 The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons , and flat floodplains . Vegetation varied from river @-@ lining forests of conifers , tree ferns , and ferns ( gallery forests ) , to fern savannas with occasional trees such as the Araucaria @-@ like conifer Brachyphyllum . The flora of the period has been revealed by fossils of green algae , fungi , mosses , horsetails , ferns , cycads , <unk> , and several families of conifers . Animal fossils discovered include bivalves , snails , ray @-@ finned fishes , frogs , salamanders , turtles like <unk> , sphenodonts , lizards , terrestrial and aquatic crocodylomorphans like <unk> , several species of pterosaurs such as <unk> and <unk> , numerous dinosaur species , and early mammals such as docodonts ( like <unk> ) , multituberculates , symmetrodonts , and triconodonts . 
