“The Cretaceous was steamy and only smaller mammals could lose heat fast enough in the hot, humid conditions to be viable as a warm-blooded lifeform over large areas of the planet. ”
This is interesting [as in strange/odd] view.
I think that large mammals could have survived perfectly fine in the climate during the Cretaceous period. There would been different animals back then as compared to now, but difference has to with evolution and the dinosaur extinction event [mostly or entirely due to a space rock about 10 km in diameter impacting what current know the Yucatan peninsula].
The extinction event mainly affected land animals. Land plants were less affected in terms going extinct.
So such trees as now exist in Temperate Zones existed prior to age of Dinosaurs
“Plant life of the Permian (248 million years ago) took on an increasingly modern “look” with the rise of a number of gymnosperm (naked seeded) plants during the late Carboniferous and their diversification during the Permian. Indeed, the late Carboniferous “extinction” is almost inapplicable to terrestrial plants.”
http://www.bomengids.nl/uk/tree-evolution.html
And of course the modern technological human can live in colder and warmer temperatures than any particular tree. And human with their need to keep warm in houses, are spending most of lives in environments with CO2 levels above 1000 ppm. And the modern human comes from a tropical region and quite comfortable if the climate always near 30 C.
As far as early large mammals:
“Pakicetids or Pakicetidae (meaning Pakistani whales) is a carnivorous mammalian family of the suborder Archaeoceti that lived during the Early Eocene to Middle Eocene (55.8 mya—40.4 mya) in Pakistan and existed for approximately 15.4 million years.”
http://en.wikipedia.org/wiki/Pakicetids
Evolution of elephants:
“The earliest known ancestors to the elephant were herbivores that lived about 40 million years ago, and were roughly the sizes of pigs and cows. The direct ancestor to the modern-day elephant is unknown, but fossils of numerous evolutionary off-shoots, such as the moerithenes (40 million years ago), the barythenes (40 to 35 million years ago), paleomastodons (40 million years ago), gomphotheres such as the mastodon, the stegodon, and the mammoth have all been found and studied. ”
http://www.ecotravel.co.za/Guides/Wildlife/Vertebrates/Mammals/Big_5/Elephant/Elephant_Evolution.htm
“The Eocene (symbol EO) epoch, lasting from about 56 to 34 million years ago (55.8±0.2 to 33.9±0.1 Ma)…
The Eocene Epoch contained a wide variety of different climate conditions that includes the warmest climate in the Cenozoic Era and ends in an icehouse climate. The evolution of the Eocene climate began with warming after the end of the Palaeocene-Eocene Thermal Maximum (PETM) at 56 million years ago to a maximum during the Eocene Optimum at around 49 million years ago. During this period of time, little to no ice was present on Earth with a smaller difference in temperature from the equator to the poles. Following the maximum, was a descent into an icehouse climate from the Eocene Optimum to the Eocene-Oligocene transition at 34 million years ago. During this decrease ice began to reappear at the poles, and the Eocene-Oligocene transition is the period of time where the Antarctic ice sheet began to rapidly expand.”
http://en.wikipedia.org/wiki/Eocene
So after the dinosaurs in the Eocene epoch was as warm and there were some fairly big mammals