I feel that there must be some physiological stressors or environmental factors that inhibit modern organisms of all domains from reaching the size, of say, a megaladon or brontosaurus.
In: 18
The biggest animal that has ever lived is currently alive as far as we know.
(Blue whale)
Something that inhibits mammals ON LAND from becoming much larger is gestation-periods and internal temperature. At least according to this [Source](https://www.youtube.com/watch?v=QHe1wmEaYWo).
There are additional sources in the description.
There are also HUGE plants and fungi on the planet right now:
Biggest landplant: [Pando](https://en.wikipedia.org/wiki/Pando_(tree))
Biggest fungi: [Armillaria ostoyae](https://en.wikipedia.org/wiki/Armillaria_ostoyae)
Biggest seaplant: [Posidonia australis](https://en.wikipedia.org/wiki/Posidonia_australis)
Bonus link: [https://en.wikipedia.org/wiki/Largest_organisms](https://en.wikipedia.org/wiki/Largest_organisms) 😀
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