page_title: Hoatzin
section_title: Fossil record
hierarchical_section_title: Hoatzin / Taxonomy, systematics and evolution / Fossil record
caption_attribution_description: THE HOATZIN INHABITS BRITISH GUIANA The newly hatched bird has claws on its thumb and first finger and so is enabled to climb on the branches of trees with great dexterity until such time as the wings are strong enough to sustain it in flight.
context_page_description: The hoatzin, also known as the reptile bird, skunk bird, stinkbird, or Canje pheasant, is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and the Orinoco basins in South America. It is notable for having chicks that have claws on two of their wing digits.
  It is the only member of the genus Opisthocomus. This is the only extant genus in the family Opisthocomidae. The taxonomic position of this family has been greatly debated by specialists, and is still far from clear.
context_section_description: With respect to other material evidence, an undisputed fossil record of a close hoatzin relative is specimen UCMP 42823, a single cranium backside. It is of Miocene origin and was recovered in the upper Magdalena River Valley, Colombia in the well known fauna of La Venta. This has been placed into a distinct, less derived genus, Hoazinoides, but clearly would be placed into the same family as the extant species. It markedly differs insofar as that the cranium of the living hoatzin is characteristic, being much domed, rounded, and shortened, and that these autapomorphies were less pronounced in the Miocene bird. Miller discussed these findings in the light of the supposed affiliation of the hoatzins and the Galliformes, which was the favored hypothesis at that time, but had been controversial almost since its inception. He cautioned, however, "that Hoazinoides by no means establishes a phyletic junction point with other galliforms." for obvious reasons, as we know today. Anything other than the primary findings of Miller 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. Going further back in time, the Late Eocene or Early Oligocene (some 34 million years ago) Filholornis from France has also been considered "proof" of a link between the hoatzin and the gamebirds. The fragmentary fossil Onychopteryx from the Eocene of Argentina and the quite complete but no less enigmatic Early-Middle Eocene (Ypresian-Lutetian, some 48 million years ago) Foro panarium are sometimes used to argue for a hoatzin-cuculiform (including turacos) link. But as demonstrated above, this must be considered highly speculative, if not as badly off the mark as the relationship with Cracidae discussed by Miller.
  The earliest record of the order Opisthocomiformes is Protoazin parisiensis, from the latest Eocene (approximately 34 mya) of Romainville, France. The holotype and only known specimen is NMB PG.70, consisting of partial coracoid, partial scapula, and partial pedal phalanx. According to the phylogenetic analysis performed by the authors, Namibiavis, although later, is more basal than Protoazin. Opisthocomiforms seem to have been much more widespread in the past, with the present South American distribution being only a relic. 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 opisthocomiform. A possible explanation to account for the extinction of Protoazin between the Late Eocene and the Early Miocene in Europe, and of Namibiavis after the Middle Miocene of Sub-Saharan Africa is the arrival of arboreal carnivorans, predation by which could have had a devastating effect on the local opisthocomiforms, if they were as poor flyers and had similarly vulnerable nesting strategies as today's hoatzins. Felids and viverrids first arrived in Europe from Asia after the Turgai Sea closed, marking the boundary between the Eocene and the Oligocene. None of these predators, and for the matter, no placental predator at all was present in South America before the Great American Interchange 3 mya, which could explain the survival of the hoatzin there. In addition to being the earliest fossil record of an opisthocomiform, Protoazin was also the earliest find of one (1912), but it was forgotten for more than a century, being described only in 2014.
  Hoazinavis is an extinct genus of early opisthocomiform from Late Oligocene and Early Miocene (about 24–22 mya) deposits of Brazil. It was collected in 2008 from the Tremembé Formation of São Paulo, Brazil. It was first named by Gerald Mayr, Herculano Alvarenga and Cécile Mourer-Chauviré in 2011 and the type species is Hoazinavis lacustris.
  Namibiavis is another extinct genus of early opisthocomoform from early Middle Miocene (approximately 16 mya) deposits of Namibia. It was collected from Arrisdrift, southe
caption: The newly hatched bird has claws on its thumb and first finger enabling it to dexterously climb tree branches until its wings are strong enough for sustained flight.[14] These claws disappear by the time the bird reaches adulthood.
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