Prior hypotheses of the relationship of these three North American groups suggested that, because of their morphology similarity to Asian stock, the stilt-legged horses were related the Asian asses, and had migrated into the Americas by way of the Bering land bridge. Hippidion was so distinctly different from other horses that it was considered to have diverged from the equid lineage about 10 million years ago. These species were thought to have spread into South America during the Great American Biotic Interchange about 2.5 million years ago, when the Isthmus of Panama formed and allowed animals to move north and south between the continents. Two competing models of these relationships, based on morphological analyses, are shown below.
The molecular data revealed a surprise, though. The tree below was constructed by analysis of mtDNA, and it groups the animals very differently. The caballines of America and Europe form a single clade, as expected. The stilt-legged horses and Hippodion, though, cluster together; Hippodion diverged only recently, and the stilt-legged horses are more closely related to the caballines than to the Asian asses. The authors also suggest that many species within the caballines ought to be lumped together, are more likely to represent regional variants than true species.
The figure is a maximum likelihood phylogeny based on mitochondrial DNA. Black numbers above/beside nodes are posterior probabilities and bootstrap values, respectively (only values > 50% are shown). White numbers on black background are divergence times as calculated from the molecular data. Numbers/letters in bold at the beginning of each specimen's name are sample numbers or GenBank accession numbers. Labels of prehistoric specimens are followed by their age, if available, in thousands of years. The outgroup (Rhinoceros and Ceratotherium) is not shown.
Tests of evolutionary hypotheses.