The waters of the vast network of channels and tributaries that pour into the great Amazon River flowing eastwards along the Equator are alive with aquatic animals of one kind or another. The forest reaches over the quiet bayous and backwaters, steeping them in a murky green gloom. Water plants root in the deep mud, reach upwards through the turbid waters and spread their leaves out over the placid surface. The undersides of the floating leaves support colonies of water snails and other aquatic invertebrates, and these are eaten both by bony fish, and by lightly-built tetrapods that crawl over the tops of the leaves and probe downwards. The floating leaves themselves are eaten by huge, placid, slow-moving water beasts that swim lazily in the shallows and engulf huge mouthfuls of the vegetation.
The largest of these river browsers is the watergulp, an aquatic thescelosaur evolved from terrestrial forms that spread into South America from North America as part of the Great American Interchange in the Pliocene. The powerful hind legs have since evolved into paddles, as have the versatile five-fingered forelimbs. These are used mainly for stabilizing the animal's great bulk, and the two claws on the forelimbs are useful for grubbing around the muddy roots. Most of the swimming action is achieved by the tail. The stiff rod-like tail of its ancestors has become more flexible, and a tall leathery fin along the upper surface can produce powerful swimming strokes.