The purported mass dinosaur die off known as the K-Pg extinction event took place during the end of the Cretaceous Period (K) during the Mesozoic Era some 65 million years ago. This scientific theory is based on the study of the fossil record, which shows evidence for the existence of dinosaurs during the Mesozoic Era but not during the proceeding Cenozoic Era. There is a clear line of demarcation, an actual line found in sedimentary rock layers known as the K-Pg Boundary, with the majority of the fossilized remains found below this line belonging to the "Age of Dinosaurs" not appearing in the space above the line, which marks the beginning of the Paleogene Period (Pg), the Cenozoic Era and the "Age of Mammals."
Though this is an accepted theory, the K-Pg extinction event is not without its complications. The idea is mostly based on data acquired from the fossil records of a limited region in North America, which allows scientists to determine that the K-Pg extinction event happened, but not necessarily how it happened. There are also some caveats to the K-Pg extinction event theory, mostly based on the idea that fossil evidence can be somewhat fickle, especially when it is limited.