Detailed fossil information on the distribution and rate of change of new equid species has also revealed that the progression between species was not as smooth and consistent as was once believed. Although some transitions, such as that of "Dinohippus" to "Equus", were indeed gradual progressions, a number of others, such as that of "Epihippus" to "Mesohippus", were relatively abrupt in geologic time, taking place over only a few million years. Both anagenesis (gradual change in an entire population's gene frequency) and cladogenesis (a population "splitting" into two distinct evolutionary branches) occurred, and many species coexisted with "ancestor" species at various times. The change in equids' traits was also not always a "straight line" from "Eohippus" to "Equus": some traits reversed themselves at various points in the evolution of new equid species, such as size and the presence of facial "fossae", and only in retrospect can certain evolutionary trends be recognized.