It flies awkwardly in the midst of vegetation along the river in the Amazon Forest, its only contemporary habitat. There it eats leaves and nothing else. It has a large beak and its digestive system is reminiscent of that of a ruminating mammal. Its feces smell of cow-dung. Taxonomists have not yet been able to reach an agreement as to how to classify it. For some of them, it’s a distant relative of the chicken, although its appearance and size somewhat remind one of the cuckoo, to which, according to other taxonomists, it is related. There are also those who place it beside the touraco, an African bird. For more than 230 years since its discovery, the hoatzin (Opisthocomus hoazin), which is typical of the Amazon and Orinoco river basins, has intrigued researchers, who nowadays tend to regard it as the only living member of an order of birds that is separate from the rest, the Opisthocomiformes. However, the discovery in Brazil of the oldest extinct species of birds related to the hoatzin — a fossil more than 20 million years old named Hoazinavis lacustris — and the confirmation, which there was at some point in Africa’s distant past, at least one life form similar to the current Amazonian bird, provided important clues regarding the likely origins of this mysterious animal. Up to this point there had not been any record of birds belonging to this order outside of South America.
The two finds were disclosed in a study published this month in the German scientific journal Naturwissenschaften by paleontologists and ornithologists from Brazil, Germany and France. According to the researchers, analysis of all the material suggests that the origins of the South-American bird lie in Africa, the oldest species related to the hoatzin had been found in the State of São Paulo. “Despite being more recent, the African fossils exhibit more primate anatomical characteristics than those present in our material,” explains the paleontologist Herculano Alvarenga, founder and director of the Natural History Museum of the city of Taubaté, in inner-state São Paulo and one of the authors of the study. If this line of reasoning is correct, it is reasonable to assume that there must be fossils that are older than those of H. lacustris somewhere on the continent. The problem is that they have not yet been found and there is no guarantee that they ever will be.
More surprising than the possible African roots of the Amazonian hoatzin is how this bird’s ancestors must have made the long migration from Africa to South America, dozens of millions of years ago. At that time there was no longer any land connection between the two continents. Africa and South America had separated much earlier and the Atlantic Ocean, although not as wide as it is today, was an obstacle to be overcome in a transcontinental crossing. Flapping its wings during a journey of thousands of kilometers and crossing the ocean by air was an impossible task for the ancestors of this South American bird, whose flying capabilities were as limited as those of the hoatzin. By exclusion, the only way they could have come was by sea. “These ancient birds must have crossed the Atlantic Ocean on raft-like remains of plants, which functioned as tiny floating islands connecting the two continents,” states Alvarenga, a specialist in bird fossils.
At the mercy of the winds and the currents
This assumption may seem fanciful to a layman, but there is scientific evidence to support it. “All reconstitutions of how the winds and the sea currents were at that time favor the displacement of species from Africa to South America, rather than the other way around,” declares the ornithologist Gerald Mayr, from the Senckenberg Museum, in the city of Frankfurt, one of the other authors of the article. There has never been a documented case of birds with limited flying capacity making this kind of intercontinental crossing, on board some kind of raft composed of plants that must have crossed the Atlantic at the mercy of the winds and the currents. However, it is possible that other animals came here in this way. “This is the most widely accepted idea as to how caviomorph rodents and platyrrhini primates got from Africa to South America,” comments the paleontologist Cécile Mourer-Chauviré, from the University Claude Bernard – Lyon 1, another one of the authors of this scientific paper. Caviomorph rodents include animals that are typical of South America, such as the capybara and the paca, and the platyrrhini primates include the so-called New World monkeys, found only in the Americas.
The formulation of this new theory that tries to explain the origins of the South American bird was only possible thanks to the discovery on Brazilian soil of the new extinct species and to the work of reviewing the little fossil material related to the Opisthocomiformes that can be found in the international museums. This double pronged approach enabled the researchers to do something that had not been possible up until now: to outline a scenario of evolutionary relationships between ancient birds and the only living member of this order of birds, the hoatzin.
The oldest of the extinct species of the Opisthocomiformes, the H. lacustris inhabited the Tremembé Formation, in the Taubaté region, which is rich in animal fossils, between 22 and 24 million years ago. Three parts of the skeleton of a single example of this bird — a complete humerus (the main wing bone), a piece of the scapula and another of the coracoid (waist bone) — were found by Alvarenga in the sediment of an ancient lake (which is where the name lacustris comes from) in 2008. “The morphology of these three associated bones leaves no doubt that this bird was related to the hoatzin,” explains the paleontologist from the State of São Paulo. The study of these fragments of the skeleton also revealed that the ancient bird must have had a large beak, which may well have contained bacteria that helped break down part of the bird’s diet before the food reached its stomach. All of that was very similar to the current hoatzin. The description of the fossil was made by the Brazilian and his European colleagues in a paper published in the journal Naturwissenschaften.
Before H. lacustris, the remains of a single example of another species of extinct bird that was apparently related to the hoatzin had been found in the late 1990s in South America. In this case it was a fragment of a skull from a Hoazinoides magdalenae, an animal that lived in the Villavieja Formation to the west of the Andes, in what is now Colombia, between 11.8 and 13.5 million years ago. Although there is little bone material to make any detailed comparison, the H. magdalenae appears to have been very similar to the current hoatzin. It was just slightly larger than its modern Amazonian relative. It is interesting to note that the two extinct species of Opisthocomiformes found in South America occupied parts of the continent outside the Amazon region, which is the hoatzin’s current home — an indication that the older forms of this bird may have been distributed over a much larger geographical area.
The reclassification of an extinct species of African bird, the Namibiavis senutae, within the hoatzin’s phylogenetic order expanded the ancient domains of this group of winged creatures, with a large beak and a vegetarian diet, even further. First described at the start of the first decade of the new millennium, the fossils of the species were found in Namibia and were originally classified as members of an extinct group of birds from that continent, the Idiornithidae. However, the analysis undertaken by Alvarenga and his European colleagues changed this classification, placing the N. senutae, which lived about 17 million years ago, with the genus Opisthocomiformes. “The fossils found in Africa are more different from the modern hoatzin than the H. lacustris found in Brazil,” states Mayr. “But they are still very like the current bird.” Among the more primitive anatomical differences that can be observed in the extinct African species, the French scientist Cécile highlights the fact that the coracoid and furcula bones — the latter consisting of the two clavicle bones connected to the sternum, a part of the structure of birds commonly known as the “wishbone” — had not yet fused, as if they were a single structure. In adult examples of the modern hoatzin the fusion of these and other bones is already complete. It was precisely these more ancestral traits of the bones of the N. senutae that supported the formulation of the hypothesis of the African origins of the genus Opisthocomiformes.
For the biologist Luís Fábio Silveira, curator of the University of São Paulo (USP) Zoology Museum’s ornithological collections, the new theory that places the birthplace of the hoatzin’s ancestors outside South America should be taken seriously and tested as and when new fossils are discovered. “The study is very interesting as well as beneficial,” states Silveira, who was not involved in the project. “The hoatzin’s origins and, consequently, its degrees of kinship are among the greatest unsolved problems in the classification of birds. Nobody knows whether this bird is more closely related to chickens, cuckoos or touracos.” This question has not been resolved by the new study. However, if the origin of this order of birds does indeed lie in Africa, then paleontologists and ornithologists may well have to shift their focus to that continent, instead of South America, which is where the hoatzin is now found.
MAYR, G. et al. Out of Africa: Fossils shed light on the origin of the hoatzin, an iconic Neotropic bird. Naturwissenschaften. v. 98, n. 11, p. 961-66. nov. 2011