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On planets with lower gravity than earth what might make carbon bases megafaun rarer than in our own planet?

The planets have a source of geothermic energy, solar energy, winds and oceans.

Sometimes the oceans are ammonia and sometimes water.

Taking this into consideration, what general reasons might stop or discourage megafauna from evolving?

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Rapid environmental changes

Large animals grow and reproduce slowly. If your environment is subject to relatively frequent changes, it could create evolutionary pressure for creatures that grow quickly and adapt/evolve in a shorter time frame.

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    $\begingroup$ I dunno. didn't those giant dragonflies still produce hundreds of eggs? $\endgroup$
    – tuskiomi
    Sep 9, 2021 at 20:06
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    $\begingroup$ @tuskiomi Is that speculation? Or was evidence actually found for that? $\endgroup$
    – DKNguyen
    Sep 10, 2021 at 4:10
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An atmosphere with low oxygen, or a low density.

Supplying the body with oxygen takes more effort for large animals

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An evolutionary path different to the one fauna on earth took.

Afaik, earth's trees came way after bushes, grasses and other growths. If plants never developed high-strength fibre "architecture", they would not be able to grow tall the way we knew it.

If there were no selective advantage to grow tall, why would a plant develop that trait? This would probably rule out sustenance from starlight though.

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Well, Mr. Canseco, keep in mind that on Earth, helium and hydrogen are not gravitationally bound. Even water vapor can escape (http://scienceline.ucsb.edu/getkey.php?key=2#:~:text=Yes%2C%20water%20does%20escape%20into%20space%20--%20there,thermal%20energies%20can%20exceed%20escape%20velocity%20relatively%20easily.). On a low-gravity planet, gasses essential to your fauna may do the same, depending on how you set it up.

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