Original articleA fossil Dendrocygninae (Aves, Anatidae) from the Early Pliocene of the Argentine Pampas and its paleobiogeographical implicationsUn Dendrocygninae fossile (Aves, Anatidae) du Pliocène inférieur de la pampa argentine et ses implications paléobiogéographiques
Introduction
The avian order Anseriformes includes a large variety of duck-like taxa, among others the screamers, geese, ducks, and swans. Living anseriforms encompass the South American Anhimidae, the Anseranatidae of Australasia, and the cosmopolitan Anatidae (Carboneras, 1992). The Anseriformes are currently cosmopolitan in distribution, and concomitantly, its fossil record is relatively well known from most continents (Howard, 1964, Brodkorb, 1964, Olson, 1985, Mayr, 2009). Its fossil record comprehend specimens ranging from the Late Cretaceous (Clarke et al., 2005) to the Pleistocene and Holocene (Brodkorb, 1964), and ranges across all landmasses, including Antarctica.
The Paleogene fossil record of Anseriformes is composed by taxa belonging to plesiomorphic forms until the Early Miocene, at least, and it is possible that the diversification of crown group anseriformes may not have taken place before the Oligocene (Olson, 1989, Mayr, 2008). In fact, as proposed by Olson (1985), it appears that the principal radiation of modern tribes and genera of Anatidae took place by the Miocene, and anatines do not appear to be a significant component of the faunas in pre-Miocene times (Olson, 1989, Worthy et al., 2007). In this way, the Anatinae or “modern ducks” lack any certain record until the early Miocene (Worthy et al., 2007).
In South America, the fossil record of Paleogene Anseriformes is still relatively patchy. Several plesiomorphic taxa referred to Presbyornithidae, Anhimidae, and Tadorninae were described from the Paleogene and early Neogene of Argentina and Brazil (Tonni, 1980, Tambussi and Noriega, 1996, Alvarenga, 1999, Agnolin, 2004, Cenizo and Agnolin, 2010).
Among non-Anatinae fossil anseriforms, the presence of an indeterminate Dendrocygninae was described from the late Miocene of Patagonia, Argentina (Acosta Hospitaleche et al., 2007, Dozo et al., 2002, Dozo et al., 2010). This report constitutes the only pre-Pliocene record for the clade. The dendrocynines, commonly named as “whistling-ducks” only include the living genus Dendrocygna Swainson, 1837, currently represented by eight species distributed across the tropical and subtropical regions of both the New and Old Worlds, this is, Pantropical distribution (Carboneras, 1992). In South America, the genus is represented by three living species (D. viduata Linnaeus, 1766, D. autumnalis Linnaeus, 1758, and D. bicolor Vieillot, 1816) distributed from the Northern limit of the continent, to the southern argentine pampas (Navas, 1977, Narosky and Di Giacomo, 1993).
The aim of the present paper is to report the presence of a new record for the Dendrocygninae, from the early Pliocene of the Buenos Aires province, Argentina. This record constitutes one of the few fossil reports for this poorly known anatid subfamily, and allows to make some observations regarding the evolution and early settlement of South American anatids.
Section snippets
Methods
Here we follow the systematic arrangement of Anseriformes proposed by Worthy (2009), and the taxonomic nomenclature of Livezey (1986). The osteological nomenclature follows Baumel and Witmer (1993) with the additions and modifications proposed by Worthy et al. (2007) and Mayr (2008).
Institutional abbreviations. MD-FM: Museo Municipal de Ciencias Naturales Carlos Darwin (Punta Alta, Buenos Aires Province, Argentina), Farola Monte Hermoso collection.
Geographical and stratigraphical context
Farola Monte Hermoso locality (39° 00′ 30′′ S; 61° 30′ 10′′ W) is located on the Atlantic coast, approximately 53 km from Bahía Blanca city and approximately 12 km southwest of Pehuen Có beach (Buenos Aires Province, Argentina) (Fig. 1). This locality was studied by numerous authors (e.g. Darwin, 1846, Ameghino, 1887, Kraglievich, 1946, Bonaparte, 1960), who described the diversity and abundance of vertebrate fossil remains yielded by this locality. In addition, Farola Monte Hermoso constitutes
Systematic paleontology
Order: ANSERIFORMES Wagler, 1831
Family: ANATIDAE Vigors, 1825
Subfamily: DENDROCYGNINAE Reichenbach, 1850
Genus and species indeterminate
Referred material. MD-FM-11-23, distal end of right humerus with slightly damaged dorsal condyle (Fig. 3).
Description. The humerus of MD-FM-11-23 indicates an individual of the approximate size of Dendrocygna bicolor. The distal end of the humerus is transversally expanded and anteroposteriorly compressed, a plesiomorphic condition for anatids, that is also
Discussion
The specimen MD-FM-11-23 differs from a wide array of basal anseriforms, as for example Anseranas Lesson, 1828, and Anatalavis Olson, 1999 in having less anteroposteriorly compressed and transversally expanded humeral distal end, smaller and shallower brachial fossa, and in having a well developed scapulotricipital groove (Olson, 1999, Worthy et al., 2007). Regarding the Presbyornithidae, the present specimen differs in a large amount of characters, including the well-developed and proximally
Conclusions
The presence of a fossil Dendrocygninae in the Pliocene of Buenos Aires province is reported for the first time. Moreover, this finding sheds some light on the evolution and early settlement of South American anatids. An overview of the fossil record of South American anseriforms suggests that Anatinae ducks immigrated from North America to the southern landmasses taking part of the Great American Biotic Interchange, probably by Pleistocene times. In spite of their late appearance in the
Disclosure of interest
The authors declare that they have no conflicts of interest concerning this article.
Acknowledgments
A.R. Caputo, the director of the Museo Municipal de Ciencias Naturales Carlos Darwin, for allowing us to study the material under his care. We also thank M.R. Derguy, N.R. Chimento, G. Lio, S.O. Lucero, and M. Sosa for their comments and discussion. We thank Marcos Cenizo for his useful comments regarding the identification of the specimen here studied.
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