The fossil record of this group is marginal. Their restriction to coral reefs means their carcasses are liable to be dispersed by scavengers, overgrown by corals, and any that do fossilize will not long survive erosion. However, "Pygaeus", a very basal fossil from the mid- to late Eocene of Europe, dates from around the Bartonian 40–37 million years ago (Mya). Thus, the Chaetodontidae emerged probably in the early to mid-Eocene. A crude molecular clock in combination with the evidence given by "Pygaeus" allows placement of the initial split between the two main lineages to the middle to late Eocene, and together with the few other fossils, it allows the deduction that most living genera were probably distinct by the end of the Paleogene 23 Mya.