tectonic, climate and evolutionary activity . The era witnessed the gradual rifting of the supercontinent Pangaea into separate landmasses that would eventually move into their current positions . The climate of the Mesozoic was varied, alternating between warming and cooling periods . Overall, however, the Earth was hotter than it is today . Dinosaurs appeared in the Late Triassic and became the dominant terrestrial vertebrates early in the Jurassic, occupying this position for about 135 million years until their demise at the end of the Cretaceous . Birds first appeared in the Jurassic, having evolved from a branch of theropod dinosaurs . The first mammals also appeared during the Mesozoic, but would remain small--less than 15 kg (33 lb)--until the Cenozoic . </P> <P> Following the Paleozoic, the Mesozoic extended roughly 186 million years, from 251.902 to 66 million years ago when the Cenozoic Era began . This time frame is separated into three geologic periods . From oldest to youngest: </P> <Ul> <Li> Triassic (251.902 to 201.3 million years ago) </Li> <Li> Jurassic (201.3 to 145 million years ago) </Li> <Li> Cretaceous (145 to 66 million years ago) </Li> </Ul> <Li> Triassic (251.902 to 201.3 million years ago) </Li>

What was the position of the continents during the mesozoic era