Simpson's Phylogeny of the Horse
Ruben A. Stirton, a student of Marsh's, is known for being the first to complete a phylogeny of the family Equidae that was branching and had no main line of descent, and for being the first to show the evolutionary relationships between species (as opposed to only between genera) (MacFadden 1992). This phylogeny, published in 1940, was modified and simplified by George Gaylord Simpson in his 1951 book, entitled Horses (MacFadden 1992). Simpson's phylogeny, while similar to that of Stirton, became more well known and is still considered by some to be valid today (MacFadden 1992). However, in Simpson's phylogeny, the Eocene-Oligocene evolutionary sequence was represented by a single unbranching line, while more recently uncovered evidence suggests that branching evolution occurred here as well (Gould 1987). Simpson also realized that his phylogeny was a much simplified version of the actual evolutonary history of the family, and noted that each branch on his figure represented many more branches in reality (MacFadden 1992).
Above, the general shape of Simpson's phylogeny.
Modified from Simpson 1951.
