Specimens of extinct species of three catfish genera found in desert areas of northwestern South America may be fossil evidence of the last temporal occurrence of the ancient Amazon River flowing in a direction quite different from its current trajectory, and discharging its waters into the Caribbean. Vestiges of these ancient types of freshwater fish have been found in sedimentary rocks in the La Victoria, Villavieja, Urumaco, Castilletes and San Gregorio geological formations in northern Colombia and Venezuela. Today this arid, riverless region lies in an Andean setting at elevations as high as 3,300 meters. According to an article published in September 2013 in the journal PLOS ONE, the catfish were among the fauna in a Proto-Amazon River that once cut through areas now included in the territory of those two countries, and they present anatomical similarities to existing fish species in the Amazon Basin.
Until about 2.5 million years ago, the nascent river, now the world’s largest, had a branch that originated in mid-Amazonia and flowed westward across the continent, where it joined another branch flowing towards northern South America (see map). According to this hypothesis, the second branch, which flowed towards the northernmost part of the continent, traversed the area between the Maracaibo Basin in Venezuela and the Magdalena River in Colombia, and drained into the southern Caribbean. “The Amazon was like a large swamp, with slowly flowing water,” notes Venezuelan paleontologist Orangel Aguilera of the Universidade Federal Fluminense (UFF), principal author of the paper. “The biodiversity of that region as we know it today had not yet emerged.”
Only after the end of the long process in which the northernmost section of the Andes uplifted is the river thought to have changed course, lost its northward-flowing branch and begun flowing eastward along its present course. The consolidation of that major mountain range purportedly pushed the waters of the Amazon far from its Caribbean branch, which permanently dried up and became an arid zone. The flow of the river then broke barriers that had prevented its access to the eastern central part of the Brazilian Amazon Region. The new eastward flow of the Amazon River is thus thought to have been strong enough to overrun two areas marked by low natural elevations and open a path to the Atlantic.
In the opinion of John Lundberg, the American curator of the Department of Ichthyology at the Academy of Natural Sciences at Drexel University, Philadelphia, the catfish fossils recovered in Colombia and Venezuela reinforce the idea that the mouth of the Amazon was in northwestern South America in the distant past. According to Lundberg, since the 1950s geologists have suspected that there was a major paleoconnection between the western Amazon and the Orinoco, the largest river in Venezuela, which flowed into the Caribbean. “The kinship relationships among many fish, reptiles and aquatic mammals living today in the watersheds of the Amazon, Orinoco, Magdalena and Maracaibo rivers are also well known,” says Lundberg, a coauthor of the PLOS ONE paper. “They suggest that there were fluvial interconnections prior to the rise of the Andes in Colombia and Venezuela.”
Although evidence is mounting and there are increasing numbers of adherents in the scientific community in recent decades, the hypothesis that the ancient Amazon flowed northward and discharged into the Caribbean remains controversial. Some scientists believe the river never followed such a trajectory. Even among those who defend the idea of a connection between the Proto-Amazon and northwestern South America, one question remains without a definitive answer: How long did the passage to the Caribbean remain open? Proof of the point in time when the river began to flow towards the Atlantic would serve as a kind of birth certificate for the present-day Amazon River.
The recent research led by Aguilera and Lundberg provides a bold response to the controversy. Based on the estimated age of the sediments in which the catfish fossils were found, the researchers maintain that the Amazon reversed its course later than other authors assert. According to Aguilera, the Amazon did not dry up in the region between the Maracaibo Basin and the Magdalena River until the end of the Pliocene geological period and the beginning of the Pleistocene, some 2.5 million years ago. Many papers on the subject tend to place the disappearance of the Caribbean connection between 12 and 8 million years ago, when the rise of the Andes in Venezuela and Colombia entered its final phase. The final ascension of that major mountain range is thought to have reorganized the drainage system in northwestern South America, cutting off the northern branch of the Amazon and paving its way eastward. Aguilera also believes that this occurred, but later than had been presumed.
Geologist Dilce Rossetti of the National Institute for Space Research (INPE), an expert on Amazonian paleorivers, regards the hypothesis advocated in the PLOS ONE paper as valid and coherent. But she maintains that the origin of the present-day course of the Amazon is a complex issue not yet supported by definitive, indisputable data. “There is no consensus that the Amazon flowed northward in the past,” Rossetti says. The existence of Amazonian catfish fossils does not necessarily mean that the river was linked to northwestern South America until that time.
The connection with northern Venezuela and Colombia may have disappeared earlier than 2.5 million years ago and left a vestige in the form of a small local basin, already separated from the large Amazon River. According to this interpretation, the newly discovered catfish fossils are therefore remnants from that separate secondary basin, which disappeared over time and led to the formation of a desert landscape—and not directly from the waters of a Proto-Amazon flowing northward across the continent.
AGUILERA, O. et al. Palaeontological evidence for the last temporal occurrence of the ancient western Amazonian river outflow into the Caribbean. PLOS ONE. Sept. 13, 2013.