Geography of Aquatic Snakes 2.

South America

South America has a rich aquatic snake fauna, undoubtedly due to the large river basins of the Orinoco and Amazon and many smaller rivers, and the ancient Pebas Wetlands.

The Orinoco River Delta of Venezuela and Colombia covers 36,000 square kilometers and is dominated by a brackish shoreline of mangrove forests. The Llanos are on the western side of the Orinoco River in western Venezuela and northern Colombia. It covers 450,000 square kilometers and is one of South America’s most extensive inland wetlands. The llanos have a winter dry season with a summer wet season that produces annual flooding. Savannas dominate the region.

Aquatic snakes in the region are numerous and include members of the genus Anilius and aquatic burrowing species estimated to have a minimum age of 72.1 Ma (Head 2015). booid genus Eunectes, and the dipsadid genera Erythrolamprus, Hydrops, Helicops, Hydrodynastes, Pseudoeryx, Tretanorhinus and the elapid genus Micrurus.

Reynolds et al. (2013, 2014) found the anacondas of the genus Eunectes to be the sister of Epicrates. Eunectes appear to have evolved by the middle Miocene (Hoffstetter and Rage, 1977; Hsiou and Albino, 2009) based on the fossil remains of Eunectes stirtoni. Epicrates and Eunectes diverged about 25.3 Ma (19.2–32.5 Ma). Thus, Eunectes appears to have separated from South American Epicrates in the Oligocene (19.2–32.5 Ma), followed by subsequent diversification into the four extant species.

There are two species of aquatic coral snakes (Elapidae), Micrurus surinamensis in the Amazon basin and M. natteri in the Orinoco basin.

The aquatic dipsadid genera include Erythrolamprus, Hydrops, Helicops, Pseudoeryx, and Hydrodynastes. The genus Erythrolamprus contains 55 species, of which 48 occur in South America. While most are terrestrial, at least a few are semi-aquatic. They tend to feed on fish, frogs, and lizards. They are oviparous and tend to be less than a meter in length. The genus is in the clade Xenodontini.

The clade Hydropsini includes three aquatic genera Hydrops, Helicops, and Pseudoeryx. Hydrops contains three species restricted to South America east of the Andes and the continental island of Trinidad.

Hydrops martii and H. triangularis have overlapping distributions and occur in syntopy, especially along the central Amazon basin and the Iquitos region. Hydrops caesurus has a distribution outside of the Amazon basin. Hydrops martii is restricted to tropical and subtropical moist broadleaf forests. The distribution of H. triangularis is associated with tropical and subtropical moist broadleaf forests, and it is present in tropical and subtropical grasslands, savannas, and shrublands in Venezuela and Bolivia. The distribution of H. caesurus is associated with flooded grasslands and savannas, in Brazil, Paraguay, and Argentina. The distribution does not overlap with the other two species. Ecological barriers of river basins likely drive the distributions of these species. One biome, tropical and subtropical dry broadleaf forest, is between the northernmost localities of H. caesurus and the southernmost localities of H. triangularis. Neither species has been recorded in this biome (von May et al. 2019).

Helicops contains 20 species with distributions in South America, mostly east of the Andes. The genus is poorly studied but interesting because some populations have been suggested to have facultative viviparity and cryptic diversity (Murphy et al. 2020; Schöneberg and Köhler 2022).

The genus Pseudoeryx occurs in Amazon lowland rainforests and the Chaco of northern Argentina and Paraguay (Giraudo and Scrocchi 2002) it is poorly known. However, the discovery of a second species of Pseudoeryx, in the Lake Maracaibo Basin supports previously hypothesized paleogeographic reconstructions of the changing course of the Orinoco River (Schargel et al. 2007).

The two species of Hydrodynastes are large (approach or exceed three meters) and widespread. Zaher et al. (2019) found Hydrodynastes to be the sister to the terrestrial and arboreal Oxyrhopus and Siphlophis. In addition, the highly aquatic Tretanorhinus was found to be paraphyletic by Zaher et al. (2019). The genus also occurs in Cuba, and Central America.
Gomesophis the Brazilian endemic is the sister to Thamnodynastes (Zaher et al. 2019). The 21 species in the genus Thamnodynastes are not monophyletic (Zaher et al. 2019) and it has and has many semi-aquatic-arboreal species specialized to feed on frogs.

The Amazon River Basin covers 300,000 square kilometers, is 3,000 km long, and floods 5-15 meters annually. It is the world’s largest river, with a flow that results in about one-sixth to one-fifth of all the world’s fresh water. It is so large that it creates its own weather patterns.

The Ancient Pebas Wetland

In what is now the Western Amazonia, an ancient wetland was likely the incubator for aquatic snake evolution on the South American continent. Hoorn et al. (2010) reviewed the characteristics of this mega-wetland and its phases of development. The region consisted of fluvial systems that originated on the Amazonian Craton and were directed towards the sub-Andean zone and the Caribbean. In the Early Miocene, the fluvial systems were primarily replaced by lakes, swamps, tidal channels, and marginal marine embayments, forming a mega-wetland.

At its maximum extent, the wetland covered over 1.5 million square kilometers comprising much of the present western Amazonian lowlands. Descriptions of the snake fossils from the Pebas system have not yet been published. Salas-Gismondi et al. (2015) report on the crocodilians from two species-rich bonebeds from late Middle Miocene proto-Amazonian deposits of northeastern Peru and some highly aquatic snakes throughout the Neogene.

In Amazonia, marine incursions are recorded as thin beds in the Middle to Upper Miocene fluvial strata and contain evidence of marine and coastal taxa (foraminifera, mangrove pollen).

The Pantanal

The Pantanal is the largest tropical wetland and the world’s most extensive flooded grasslands. The region is a vast alluvial floodplain in southwestern Brazil’s upper Paraguay River basin. The subsidence that created the basin probably occurred about 2.5 Ma and was subject to severe climatic changes during the Quaternary. It was a sandy desert during the last glacial period 13,000 years ago. About 80% of the Pantanal floodplains are submerged during the rainy seasons, supporting a diverse collection of aquatic plants and animals.
The gradual slope of the basin receives runoff from the upland Planalto highlands and slowly releases the water through the Paraguay River and its tributaries. The concave formation resulted from a pre-Andean depression of the earth’s crust, related to the Tertiary Andean orogeny. It constitutes a huge inland delta, in which several rivers flowing from the surrounding plateau merge, depositing their sediments and erosion residues.

The Pantanal is bounded by mostly arid habitats – the Chiquitano dry forests to the west and northwest, the arid Chaco dry forests to the southwest, and the humid Chaco to the south. The Cerrado savannas lie to the north, east, and southeast. Throughout the year, temperature varies about 6.0 °C, with the warmest month being November (with an average temperature of 26 °C) and the coldest month being June (with an average temperature of 20 °C).