Lesson 3. How did the habitat at the Gray Fossil Site change over the course of the Pliocene?


Now that we've reconstructed the habitat of the early Pliocene Gray Fossil Site and compared it to the modern landscape, let's take a quick look at what scientists think happened in the last 5 million years to get to this point.  

Sinkholes do not last very long in geologic time. Scientists think that the Gray Fossil Site sinkhole slowly filled with sediments washed in from the edges over a few thousand years. Over time, the limestone walls surrounding the sinkhole eroded away, leaving the clay plug of sinkhole sediments–and the fossils buried within them–in place. 

The climate conditions experienced by plants and animals preserved at the Gray Fossil Site did not last long. By the mid-Pliocene (3.3 to 3.0 million years ago), global climates warmed. These warmer temperatures caused ice sheets at the poles to melt, raising sea level by 80 feet compared to today. This period of warmer climates is not preserved at the Gray Fossil Site. However, we would have expected plants and animals that preferred a more temperate climate to move north, to be replaced by more warm-climate species moving into northeastern Tennessee from the south. 

This warmer climate was followed by the beginning of the Pleistocene, a period of global cooling. The Pleistocene is also called the Ice Age because it was a period when glaciers covered up to 25% more of the Earth’s land area. Because more of the Earth’s water was locked up in ice on the continents, the sea level was as much as 400 feet lower than it is today. These cooler conditions are evident in the fossil record in northeastern Tennessee. Caves near the Gray Fossil Site have preserved Pleistocene animals that were adapted to much cooler climates, like taiga vole and caribou. Mammoths, mastodons, muskoxen, dire wolves, and saber-toothed cats are also present in northeastern Tennessee at this time. 

Near the end of the Pleistocene a new animal arrived, humans. Scientists generally agree that humans entered North America by ~15,000 years ago, although the earliest evidence for human communities in northeastern Tennessee dates to ~13,000 years ago. These people are the ancestors of modern indigenous peoples such as the Cherokee and the Muskogeon who lived in the area when it was first settled by Euro-American colonists.