Did you ever wonder why it was always the littlest kid in your class in school who could do the most pushups and pullups on the chinning bar or why you never see 200-lb athletes competing in gymnastics?Weight is basically proportional to volumn, which is a cubed figure (width times breadth times height) while strength is proportional to cross section of bone and muscle which is basically a squared figure. Double your physical dimensions, and you have a factor of two which gets figured three times for volumn and weight (you'll be eight times heavier), and only gets figured twice for cross section and strength (and you'll only be four times stronger). You'll have cut your power/weight ratio in half. Clearly you can only halve your power/weight ratio so many times and still stand up and walk; the mathematical limit for that sort of thing in our present world and gravity is about 20,000 lbs, indicating that the largest elephants at something like 15000 lbs are the largest animals which in actual fact are possible in our present world.
A few snapshots from the recent Japanese documentary on these topics.
The Basic Holden Thesis regarding sauropod dinosaurs, and why animals of such sizes are not possible in our present world.
Sauropod Thesis Defense wherein the infamous Holden thesis wrt sauropod dinosaurs is further amplified, and the Hokkanen paper often cited by the talk.origins regulars is shredded.
Other kinds of ancient square/cube problem anomalies
Amongst other indications of lesser gravity in antique times are Cro Magnon images of mammoths galloping. Elephants in our present world cannot run or gallop and can manage only a stiff-legged walk.
Flying creatures significantly beyond the present 30-lb limit for flying creatures
Similarly, flying birds in our present world and gravity are limited to something like 25 - 35 lbs. In prehistoric times, 200-lb birds with 25' wingspans flew, and other flying creatures weighing around 1000 lbs flew with 60' wingspans. When gravity changed to its present value, birds beyond a certain size which managed to survive did so by adopting a lifestyle not involving flight, and their wings became vestigial as we observe with the ostrich.
The original article on the Argentinian teratorn from Bioscience, December 1980.
The teratorn was a modern bird with a 25' wingspan. That only sounds big until you consider the Texas pterosaurs
Wann Langston originally wanted to reconstruct a Big Bend pterosaur with a 60' wingspan and the engineering department at UT wouldn't let him. Recent finds in Mexico and Israel however have more complete wing bones and do in fact indicate that the original estimate of wingspan was closoer to reality than the 40' limit they imposed on Langston at the time.
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