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I can think of only four ways that that Diplodocus could have taken a single step without a total and soon fatal collapse:

He lived on a planet with a different universal gravitational constant than we have now.
He lived on a planet with a much, much smaller mass than present day earth.
He lived on a planet with a much larger radius than present day earth.
He lived on a rapidly spinning planet.

How about this one:

He lived on a planet with a huge polar aligned tidal force opposing the pull of gravity as his planet orbited around a large gravity producing mass.

? We also have the distinct problem that Diplodocus' difficulty with his morning amble 67 million years ago is not the only mega-fauna with problems... the Teratorns of both North and South America must have been having a large problem with flying to look for that early sloth within the last 20,000 years... going extinct only 12,000 years ago.

Was the air denser then? No, that can't be the answer because the keeled breastbone of the Teratorns is NOT a proportionately super large adaptation compared to modern birds... indicating that it didn't have to anchor huge flight muscles (27 times wider and thicker) to overcome his 27 times non-proportional increase in mass than the smaller birds. In addition, the wing area, although larger than modern Condors, is only 9 times larger.... to support 27 times the mass in flight. Not a very good aerodynamic design... and one that shows no adaptation to having to support a lot more mass. Ergo, the force that the bird was designed to counter... gravity... had to be less.

Could a humming bird scaled up just three times its normal size and 27 times its normal mass still fly? How much energy would it need to consume to stay in the air and hover... and how large would the wings have to be?

He lived on a planet with a huge polar aligned tidal force opposing the pull of gravity as his planet orbited around a large gravity producing mass.

That one seems unlikely to me, but what do I know.

Changing the earth's rotation rate is kinda easy, if you can change its diameter.

There is intriguing geological evidence that the earth's diameter was quite a bit smaller at one time. If that was the case, it -would- have been smaller, hence rotating faster, hence supporting more massive land animals.