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

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

The initial impact event wiped out most dinosaur within about 5000 miles within minutes. Beyond that very irregular and fuzzy expanding zone the immediate impact still killed but with a slow gradient of less severity. Dinosaurs still existed far away from the impact years after the impact but the world climate was severely altered by the atmospheric results of the subsequent fires and initial dust and debris. It was a slow but steady catastrophe over the next many thousands of years. Populations of all large animals slowly died off and went extinct. Eventually, by about 100k years after (some say sooner like 10k years, some say later like 1 mil years) the few remaining small dinosaurs that still existed had started to evolve into what became modern birds. More on reddit.com

7:35am on a Tuesday Seriously though, our estimate gets more accurate as we learn more, currently it's roughly 66 million. More on reddit.com