Cretaceous–Paleogene extinction event

I actually doubt the extinction were as total or global, on no less than three fronts. First up is palynology shows global disruption to food webs at the K/Pg, though it was not equally severe everywhere. Floras of the Antarctic Kingdom bounced back soon enough. Second is the matter needs 'slices of time' recorded in the sedimentary rocks, that must be suitably fossil yielding, from at least the late Masstrichtian to early Danian - Hell Creek is very informative for this reason, but cannot represent a global picture. Third is the size filter selecting K/Pg survival, really ought to have let through a number of Campanian-Maastrichtian coelurosaurs. The smaller alvarezsaurs were apt to survive, as were petite long tailed pennaraptorans such as Hesperonychus and Rahonavis. Not a lot is known of Palaeocene birds still. It's still possible that toothed Ichthyornis type forms, and even enantiornitheans survived. It's obvious the latter included more than only avisaurids, in the latest Cretaceous. No one same would deny dinosaurs were victims of the K/Pg but the way people think about that, involves a fair few assumptions that can't be justified simply by repetition. More on reddit.com

There were a few different reasons, such as size, mobility, diet etc. Many of the species that survived the end of the Cretaceous were generally smaller than non avian dinosaurs and so this gave them an advantage as they would have required less food to survive. As for the birds their mobility went along way in ensuring their survival as it is a massive advantage being able to fly to areas with more food or better conditions. But, that is not to say this guaranteed survival as many species of birds, mammals and reptiles etc would have gone extinct too, especially if they were too highly specialised for a certain environment or lifestyle. More on reddit.com

Dinosaurs weren’t necessarily hit hardest (though larger creatures definitely were hit hard). Even among mammals it was brutal. Whole groups were wiped out (North American metatherians, for example). Pterosaurs were wiped out. Only half of the crocodyliforms made it. Belemnoids and ammonoids got wiped out. Mosasaurs and plesiosaurs were gone. People focus on the dinosaurs, but pretty much anything big got hammered, as did most of the specialists. The dinosaurs that made it (a small fraction of the birds) were small generalists able to eat seeds and bugs. That’s how extinctions go. Niches get emptied and then refilled. More on reddit.com

The simple answer is the rocks they are found in. The earth is in layers, the deeper you go the more layers there are, and within these layers are smaller layers. If you pay attention to them and study them you can start to be able to piece Earth's history together, and with tools like carbon dating, we can guesstimate when and where they would've died off. Some are more obvious than others, but in the end I hope I was able to answer your question mate More on reddit.com