Reproductive isolation from a cross between a horse and a donkey or between a mare and a donkey, respectively. These animals are nearly always sterile due to the difference in the number of chromosomes between the two parent species. Both horses and donkeys belong to the genus "Equus", but "Equus caballus" has 64 chromosomes, while "Equus asinus" only has 62. A cross will produce offspring (mule or hinny) with 63 chromosomes, that will not form pairs, which means that they do not divide in a balanced manner during meiosis. In the wild, the horses and donkeys ignore each other and do not
Equus conversidens New Mexico, Oklahoma, Kansas and Florida, synonymizing "Equus francisci", "Equus tau", "E. littoralis", "E. achates", and "E. barcenaei" with "E. conversidens". However, Winans (1985) and MacFadden (1992) challenged the validity of "E. conversidens" due to its minimal diagnostic value, and treated "E. francisci" as valid. Subsequent study confirmed the "nomen dubium" status of "E. conversidens" because the holotype was undiagnostic besides its small size. Equus conversidens Equus conversidens (Owen 1869), or the Mexican horse, is a dubious Pleistocene species of horse, now extinct, that inhabited North America. The holotype of "Equus conversidens", a partial palate, was unearthed in Pleistocene deposits