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.