The problem with the huge dinosaurs that used to exist on Earth and whose fossilized bones have been excavated all over the planet – is that they just could not exist in our present physical conditions, they were just too Big.
One problem is blood flow. A single heart could not pump sufficient blood to sustain a big dinosaur like a T. Rex or more so a brontosaurus. The heart would have to be immense and blood pressure would have to be so high that common flesh could not withstand it. Wildlife veterinarians have found in sedating much shorter giraffes, that the head can’t be in a prone position for long because of the animals high blood pressure necessary to reach the 15 foot head, and risk of stroke in a prone position – blood vessels will burst in the brain. A large dinosaur would require several hearts to pump blood to all parts of the immense body, and this is just not a likely or practical possibility mechanically or evolutionary-wise. No animal today has more than one heart.
Another problem is the weight of internal organs. The same vet cited above stated that when sedating a rhino, because of its weight the rhino could not be left laying in one position for more than 20 minutes, as its weight would crush bottom organs and stop blood supply. Continual movement when awake must alleviate this, but dinosaurs being much, much larger the a rhino, the problem of gravity would be exponentially worse. How would lower internal organs work at all, movement or not, with 5 tons of liver, spleen, et. al. resting upon them?
The sheer weight of some dinosaurs and the necessary bone structure provides perhaps the biggest impossibility. Theosophist and engineer Victor Endersby pondered this question some years back.  He calculated the weight of a T. Rex from figures given (weight or mass increases at the cube of height for the same type of animal, not just the square.) He calculated that one might weigh 90 tons. From bones, and taking the T. Rex’s ankle diameter, and comparing it with the weight and ankle diameter of an average 6 foot man, he figured proportionally the man would weigh 1500 lbs. – which would crush the ankle by just standing still. Extra height and size also puts much more stress on the ankle and other bones for the same reason that it is much more difficult to pick up a weight on the end of a stick rather than being directly over it – a “lever” effect. So, large as they are, the bones of dinosaurs are no where near being big enough to be able to support their huge mass in our present conditions of matter and gravity.
There’s no real scientific solutions to these conundrums. One unlikely solution presented is that the planet was spinning much faster and the centrifugal effect offset gravity by a sufficient amount.
While the problem wasn’t presented in her day, a solution is provided in Blavatsky Theosophy, in that according to her system the conditions of matter gradually change over huge spans of time. She delineates cycles of time, calling them Rounds and Races, etc., and states the Dinosaurs were creatures of the Third Round, while we are now in the Fourth Round.  The cycles carry Earth through periods where matter is more dense or more ethereal, and when the dinosaurs existed the Earth and they were in what she called the Astral state. As the Earth gradually descended to a more dense present state of matter, their bones materialized along with everything else. As Endersby said, the conditions or density of matter could change ten times in ten minutes – as long as relationships stayed proportionally the same – and we would never know the difference.
In Blavatsky’s system we have reached the crux of materiality or “density” a short few million years ago, and are now again on the upward cycle towards a more ethereal state of matter, so to speak. Theosophical teacher G. De Purucker says evidence of this is provided in the natural radioactivity of some of the heaviest elements, like uranium. The densest matter is gradually disintegrating into less dense
matter or a less dense element, which is what radioactivity causes to occur. Heavier radioactive Uranium is gradually transforming itself into less-dense Lead.
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 “Theosophical Notes”, No. 1, 1964, pp 11-12.”
“…. Those big lizards, said by the S.D. to be from the Third Round of less dense matter. They are just not practical animals for our era. The locomotive abilities of animals of any time and place are conditioned by the cube-square ratio. That is to say, as height increases, weight increases as the cube of height and the ability to carry the increases as the square of the diameter of the ankle. We took a 50-foot Tyranosaurus Rex, which was a ferocious, carnivorous and hence active biped, lending itself well to comparison with man: sliced, him up, and calculated him. He came out 90 tons, and in proportion to the writer, said writer would have to weigh about 1500 lbs or three quarters of a ton in order to have an equal problem getting around in swamps and latching on to live meat.
“It just won’t work. Scientists are aware of this – rather uneasily – but they have not even tried to solve it. They can’t find a handle to get hold of. Not only this, those animals ran to 70 feet; in which case the problem would increase by a factor of about 36 percent.
“This is the only sort of thing in sight at present which could indicate anything about these changes in matter; because it could change its density, etc., now, ten times in ten minutes and nobody could tell the difference, because everything would change alike. But these fossils are likely to turn out to be fossilized cosmic history in more senses than one. (This question of the previous existence of a proto-matter of the same general pattern as ours, but less condensed, as we said, is tied up with quantum physics.) ….”
 “The Secret Doctrine”, H.P. Blavatsky, vol. II, p. 68:
“The astral prototypes of the mineral, vegetable and animal kingdoms up to man have taken that time (300 million years) to evolve, re-forming out of the cast-off materials of the preceding Round, which, though very dense and physical in their own cycle, are relatively ethereal as compared with the materiality of our present middle Round. At the expiration of these 300 million years, Nature, on the way to the physical and material, down the arc of descent, begins with mankind and works downwards, hardening or materialising forms as it proceeds. Thus the fossils found in strata, to which an antiquity, not of eighteen, but of many hundreds of millions of years, must be ascribed, belong in reality to forms of the preceding Round, which, while living, were far more ethereal than physical, as we know the physical. That we perceive and disinter them as tangible forms, is due to the process of materialization or crystallization referred to, which took place subsequently, at the beginning of the Fourth Round, and reached its maximum after the appearance of man, proceeding parallel with his physical evolution. This alone illustrates the fact that the degree of materiality of the Earth changes pari passu with that of its inhabitants. And thus man now finds, as tangible fossils, what were once the (to his present senses) ethereal forms of the lower kingdoms.”