The hoatzin is a primitive chicken-sized bird of South American swamps, principally in the Amazon and Orinoco river basins, first described scientifically in 1776. It has been given ordinal rank because it combines primitive and highly specialized characteristics and still could not be confidently placed in phylogenomic studies.

The young possess two large claws on each wing which are shed during growth , a trait that has led some scientists to link the species with Archaeopterix, the fossil of the dinosaure era.

The hoatzin is the only bird with a digestive system that ferments vegetation as a cow does, which enables it to eat leaves and buds exclusively. Hoatzins feed on swamp plants, grinding foliage in a greatly enlarged crop (not the gizzard, as in other birds). A large rubbery callus on the bird's breastbone acts as a tripod to keep it from falling over when its stomach is distended.

They swim well and can dive. Adults can fly clumsily for short distances, but they spend most of their time perched, digesting their leafy food.

The hoatzin is about 65 cm (25.6 inches) long but weighs less than 1 kg (2.2 pounds). Sexes look alike, and both parents, as well as older siblings, cooperate to raise two to five young. After four weeks of incubation, the eggs hatch, and adults feed the chicks a leaf paste regurgitated from the crop. Adult hoatzins hiss, hoot, and yelp at predators, such as tayras and capuchin monkeys. Nests are built over water, and if danger threatens, the young, which are excellent swimmers, will plunge to safety, return to shore, and use their claws to climb back up to the nest.

hoatzin (Opisthocomus hoazin) (2012) Encyclopædia Britannica Ultimate Reference Suite.  Chicago: Encyclopædia Britannica

Hackett SJ et al (2008): Phylogenomic Studies of Birds