Opisthocomus hoazin (Müller 1776, Hoatzin) The extant hoatzin is chiefly arboreal bird capable of short flights. A sister to Gallus and Dinornis, Opisthocomus is famous for having chicks with claws on two of the wing digits that metamorphosize into the standard fused bird hand as adults. This appears to be an atavism, or reappearance, of a trait that is lost in all other birds.

The hoatzin is an herbivore. The premaxilla does not reach the frontals. The mandible has a fenestra. The jugal is straight, but descends from fore to aft. Like Gallus, both the frontals and parietals are fused. The feet are large. A pygostyle tips the tail. The upper temporal fenestra is not separate from the orbit and appears on the lateral side of the enlarged cranium. The lower temporal fenestra is likewise coincident with the orbit, benath the anterior process of the quadrate.

The taxonomic positiion of Opisthocomus has been debated, but it appears to be a primitive member of extant birds, close to the much larger Dinornis (below).