Inland sea
After the breakup of Gondwana, the Amazon found its exit blocked by the rise of the Andes about 15 million years ago (mya). A great inland sea developed, at times draining north through what is now Venezuela before finding its present eastward outlet into the South Atlantic. Gradually this inland sea became a vast freshwater lake and wetlands where sediment flattened its profiles and the marine inhabitants adapted to life in freshwater. Over 20 species of stingray, most closely related to those found in the Pacific Ocean, live today in the freshwaters of the Amazon, which is also home to a freshwater dolphin.