And yet, we have elephants as recently as 10K-y ago, which weighed over 40,000 lbs. Was the Earth’s gravity also much weaker as recently as a mere 100 centuries ago in your “theory?”
Actually, yes. Eleven thousand years ago Teratorns flew in the Andes. Their fossils exist. These were birds indistinguishable from modern eagles except for one major and decided difference.. . . teratorns had wingspans up to 45 feet and weighed an estimated 450 pounds! The largest modern birds capable of flight are the Albatross with an up to 11 foot wingspan and weigh only 30 lbs. or some reports offer the Andean Condor with a reported 10.5 feet and 25 pounds. Both birds have trouble getting off the ground under one G, which is why the Albatross is called the Goony Bird and the Condor is prey to land animal while feeding and needs a downhill, running start of at least forty feet to get off the ground.
Biologists and Aeronautics engineers tell us that a scaled-up eagle with muscular engines simply could not fly under its own power without a complete redesign of its "airframe and power plant" . . . Just to flap the wings the muscles would have to be far larger and the bones would have to be greater in cross-sectional strength, if not made of carbon fiber, and the wishbone—the anchoring keel that those flight muscles attach to—would be so large, thick, and deep to handle the stress, that the bird’s shape, it's form factor, would be unrecognizable as an eagle.
Speaking of flying animals, the Cretaceous had flying dragonflies, completely indistinguishable from modern dragonflies, except the Cretaceous model had FOUR FOOT WINGSPANS. Such an insect could not fly under modern gravity. . . cube/square law, again. . . and its form factor would be far different at four feet than at four inches.
And the facts are that Teratorn and four foot dragonflies flew at their sizes and form factors 12,000 years ago and in the Cretaceous. These are facts. Sticky things facts. They didn't fly underwater, Fred. Either these animals magically defied all laws of aerodynamics or somehow magically broke the cube/square law, or something was drastically different about the environment in which flew undoubtedly flew a mere 12,000 years ago and back in the Cretaceous. . . and Fred, there are one Hell of a lot more anomalies from those eras, such as your 40,000 pound pachyderms that NO LONGER, and can no longer exist today.
Megafauna exists nowhere on land today yet millions of years of evolution seems to show that mega size is a survival factor. Why not today? What has changed? The best answer seems to be gravity increased
The fact is that above a certain size, if a modern elephant stumbles, it cannot get up. . . and unless an outside force provides assistance, it is effectively dead. Gravity prevents it from lying down and then getting above a certain size. . . . and most adult elephants spend their lives standing up.
Under modern gravity, the blood pressure to get the blood the mere SEVEN FEET from a Giraffe's 24lb heart to its brain is so high—300 over 200—that it would most likely cause any other animals' arteries to blow out before long! Even then, the arteries in the Giraffe's neck have evolved special one-way valves to prevent gravity from pulling the blood back down. Yet what was the size and weight of the blood pump possessed by Argentinosaurus—which weighed an astounding 400,000 pounds, was 125 feet in length, and had a 65 foot long neck—to have blood reach what ever sized brain it used? The mind boggles thinking about it.
NOW, do you begin to grasp the problem? This is an issue that simply cannot be swept under the proverbial rug by assuming facts not in evidence such as claiming a non-existent semi-aquatic lifestyle for these animals. (Incidentally, they unearthed a vertebra of another sauropod (dubbed Amphicoelias) that is TWICE the size of the corresponding vertebra in the Argentinosaurus, which implies its from a dinosaur that is twice Argentinosaurus' size!)
By the way, working the math backwards to find what level of gravity acceleration WOULD allow such megafauna to safely exist, we find that approximately 30% of today's gravity would permit such huge animals to live and survive. . .