The western Andes mountain range that forms the backbone of South America traps moist air blowing in from the Pacific Ocean and is always drenched in rain. The precipitation produces streams that cascade steeply back to the Pacific, or form small rivers that develop into the tributaries of the huge rivers that meander across the lowlands to the east. These streams are full of different types of bony fish and invertebrates. The fish are eaten by various terrestrial animals including birds, pterodactyl pterosaurs and specialized hunting non-avian theropods.
One of the most interesting of these particular animals is the dip, a fish-eating coelurosaur. Like many of the modern non-avian theropods of the South American continent, the dip is descended from the mountain leaper that migrated southwards when the Isthmus of Panama land bridge to North America was established about 4 million years ago in the Pliocene, resulting in the Great American Interchange. It retains the long silky fur-like feathers of its relatives that is necessary to protect it from the harsh mountain climates. The head has evolved the shape that has developed in many groups of fish-eating land animals. The jaws are long and narrow and furnished with many fine-pointed teeth. The eyes are set so that they are directed forward and give stereoscopic vision. Like the mountain leaper, the dip is very fleet of foot and can run acrobatically along the sides of crags to escape the predatory birds and pterodactyl pterosaurs, like the harridan, that abound in the region.