As in many aquatic vertebrates, the watergulp's ribs are very heavy, giving it enough weight to keep it submerged. It also swallows stones from time to time to adjust its buoyancy.
The watergulp, Fluvisaurus hauristus, is an aquatic, viviparous, manatee-like thescelosaur from the Amazon River, in The New Dinosaurs: An Alternative Evolution. It is similar to the Amazonian manatee.
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 evolution of the legs and tail of the watergulp mean that it can move only in water. Its eyes and nostrils are high on the head, allowing it to see above the water surface when submerged. The mouth is broad, and the sharp-edged horny beak can cut through the stems and leaves of the toughest water plants. At about 2.5 meters (8 feet) long it is one of the largest river animals. Each watergulp also supports a large colony of parasites and other companions that feed on algae growing on its flanks, and on small creatures disturbed from the mud by its passage.
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.