The first recorded time that mammals shrank significantly in size occurred 55 million years ago during a 200,000 year period known as the Paleocene-Eocene Thermal Maximum (PETM). Now, University of Michigan paleontologist Philip Gingerich has discovered that another period of shrinking occurred 2 million years later during the Eocene Thermal Maximum 2 (ETM2). The fact that this evolutionary reaction to extreme global warming has already happened twice has led scientists to believe that it could happen again if temperatures on Earth continue to increase as they have been.
Fossilized teeth and jaw bones of primates and hoofed animals from the ETM2 period were found in Wyoming’s Bighorn Basin and used to determine the size of animals at the time. Researchers referenced Hyracotherium, an early horse about the size of a small dog, as an example of just how much mammals shrunk during these two time periods. The initial “dwarfing” of the PETM period was more significant, during which the horse shrunk by 30 percent. ETM2 was less severe, but still saw a 19 percent decrease in the body size of Hyracotherium.
Gingerich spoke about the significance of animals shrinking in size more than once: “The fact that it happened twice significantly increases our confidence that we’re seeing cause and effect, that one interesting response to global warming in the past was a substantial decrease in body size in mammalian species.”
These adaptations more than likely helped various species of mammals survive the extreme changes in temperature, after which they returned to their original size. While some people may think this means we can handle changes caused by global warming unharmed, there is one significant difference: Ancient warming events were probably caused by the release of seabed methane clathrates—a kind of methane ice found in ocean sediments. Today, fluxes have been caused by the burning of fossil fuels—something that is much more harmful and difficult to reverse.