They are markers for extinction events. PT relates to an extinction event that marks the end of Paleozoic era and Permian period (P) and begins the Mesozoic era with the Triassic period (T). KT relates to an extinction event that marks the end of the Mesozoic era and Cretaceous period (K for the German word Kreide, which translates to Chalk- You know, because a team of North Americans found the first evidence for the event in a 'chalky' profile while in Italy) and begins the Cenozoic era with the Paleocene period (T because it was previously called the Tertiary period but has been officially replaced by the periods Paleocene (now Paleogene) and Neogene).