Elsevier

Annales de Paléontologie

Volume 98, Issue 3, July–September 2012, Pages 191-201
Annales de Paléontologie

Original article
A 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

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Abstract

In the present paper, the distal end of a humerus referable to a Dendrocygninae anseriform bird is reported. The specimen was collected at the Monte Hermoso Formation (early Pliocene) of the Farola Monte Hermoso locality (Buenos Aires Province, Argentina). This record constitutes one of the oldest for the group in South America and elsewhere. The overview of the anseriform record in South America indicates a similar pattern to that of other localities across the world. Paleogene localities are dominated by non-Anatinae taxa, whereas more recent faunas are dominated by anatines. Following this pattern, in South America the Anatinae appears in the fossil record as recently as in the Pleistocene, probably arriving from North America during the Great American Biotic Interchange. Pleistocene and Recent anseriform avifaunas are dominated by Anatinae taxa, and this dominance is probably due to some “key characters” regarding reproductive biology. In fact, the anatines exhibit a high reproductive success probably due to the capability of having two molts per year, and that the parental care of the downy young is only conducted by the female. The combination of characters may have allowed anatines to dominate most aquatic environments, surpassing in diversity more plesiomorphic taxa.

Résumé

L’extrémité distale d’un humérus appartenant à un Dendrocygninae est décrite. Le spécimen a été trouvé dans la Formation Monte Hermoso (Pliocène inférieur) de Farola Monte Hermoso (province de Buenos Aires, Argentine). Cette présence constitue l’un des plus anciens fossiles connus pour le groupe en Amérique du Sud. Le registre fossile des Anseriformes en Amérique du Sud indique une tendance similaire à celles qui sont reconnues dans d’autres parties du monde. Les localités du Paléogène sont dominées par des non-anatinés, tandis que les plus récentes sont dominées par des Anatinae. Suivant cette tendance, en Amérique du Sud les Anatinae apparaissent dans les archives fossiles au cours du Pléistocène, peut-être suite à une arrivée en provenance de l’Amérique du Nord au cours du Grand échange biotique américain. Les Avifaunes d’ansériformes du Pléistocène et de l’Holocène sont dominées par les Anatinae, et cela a sans doute été favorisé par plusieurs caractéristiques avantageuses liées à la biologie de la reproduction de ces oiseaux. Ainsi, les Anatinae montrent un taux de reproduction élevé, ayant la capacité de produire deux portées par an avec des soins parentaux dispensés par la femelle uniquement. La combinaison de ces éléments aurait pu permettre aux Anatinae de dominer les écosystèmes d’eau douce, surpassant en diversité le reste des ansériformes.

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, Dautumnalis 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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