Out of Time's Abyss by Edgar Rice Burroughs

This is the tale of Bradley after he left Fort Dinosaur upon the west coast of the great lake that is in the center of the island. About them upon the ground, among the trees and in the air over them moved and swung and soared the countless forms of Caspak's teeming life. Always were they menaced by some frightful thing and seldom were their rifles cool, yet even in the brief time they had dwelt upon Caprona they had become callous to danger, so that they swung along laughing and chatting like soldiers on a summer hike.