Taxonomy and evolution
Research into the American cheetah has been contradictory. It was originally believed to be an early cougar representative, before being reclassified in the 1970s as a close relative of the cheetah. This suggested that the ancestors of the cheetah diverged from the Puma lineage in the Americas and migrated back to the Old World, a claim repeated as recently as 2006 by Johnson et al., and in 2015 by Dobrynin et al. However, other research by Barnett and Faurby, through examining mitochondrial DNA and reanalyzing morphology, has suggested reversing the reclassification: the American cheetah developed cheetah-like characteristics through parallel evolution, but it is most closely related to Puma and not to the modern cheetah of Africa and Asia. Moreover, Faurby notes that no Acinonyx fossils have been found in North America, and no Miracinonyx fossils elsewhere. However, O'Brien et al. (2016) posit that the supposed homoplasy between the genera is controversial, as it is asserted that is not necessarily any conclusive anatomical or genetic basis for dismissing a homologous relationship between Acinonyx and Miracinonyx. The veracity of the origin of the modern cheetah is also debated; however, Miracinonyx is believed to have evolved from cougar-like ancestors, regardless of whether in the Old World or the New World.
The cougar and M. trumani are believed to have split from a cougar-like ancestor around three million years ago; where M. inexpectatus fits in is unclear, although it is probably a more primitive version of M. trumani.
M. trumani was the most similar to true cheetahs in morphology. Living on the prairies and plains of western America, it was likely a predator of hoofed plains animals such as the pronghorn. In fact, predation by Miracinonyx is thought to be the reason that pronghorns evolved to run so swiftly, their 60 mph top speed being much more than needed to outrun extant American predators such as cougars and gray wolves.
The similarity between M. trumani and the cheetah is an example of parallel evolution. As grasslands became more common in both Africa-Eurasia and North America, cougar-like cat species from both continents evolved to catch the new fleet-footed herbivores. The claws of M. trumani had even become only partially retractable, to be used for better grip at high speeds.
M. inexpectatus was more similar to the cougar, its proportions between that of the cougar and M. trumani. It had fully retractable claws, and although it was likely faster than the cougar due to its slim build, it is also thought to have been more adept at climbing than M. trumani.