A strange bird: Fossils shed Light on the Origin of the South American Hoatzin
The Ornithological Section of the Senckenberg Research Institute in Frankfurt is one of only a few institutions throughout the world in which palaeornithological research has been carried out for more than 30 years. Especially in view of the historical distribution of modern avian groups, research on fossil birds has produced a number of surprises in recent years.
The fact that the recent distribution of many bird groups is relictual has been known for a long time, and fossil remains of parrots and trogons were found in Europe as early as the nineteenth century. However, the disappearance of both these groups can be explained quite simply by climatic changes that lead to cold northern hemispheric winters. Being incapable of seasonal migration over long distances, these insectivorous or frugivorous birds could not find enough food to survive. On the other hand, the relictual distributions of other bird groups that have been described in recent years cannot be so readily accounted for by pure changes of climatic conditions. Early ancestors of hummingbirds (Trochilidae), todies (Todidae) and potoos (Nyctibiidae) that lived in the Paleogene of Europe greatly resemble their modern relatives, which are now found only in South, Central or North America (a few hummingbird species). If the fossil species became extinct because of the significant drop in average temperatures that occurred during the Cenozoic era, it would be puzzling that no modern representatives of these groups are to be found in the tropics of the Old World.
Whereas the avian fossil record of Europe and North America is quite comprehensive, our knowledge of Cenozoic birds of the southern hemisphere is, unfortunately, still sketchy.That this region also produces surprising palaeornithological findings was amply demonstrated by descriptions of hoatzins that were recently published by a group led by the author.
Fossil bones lead to new insights
The hoatzin (Opisthocomus hoazin) is the only living representative of a strange group of birds whose genetic relationships have yet to be satisfactorily explained, even with the aid of modern molecular genetic methods. This bird, somewhat reminiscent of a chicken, lives in the tropical rainforests of the northern part of South America. It is the only known avian species that almost exclusively feeds on leaves. In order to enable preprocessing of this rather indigestible food, the crop is extremely large. This has brought about major changes in the skeleton in the region of the pectoral girdle and the sternum. As in mammalian ruminants, the crop contains a rich bacterial and protozoan fauna, which degrades the plant cellulose into components that are more easily digestible for the birds.
Until recently, practically nothing about the evolutionary history of this bird was known. A skull fragment from the mid-Miocene (around 11.8 – 13.5 million years ago) found in Colombia shows that hoatzins once lived west of the Andes, which is not the case today. Apart from this, the specimen yielded little further information, and as it is in poor condition, its identification was subject to some debate.
Then, in 2011, the first fossils that could unequivocally be identified as hoatzin fossils were found in South America by Brazilian colleagues. The finds come from the Tremembé Formation (Oligocene/ Miocene, 22 - 24 million years ago) in Brazil. They belong to a new species, Hoazinavis lacustris, that hardly differs from recent hoatzins except in its smaller size. The fact that the fossil material consisted of parts of the wing and the pectoral girdle was particularly informative, because these are exactly the bones that have a characteristic shape in recent hoatzins on account of the huge crop. As the corresponding skeletal elements of Hoazinavis are practically identical with those of its modern relatives, it may be assumed that the fossil species likewise had a greatly enlarged crop. From this, we deduce that hoatzins have maintained their highly specialized feeding habits for over 20 million years.
Other fossils described in the same study have yielded even more information on the evolution of the hoatzin.These are bones that were attributed as early as 2003 to a new species, Namibiavis senutae, that lived in the early Miocene (17 – 17.5 million years ago) of Arrisdrift in Namibia. The fossils were erroneously assigned to an extinct taxon of the South American seriemas, which were also abundant in Europe during the early Cenozoic Era. In fact, Namibiavis senutae belongs to the hoatzins. The humerus and pectoral girdle of this species have the characteristic morphology of hoatzins and are very similar to the corresponding bones of the Brazilian Hoazinavis. Despite the fact that the African species is more recent, an analysis of the interrelationships showed that Namibiavis senutae has more primitive features than the two more closely related species Hoazinavis lacustris and the extant hoatzin, Opisthocomus hoazin.
From Africa to South America on floating islands ?
Hoatzins are very poor flyers, only capable of a few hundred metres at a time. The morphology of the humerus of the fossil species indicates that they were not long-distance flyers either. However, Africa and South America were already fully separated as early as by the Miocene era, and even at its narrowest point the South Atlantic was too wide for a hoatzintype bird to cross it. Therefore, the distribution of hoatzins presents us with similar biogeographic conundrums as those connected with the South American monkeys and porcupine relatives (the so-called hystricomorphic rodents), whose relatives also colonized the New World from Africa during the Paleogene. Most researchers now assume that this colonization must have taken place by rafting over the Atlantic, i. e. by a small founding population drifting over on flotsam. These “floating islands” should not be conceived as tree-trunks wallowing in the ocean, carrying individual, frightened animals, which would indeed have had no chance of survival on the open ocean. Instead, it has been observed that in coastal areas, especially in the estuaries of large rivers, plant mats can accumulate to a considerable size, carrying uprooted bushes and shrubs that would provide food for herbivores. As hoatzins eat leaves and live in riparian forests, they are presumably the most likely avian candidates to find themselves on a raft of plant material heading towards another land. Although the distribution paths for earlier hoatzins cannot yet be exactly charted, the fossil finds show that these birds were once more widely distributed and that their present habitat in South America is relictual.
Dr. Gerald Mayr studied physics and biology in Munich and Berlin. He has headed the ornithological section of the Senckenberg Research Institute in Frankfurt since 1997. His research focuses mainly on avian higher-level systematics and the fossil history of birds.