..."Mammals, including humans, are warm-blooded and generate a lot of heat internally," he explained. "This becomes a problem at large body sizes as there is a danger of overheating. It's possible that many extinct archosaurs, including dinosaurs, were intermediate between cold-blooded and warm-blooded physiologies."
...and sweet little birds were warm-blooded.
NEWS: Blue Whales Keep Getting Bigger
"The large herbivorous dinosaurs undoubtedly spent much of their day feeding," McNab told Discovery News. "One should notice that the heads of dinosaurs related to the size of the bodies were very small, which means that the dinosaurs spent little time chewing the food, so most processing occurred in the gut, therefore the process of eating was probably inexpensive."
"This is very different from the behavior of most herbivorous mammals, which have large heads that house many teeth and spend much time chewing," McNab explained.
Benson thinks it's unlikely that any land animals today, including humans, could ever evolve to become as large as the biggest dinosaurs were.
"Mammals, including humans, are warm-blooded and generate a lot of heat internally," he explained. "This becomes a problem at large body sizes as there is a danger of overheating. It's possible that many extinct archosaurs, including dinosaurs, were intermediate between cold-blooded and warm-blooded physiologies." "If so, then temperature physiology would not have imposed limits on their body size," he added. "But it would certainly limit that of giant humans."
(Source)
Maybe birds would've became (large as) dinosaurs.. but couldn't. The dinosaurs kept eating them if they wandered out in open terrain. They could never diversify until the dinosaurs went extinct.
Largest Flying Bird Could Barely Get Off the Ground
"Lived in the Andes mountains and the pampas of Argentina about six million years ago.
"Takeoff capability is the limiting factor for the size of flying birds, and Argentavis almost reached the upper limit," Chatterjee said.
"Heavier birds such as the ostrich had to give up flight."
(Source)
Terror Birds: Predators with a Kung Fu Kick
South America 65-2.5 million years ago... These monstrosities arose AFTER dinosaurs died out enabling birds to exploit niches that they had not before.. which includes growing to heights that they had not in the past. Birds this size... had they evolved during the Cretaceous or Jurassic... would have ran into a dinosaur who was larger and made for a delicious Bucket o' Bird.
"The largest terror bird was the gargantuan Brontornis burmeisteri, which stood nearly 10 feet (3 meters) tall and weighed a whopping 1,100 pounds (500 kilograms)."
(Source)
Did these bird monstrosities give rise to modern birds like the hummingbirds... the geese, the parrot? Does it sound like they did?
They went extinct.
And note: National Geographic is guilty of reprinting the lie:
"Researchers still do not know if terror birds hunted in groups—as velociraptors..."
Fossil evidence shows Velociraptors were most likely solitary predators.
And THIS is probably why:
"...If relative brain size is any measure of intelligence, dromaeosaurs were just a little smarter than typical dinosaurs. Not geniuses by mammalian standards."
(Source)
No bucket o' brains.
Biggest Dinosaurs Had Brains the Size of Tennis Balls
"An advanced member of the largest group of dinosaurs ever to walk the Earth still had a relatively puny brain, researchers say."
(Source)
Now birds (in proportion to body mass) are complex and intelligent creatures.
Bird Brains (Source)
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