377: EVOLUTION: Fossil Horses--Evidence for Evolution -- MacFadden 307 (5716): 1728 -- Science
Fossil horses have held the limelight as evidence for evolution for several reasons. First, the familiar modern Equus is a beloved icon that provides a model for understanding its extinct relatives. Second, horses are represented by a relatively continuous and widespread 55-My evolutionary sequence. And third, important fossils continue to be discovered and new techniques developed that advance our knowledge of the Family Equidae. The fossil horse sequence is likely to remain a popular example of a phylogenetic pattern resulting from the evolutionary process.The article discusses some of the scientific history of the study of horse evolution, and presents a thorough review of the state of knowledge about the evolution of horses. More at Pharyngula.
What's interesting about horses is that they are a once diverse lineage now represented by a few species. That gives the impression of directionality to their evolution, a pattern not supported by the data or the theory.
In particular, while the story of horse evolution was once seen as a linear increase in size, it is now clear that there was a period of relative stasis in sizes, followed by a wide diversity of horse sizes. Extinctions eliminated much of that variation along the way, leaving the impression of direction instead of diversity.