George Gaylord Simpson (16 June 1902 – 6 October 1984) was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern evolutionary synthesis.
He was an expert on extinct mammals and their migrations, especially the Great American Interchange between the Americas. Simpson opposed Alfred Wegener's theory of continental drift, so his work on animal migration lacked this idea.
Simpson dispelled the myth that the evolution of the horse was a linear process culminating in the modern Equus caballus. He coined the word hypodigm in 1940, and published extensively on the taxonomy of fossil and living mammals.