How did tuataras survive when dinosaurs became extinct?

The truth is, nobody is really sure why none of non-avian dinosaurs seem to have survived past the end of the Cretaceous. The larger ones, both herbivores and carnivores, certainly needed a lot of food to survive, which was likely very scarce following the K-T event, and popular theories suggest that that smaller dinosaurs were warm blooded, and therefore could not go long periods of time without food, whereas the tuatara is cold blooded and needs much less food than a similarly sized warm blooded animal might. However, the survival of other warm blooded animals such as mammals and possibly also Flightless Birds (which would have been extremely similar to maniraptoran dinosaurs) calls such sweeping theories into question. Certainly, the survival of the tuatara might be more due to luck than anything else, as their once prolific family, the Sphenodontids, suffered greatly since their heyday in the early Mesozoic (around 200 million years ago), when many relatives of the tuatara existed in a variety of environments, some even being aquatic (the pleurosaurs) and the two modern species of tuatara are all that's left of their lineage. However, the tuatara do have a certain degree of adaptation to cold weather, which might have enabled their ancestors to survive the hypothetical "nuclear winter" which followed the impact event which is implicated in the K-T extinctions, assuming it was present in their ancestors at that time, and isn't a more recent evolutionary adaptation.