Let's add something to compare the 300' monster with, a 100' creature that exists today.
Blue Whale:
The largest mammal on earth is the blue whale and it is actually the
largest animal to ever have lived on this earth, even bigger than the
largest recorded dinosaur. Adults can grow up to 100 feet long and weigh
100 tons.
a passage about blue whales from the book "Among Whales" by
Roger Payne, President of the Whale Conservation Institute:
"In a large blue whale the heart weighs about four thousand pounds (2 tons)
and probably pumps about sixty gallons with each beat. Its valves would be
about the size of a hub cap, and a child could crawl through the aorta, the
largest blood vessel leaving the heart. So large are the arteries that even
sixty feet aft of the heart, where the main artery has narrowed down to a
diameter needed to support just the last few muscles attached to the last
few vertebrae, this vessel is still about the size of the sewer pipes that
carry waste water away from the average house - large enough, that is, to
allow a good sized trout to swim leisurely along it." The heart contracts
about 18-20 beats per minute or once every three seconds.
For further information search WhaleNet's educational resource pages.
A 300 foot dinosaur would be extremely large but maybe, just maybe, this was an oceangoing creature.

