 The animals singled out by these formulations, the amniotes other than the mammals and the birds, are still those considered reptiles today.The synapsid/sauropsid division supplemented another approach, one that split the reptiles into four subclasses based on the number and position of temporal fenestrae, openings in the sides of the skull behind the eyes.This classification was initiated by Henry Fairfield Osborn and elaborated and made popular by Romer's classic "Vertebrate Paleontology".