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So a friend is working on a worldbuilding/rpg project and I'm "beta-testing". He drew up an earth-like map and set the players start locations. Then I ask him about fauna. Will it be modern earth? Prehistoric megafauna? Fantasy monsters? Dinosaurs? To which he promptly replies, "yes". After some needling, we agree that monsters and dinos will be rare and mostly regional. But he does want megafauna to be more common and live alongside normal modern species. My main question: is this feasible?

Or more specifically: Do we know much about species overlap between extant and extinct species? Could there be enough niches for everyone to survive without over-crowding the world with such a menagerie? What about cases like the many giraffe cousins, did they all live in the same area with different niches, did they compete or did they live apart?

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  • $\begingroup$ If they were of the same species, the giraffes would have to have lived apart, or interbreeding would eliminate any physical differences between them. $\endgroup$ – Bilbo Baggins Jun 17 at 23:10
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    $\begingroup$ Your title gives the answer. (Or at any rate, one possible answer.) Pleistocene megafauna such as mammoths, gompotheres, sabertooth cats, and so on co-existed with most modern species near the end of the period. $\endgroup$ – jamesqf Jun 18 at 0:13
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A certain species will not come in two different sizes in the same time period and the same place. It would be literally impossible to make a large version of the species because the larger specimens would keep breeding with regular sized specimens.

Here is a realistic alternative: Insular gigantism. This is a case where a few members of a species are stranded on an island, and the species on the island gradually grows bigger and bigger. A real example is that there was a certain species of rat of which a few members were stranded on the Canary Islands. They adapted to bigger and bigger sizes, and eventually grew to be over a meter long.

In your game, the players could spawn on a small island or isolated area (this is the case for most RPGs) where all the species are exhibiting island gigantism, and regular species of the animals had been introduced after the original animals had already undergone the transformation and could no longer interbreed with the regular species.

However, by definition, the giant and regular species would not be in the same species since interbreeding would destroy the size differences.

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  • $\begingroup$ While insular giantism is definitely something we can use, my primary concern was about the vast menagerie of species potentially creating unnaturally high levels of biodiversity. Similar to Jurassic park, where they compiled animals from the Triassic, Jurassic and cretaceous periods all in the same place and time. Only this time we're considering the ramifications of compiling animals from the entire cenozoic, from paleocene to holocene. $\endgroup$ – Riggy Jun 18 at 18:02
  • $\begingroup$ WRT different sizes within a species, there are certainly sex-related size differences. A well-known example is the walrus, where males are about twice the size of females. $\endgroup$ – jamesqf Jun 19 at 17:08
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Some combinations will not be possible because of reasons

Large dinosaurs specifically would have a problem coexisting with modern animals for one very important reason. The atmospheric pressure needed for modern organisms is much less than it was for ancient ones. This is particularly relevant to any flying creature and any dinosaur of the long neck variety. The details of why are enumerated in this scientific paper. If the atmospheric pressure is similar to the present earth's, then the flying dinosaurs could not fly, and the largest dinosaurs would die of lack of blood getting to their brains.

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Megafauna existed along side their smaller brethren, examples include dire wolves coexisted with wolves, giant ground sloths coexisting with regular three toed and four toed tree dwelling sloths similar to those existing today.

As long as there is habitat and food for the different subspecies, and there isn’t some disease that wipes it out, then the subspecies will continue differentiating and diverging in that Darwinian struggle for survival. Some will live side by side and others will adapt to a different habitat where their kind aren’t prevalent.

And, we know that mammals and dinosaurs coexisted, except the mammals were small shrew-like creatures. And, we know some dinosaurs evolved into birds. But, given the widely separated period between the age of homo sapiens and dinosaurs, it's hard to imagine that if an asteroid hadn’t kicked their butts that they wouldn’t have continued to evolve, especially if they had to compete with fast warm-blooded saber tooth tigers. So I don’t think ancient dinosaurs would exist side by side with modern, or relatively modern creatures. But, I could imagine things, besides birds, that might exist in your world that evolved from dinosaurs.

Imagine velociraptors with big puppy dog eyes — outside of a velociraptor, a book is a man’s best friend. And, inside a velociraptor its too dark to read.

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