170 million years ago during the Middle Jurassic
Middle Jurassic is the second geological epoch in the Jurassic. It began 174.1 million years ago, and ended at 163.5 million years ago.
In the Middle Jurassic,
Pangaea began to split apart. It began to separate into Laurasia and Gondwana, and the Atlantic Ocean formed. Tectonic activities closed the Paleo-Tethys Ocean. A subduction zone  on the coast of western  North America continues to create the ancestral Rocky Mountains.
During this time, marine life (including
ammonites and bivalves) flourished. Ichthyosaurs, although common, are reduced in diversity. A type of crocodilians first appeared.
Plesiosaurs became common at this time. The top marine predators, the pliosaurs, grew to the size of killer whales and larger e.g. , Pliosaurus . Liopleurodon
New types of
dinosaurs evolved on land. was an early sauropod found in England in the mid-19th century. Cetiosaurus  
The environment in which
Cetiosaurus lived was floodplain and open woodland. Brachiosaurs, and Megalosaurus also lived in this Hypsilophodon environment.
cynodont therapsids flourished with the dinosaurs, but they were only shrew-sized. None grew larger than a badger. One group of cynodonts, the Trithelodonts were becoming rare and eventually became extinct at the end of this epoch. The Tritylodonts were still common, though. In this epoch "true" mammals evolved from a group of cynodonts.
Conifers were dominant in the Middle Jurassic. Other plants, such as ginkgoes, cycads, and ferns were also common. These are the trees which the large herbivores ate.
↑ The Tethys Ocean: an ancient seaway approximately in the position of the Mediterranean.
↑ Formed by an oceanic plate sliding under a continental plate.
↑ "Cetiosaurus." In: Dodson, Peter et al. The Age of Dinosaurs. Publications International, p65. ISBN 0-7853-0443-6.
↑ Upchurch P & Martin J (2002). "The Rutland Cetiosaurus: the anatomy and relationships of a Middle Jurassic British sauropod dinosaur.". Palaeontology 45 (6): 1049–1074. doi: 10.1111/1475-4983.00275.