There would be an awful lot of contradiction and overlap.
Not to mention that many species from earlier epochs such as the Precambrian would find a “modern Earth” biosphere to be literally poisonous to them. (Many don’t like oxygen or require amounts that are insufficient with today’s atmosphere.)
You would see the greatest mass extinction event playing out in real time if it all manifested all at once.
It would also be proving evolution or at least Darwinian style survival of the fittest in real time.
Speaking of evolution wasn’t this concept basically the plot to Evolution? Albeit alien life?
You don't even need to go to earlier epochs. The larger dinosaurs alone would be behaving like a human trying to hike Everest without an oxygen mask.
The They who said it
Actually the oxygen content of the atmosphere is probably not why they were so large.
After all, whales do fine and they're just as big and breathe significantly less, so it's clearly possible to be huge and oxygen efficient.
Same with the giant arthropods, while the oxygen content may have helped them be large, most of them lasted well past when it fell, and most likely went extinct due to the massive marshes and swamps in which they lived fragmenting and no longer being large enough to support substantial communities of obscenely large millipedes and spiders and gryphonflies.
They haven't come back because their particular niche has been filled and remained filled by other things, and the swamps where they'd do well are gone, but if you put them on a continent that's extremely swampy and warm and had no other animal life with todays oxygen levels, they'd probably come back if you waited a few million years. The giant millipedes were there to fill the large herbivore niche, for example, and now they fill the tiny herbivore niche. But they could just as easily claim their old niche back if there was nothing to stop them.
Now mind, we actually have a bunch of precambrian life around today. For example, sea pens, sponges, and jellyfish are all from around then, and looked pretty much the same. Starfish were around, but they were still basically sea anemones back then, they lived on what we'd now consider their back. (Sea anemones were also a thing.)
But the land plants and fungi from then are almost all gone, for good reason. Most plants were basically moss. There were enormous fungi, as big as trees which had their own boring beetles, but they wouldn't survive for very long in an environment where shade exists, because low lying fungi doesn't need to weather the sun, which is one of the most dangerous things for life to be exposed to. Light-sensitive pigment was selected for specifically to know when there was sun so it could be hid from.
Most plants and animals from the past would get quickly outcompeted in their own niche and die out in a matter of decades. Further, most early sea life was adapted for a pre-sand ocean. Sand didn't really exist on the ocean floor until worms showed up, and they effectively destroyed nearly all species of sea pens and other anchored animals aside from the few that managed to survive to today. So much of that life wouldn't even last a year. In contrast, most relatively modern animals are adapted for a sand sea floor and couldn't survive in a rock-floored ocean.
I just recently have Idea for Alternate Universe based on inspired by Land Before Time & Ice Age with concept of this universe is based on Site B Mode of Jurassic Park Operation Genesis but I downplayed Speculative Biology tropes in this world so they are similar to our world lifeform except this world don't have modern homo sapien sapien but only archaic human and story is tell in Documentary style similar to Walking with Series