F.P. Wesselingh

Amazonia: landscape and species evolution: a look into the past

The book focuses on geological history as the critical factor in determining the present biodiversity and landscapes of Amazonia. The different driving mechanisms for landscape evolution are explored by reviewing the history of the Amazonian Craton, the associated sedimentary basins, and the role of mountain uplift and climate change. This book provdes an insight into the Meso- and Cenozoic record of Amazonia that was characterized by fluvial and long-lived lake systems and a highly diverse flora and fauna. This fauna includes giants such as the ca. 12 m long caiman Purussaurus, but also a varied fish fauna and fragile molluscs, whilst fossil pollen and spores form relics of ancestral swamps and rainforests. Finally, a review the molecular datasets of the modern Amazonian rainforest and aquatic ecosystem, discussing the possible relations between the origin of Amazonian species diversity and the palaeogeographic, palaeoclimatic and palaeoenvironmental evolution of northern South America. The multidisciplinary approach in evaluating the history of Amazonia has resulted in a comprehensive volume that provides novel insights into the evolution of this region.

Reconstruction of the Miocene western Amazonian aquatic system using molluscan isotopic signatures

Abstract Strontium isotopic ratios and Sr contents of molluscs from the Miocene Pebas Formation in Colombian Amazonia indicate that the bulk of the Pebas Formation was deposited in a freshwater environment, predominantly draining the Andes, with minor contribution from the shield regions. The molluscan oxygen and carbon isotopic composition does not provide a palaeosalinity proxy, but points to a lacustrine palaeoenvironment. The faunal character confirms such an interpretation. Sr isotopic ratios further suggest that a marine incursion that reached the area was composed of a mixture of shield-derived freshwater and seawater, with a maximum salinity of about 5 psu. Faunistic components of this incursion-level support a shield–seawater mixing. The incursion most likely reached western Amazonia from the north, probably along the western Guyana shield.

Paleogeography of Miocene Western Amazonia: Isotopic composition of molluscan shells constrains the influence of marine incursions

Strontium, oxygen, and carbon isotope compositions of well-preserved mollusks (bivalves) indicate a dominantly freshwater depositional setting for the lower Miocene–upper Miocene Pebas Formation in Western Amazonia. Molluscan 87Sr/86Sr ratios identify different freshwater sources. Andean runoff was the dominant water source in Miocene Western Amazonia, though there was occasional influx of waters from cratonic catchments.nnAt only one stratigraphic level, isotope signals indicate increased (mesohaline) aquatic salinities, in concert with a clearly more saline molluscan faunal assemblage.nnStrontium isotope–based salinity estimates are surprisingly low when compared to other paleosalinity estimates based on the interpretation of (ichno)faunal assemblages and sedimentological structures. We propose that these seemingly contrasting observations can be unified if Miocene Western Amazonia was occupied by a long-lived (lacustrine) wetland system with a restricted connection, via the Los Llanos Basin, to the Caribbean Sea. Abundant runoff supplied fresh water to this system, which effectively blocked the influx of saline waters through the restricted marine connection to the north. Much like modern Lake Maracaibo, such a system could have been the site of microtidal currents and thus could have hosted brackish-water fauna in a dominantly freshwater depositional system.

NEOGASTROPOD MOLLUSCS FROM THE MIOCENE OF WESTERN AMAZONIA, WITH COMMENTS ON MARINE TO FRESHWATER TRANSITIONS IN MOLLUSCS

Abstract Two neogastropod species occur in brackish intervals in the Pebas Formation (late Middle to early Late Miocene) of Peru and Colombia in western Amazonia. Purpura woodwardiRoxo, 1924, is assigned to MelongenaSchumacher, 1817 (Melongenidae), and ?Nassarius reductus (Nassariidae) is recognized as a new species. These gastropods are among the very few marine invaders in the otherwise freshwater Pebas fauna. The small number of marine to freshwater transitions among South American molluscs contrasts with the situation among South American fishes and southeast Asian molluscs. It may be related to seasonal fluctuations in water level and anoxia in present-day South American freshwater environments, as well as to predation and productivity.