To narrow this down, I'm looking only for climactic specifications. Not cultural responses, etc.
During Ernest Shackleton's expedition to the Antarctic continent, he found a huge canyon/crevasse that spanned a very large area of tectonic activity. In fact, with hot springs, volcanic activity and geysers, it remained a humid subtropical and temperate tropical version of Yellowstone or Gobustan.
This area was protected from climate changes over millions of years and still boasts the remnants of the Cretaceous Period. Due to heat from the planet, it maintains a humid and humid-subtropical climate to have allowed ancient plants and animals to thrive.
In a magnitude of order (no hard science needed), how big should my sheltered canyon or basin be, in order that Mr. Shackleton has stumbled upon dinosaurs and ancient plants, that were able to survive in a 'biome' for 66+ million years?
Post Script: I know this to be impossible; I'm asking only for the physical size, in a magnitude of order, needed to support such a large ecosystem over 66+ million years in an Alternate History story. Are we talking, "The Whole Continent," or can we have a good "Thousand Acres," et. al.
I don't care if the dinosaurs have evolved a bit. I want Shackleton to have stumbled upon a Cretaceous remnant that includes 'dinosaurs'