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.
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The blue whale is a modern organism and is much bigger than a megaladon or brontosaurus. As far as we know, it’s the largest animal to ever exist.
If I remember correctly from science class there was a lot more oxygen back then because plants had been around for millions of years pumping O2 into the atmosphere.
So nature needed a way to use up all that O2 and make CO2. Hence really big dinosaurs with really big lungs sucking up lots and lots of Oxygen to make carbon dioxide for the plants.
Outside of animals, check out Pando. Its a colony of aspen that all grow from the same root system so it is actually one tree, although from above ground it looks like many trees. It weighs about 30x a blue whale
Dunno how that compares to extinct plants but it’s still pretty huge
https://en.m.wikipedia.org/wiki/Pando_(tree)
The bird-like features most likely allow dinosaurs to get as big as they were.
They laid eggs instead of giving birth, which allow them to give birth more quickly and their offsprings were able to feed on their own very quickly, allowing the mothers to feed and produce more children etc.
They respiratory system allowed them to get big. Mammals cant get this big, because their weight would actually crush them. Whales need buoyancy of the water, but the bird-like air sacks allowed them to grew larger and larger. That probably also had evolutionary advantages, becuase the type of food available for them (some theories how most dinosaurs had to eat foliage from plants and trees, meaning the larger they got the higher positioned foliage they could eat which allowed them to get bigger which required more food which allowed them….) being bigger was better.
Climate stability is what allowed them to grow so large. There was a very stable climate for many millions of years, which allowed species to become hyper developed for those environments. When climate changes, the smaller species with a higher generation rate consistently fare better than larger species, often due to food scarcity and rate of change. The same thing happened in the past 15 thousand years – rapid climate change from an ice age caused most of the megafauna to go extinct.
The issue is that large species usually need an abundance of food, which requires a stable ecosystem. If 80% of the plants in an ecosystem die, all the larger herbivores and predators are going to die off, but the small herbivores will likely survive, as will their predators.
This sort of thing can be seen in real time by watching the arctic circle. When the flora freezes over and food becomes scarce, all the large fauna go away. The polar bears hibernate, the wolves and caribou move south, etc. But the foxes and the hares stick around – the hares can survive on the scarce flora, which supports a small population of foxes. Once it warms up and the flora comes back, so do the big fauna.
Were they though? We currently have the largest animal to have ever existed on the planet living in our lifetime. The Blue Whale
Mostly there just weren’t any humans to extinct them. We are an existential threat to all sorts of fauna, but more so for the larger species. It’s either domestication or eradication, humans don’t tolerate competition.
One theory is that as humans colonized the globe, we hunted every large animal we came across to extinction. Africa is the one big exception because the large animals there coevolved with humans. Everywhere else, we wiped out the largest species, leaving only species that could reproduce fast enough to keep up with human predation.
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) 😀
Was a far more balanced ecosystem, mankind has truly wreaked havoc upon it. The oxygen enrichment in the atmosphere was at a much higher percentage. I could get theological about it if you want. But even from a darwinian pov as the limate changed from human burnin carbon/lignum fuel sources (wood,leaves,seeds and seed pods) the CO2 slowly accumulated to levels where such heavy animals ,with slow respiratory rates, could not as easily get sufficient oxygen to hunt and outrun human predation. Eventually ts caused them to bear smaller offspring which eventually caused changes to the genome of these species until we have the sall faster animals (most of which are mammalian and have higher respiratory rates).
Lemme know if you would prefer a theologically based answer.
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