23 million to five million years ago, possibly creating an environment where hosts of new species could evolve. Scientists agree that sections of the Amazon have been underwater in the past, but there is no consensus as to the mechanism or extent of the flooding.

A study published earlier this year in Science Advances indicates that the Amazon flooded during two separate intervals in the early and mid-Miocene (roughly 18 million and 14 million years ago, respectively). Study co-authors Carlos Jaramillo, a pollen expert at the Smithsonian Tropical Research Institute in Panama, and Jaime Escobar, a paleoclimatologist at the University of the North in Colombia, and their colleagues contend that the Caribbean Sea breached the South American coast by way of Venezuela and Colombia, covering massive tracts of ancientAmazonia with a tongue of saltwater that reached into the continent. As the water advanced, it lost depth and became a marine/brackish ecosystem, then a watery transitional zone, which finally gave way to dry land. Distinct layers in the sediment cores show that in Colombia, the flooding events lasted for 900,000 and 3.7 million years, respectively, while in western Amazonia, which is farthest inland, they endured for 200,000 and 400,000 years.

The researchers also describe how sediment cores have yielded fossil evidence supporting the occurrence of these marine inundations. The most interesting macrofossils found in the several-centimeter-wide core from Colombia's Saltarin region are a shark tooth, possibly from a blacktip or a hammerhead, and a mantis shrimp, a marine organism that burrows into the sandy and muddy bottom of