The hoatzin is notable for its chicks having primitive claws on two of their wing digits; the species also is unique in possessing a digestive system capable of fermentation and the effective breaking-down of plant matter, a trait more commonly known from herbivorous ungulate-ruminant mammals and some primates.

This has inevitably led to comparisons to the fossil bird Archaeopteryx, but the characteristic is rather an autapomorphy, possibly caused by an atavism toward the dinosaurian finger claws, whose developmental genetics ("blueprint") presumably is still in the avian genome.

Modern researchers, however, hypothesize that the young hoatzin's claws are of more recent origin, and may be a secondary adaptation from its frequent need to leave the nest and climb about in dense vines and trees well before it can fly.

At various times, it has been allied with such taxa as the tinamous, the Galliformes (gamebirds), the rails, the bustards, seriemas, sandgrouse, doves, turacos and other Cuculiformes, and mousebirds.

[5] A whole genome sequencing study published in 2014 places the hoatzin as the sister taxon of a clade composed of Gruiformes (cranes) and Charadriiformes (plovers).

[12] In 2015, genetic research[13] indicated that the hoatzin is the last surviving member of a bird line that branched off in its own direction 64 million years ago, shortly after the extinction event that killed the nonavian dinosaurs.

[14] Another genetic study from 2024 instead suggested a Late Cretaceous origin (around 70 million years ago), but found that this early divergence is shared with a majority of extant bird orders, making it no more primitive than them.

[12] With respect to other material evidence, an undisputed fossil record of a close hoatzin relative is specimen UCMP 42823, a single cranium backside.

Anything other than the primary findings of Müller are not to be expected in any case, as by the time of Hoazinoides, essentially all modern bird families are either known or believed to have been present and distinct.

By the Early to Middle Miocene, they were probably extinct in Europe already, as formations dated to this time and representing fluvial or lacustrine palaeoenvironments, in which the hoatzin thrives today, have yielded dozens of bird specimens, but no opisthocomiforms.

[19] One of this species' many peculiarities is its unique digestive system, which contains specialized bacteria in the front part of the gut that break-down and ferment the foliar material they consume (much like cattle and other ruminants do).

Because they lack the teeth of mammals, hoatzins do not chew the cud; instead, a combination of muscular pressure and abrasion by a “cornified” lining of the crop is used as an equivalent to remastication, allowing fermentation and trituration to occur at the same site.

[20][22][23][24] With a body weight as low as 700 grams (1.5 lb), the adult hoatzin is the smallest known animal with foregut fermentation (the lower limit for mammals is about 3 kilograms or 6.6 pounds).

[25] Because of aromatic compounds in the leaves they consume, and the bacterial fermentation required to digest them,[26][27] the birds have a disagreeable, manure-like odor and are only hunted by humans for food in times of dire need; local people also call it the "stinkbird" because of it.

[8] Much of the hoatzin’s diet, including various types of Monstera, Philodendron and other aroids, contains a high concentration of calcium oxalate crystals, which, even in small amounts, can be greatly uncomfortable (and even dangerous) for humans to consume.

In Brazil, indigenous peoples sometimes collect the eggs for food, and the adults are occasionally hunted, but it is generally rare to consume mature birds, as hoatzin meat is reputed to have a bad taste.