Mesozoic that would move into their current positions during the next era. 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.
Robert Frost Trail (Massachusetts) Hadley, South Hadley, Amherst, Granby, Belchertown, Pelham, Shutesbury, Leverett, Sunderland, Wendell and Montague. The Robert Frost Trail might be divided into three general geo-ecological landscapes. Both the Holyoke Range and Mount Toby were formed 200 million years ago between the end of the Triassic period and the beginning of the Jurassic. The Holyoke Range, part of the Metacomet Ridge that extends south to Long Island Sound, are composed of basalt, an extrusive volcanic rock. This basalt ridge is the product of several massive lava flows hundreds of feet deep that welled up in faults created by the rifting apart of