Hsiou & Albino, 2009 - Presence of the Genus Eunectes (Serpentes, Boidae) in the Neogene of Southwestern Amazonia, Brazil

Hsiou & Albino, 2009 - Presence of the Genus Eunectes (Serpentes, Boidae) in the...

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Fossils of the genus Eunectes have only been reported from the Middle Miocene of La Venta (Colombia), represented by the extinct species E. stirtoni Hoffstetter and Rage, 1977. The material was described based on a right prootic and an incomplete basisphenoid. According to Hoffstetter and Rage (1977), they are distinct in morphology from extant E. murinus. The authors also assigned an incomplete series of vertebrae associated with the holotype to E. stirtoni, without providing a formal description. Later, Hecht and LaDuke (1997) examined these vertebrae and questioned the allocation to the genus Eunectes. They argued that the size and shape, and other characters, do not match with the vertebral morphology of Eunectes, but they did not provide a discussion of these features. These authors also assume as Eunectes sp. a series of eight fragmented presumably posterior trunk vertebrae from La Venta that have a very protuberant paradiapophyses; nevertheless, the composite drawing provided by the authors does not show the projected paradiapohyses as in the posterior vertebra of the Solimoes Formation studied in this paper.

The extant species of Eunectes are adapted to a semiaquatic lifestyle, living in rivers, lakes, lagoons, swamps, temporary pools, and flooded forests (Strimple, 1993). They have morphological adaptations consistent with the lifestyle: dorsal nostrils and eyes fairly close to each other on the top of the head, relatively small eye

NEOGENE EUNECTES FROM BRAZIL 617 diameter, and narrower ventral scales (Scartozzoni, 2005). The aquatic habit arose only once among the Neotropical boids, in the snakes of the genus Eunectes, despite the great diversity of habits found in other genera (Murphy and Henderson, 1997). This statement could support the hypothesis proposed by Latrubesse et al. (1997, 2007) that a large fluvial megafan complex would be present in the Late Miocene of the Solimoes Formation. The environment proposed for the region is suggested by the presence of vertebrate fauna (rodents, crocodiles, turtles, and freshwater fish) and palynological data that indicate open areas and forest galleries along rivers, swamps, and shallow lakes that would be subject to variation in the water level in a seasonal dry-humid tropical climate (Latrubesse et al., 2007).

Presence of the semiaquatic Eunectes in the

Solimoes Formation represents the first fossil record of snakes from southwestern Brazilian Amazonia. It would indicate a general ecological similarity with the Miocene fauna of La Venta (Colombia), extending the distribution of the genus toward the southeast. It also would indicate at least some similarity between the Miocene faunas of Talisma (Brazil), La Venta (Colombia), and Socorro and Urumaco (Venezuela). Although the fossils of boids found in the Socorro and Urumaco areas may not be assigned to the generic level (Head et al., 2006), the presence of these kind of snakes in the Miocene faunas of Brazil, Colombia, and Venezuela would demonstrate that the boids were broadly distributed in northern South America as early as the Middle and Late Miocene.

Recent phylogenetic analyses based on molecular data of Neotropical boids, conclude that Eunectes and Epicrates form a strongly supported clade (Burbrink, 2005; Noonan and Chippindale, 2006). According to Noonan and Chippindale (2006), Epicrates is recovered as paraphyletic with respect to Eunectes, suggesting that the Caribbean Epicrates are the sister group to Eunectes + South American Epicrates clade. The molecular data sets are consistent with Caribbean dispersal of Epicrates prior to the divergence between the representatives of South American Epicrates and Eunectes (Noonan and Chippindale, 2006). The authors propose that after the isolation of South America at the end of the Late Cretaceous (or Paleocene, Gayet et al., 1992), the Neotropical boids underwent little diversification until the invasion of the Caribbean at the beginning of the Oligocene, where the connectivity of what today represents the Greater Antilles reached its peak. If this hypothesis is correct, the divergence of the genus Eunectes fromtheSouth AmericanEpicrates may have occurred immediately after that dispersal event. The earliest fossil record from the

Middle and Late Miocene of northern South America is congruent with this supposition and corroborates the probable origin of the genus as early as the Miocene.

Acknowledgments.—The authors wish to thank for the financial support provided by CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico) to ASH for studying at Universidade Federal do Rio Grande do Sul (PPGGeociencias/UFRGS). Thanks also to J. Ferigolo (MCN/FZBRS) for all the incentive and help; to J. P. Souza-Filho, A. Maciente, and M. B. de Souza (UFAC) for loan of the fossil Squamata material from Solimoes Formation; to H. Zaher (MZUSP) and F. L. Franco (IB) for the permission to visit the Herpetological collections; to M. L. Araujo and M. L. M. Alves (MCN/FZBRS) for the loan of the Boidae specimens; to J. C. Cisneros (UFRGS) for English revision of the manuscript; and to J. C. Rage and an anonymous reviewer for their careful reviews and helpful suggestions.

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