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I'm writing about a fictional world where reptiles evolved and became the dominant species on a planet. on this planet an extinction event never occurred and reptiles still remained the dominant species , despite the absence of mammals , through convergent evolution some reptiles managed to evolve like primates such as apes and managed to use stone tools where they eventually started entering the stone age.

I'm wondering whether such a scenario would be possible.

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    $\begingroup$ What's a "reptile"? This is not a trick question; it actually is hard to find a good definition for a reptile. The best attempt is to define reptiles as those amniotes which are neither mammals nor dinosaurs (including birds). How does your definition of reptiles exclude Archaeothyris and the other basal synapsids, which most definitely looked very very much like stereotypical reptiles? Because if it does not exclude them, then reptiles did actually evolve into primates. $\endgroup$
    – AlexP
    Jan 28, 2020 at 22:19
  • $\begingroup$ According to several studies and anecdotes, monitors, tegus, and anoles are already quite smart. So, brain size wouldn't be that big of an issue. $\endgroup$
    – Aezyc
    Jan 29, 2020 at 0:00
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    $\begingroup$ define "like primates" there are plenty of arboreal lizards. $\endgroup$
    – John
    Jan 29, 2020 at 0:33
  • $\begingroup$ They did, it just took them a few hundred million years. That is, primates, like all mammals, are direct evolutionary descendents of one particular group of synapsid reptiles. The biggest problem, I think, is evolving endothermy. $\endgroup$
    – jamesqf
    Jan 29, 2020 at 0:38

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Do Drepanosaurs count?

Drepanosaurs might be described as bird-monkey-chameleon-things, or monkey-lizard if you're in a hurry. They were arborial reptiles during the Triassic, characterized by bird-like facial features, primate-like prehensile tails and grasping appendages, chameleon-like skeletal structure, and sometimes a tail-claw.

Adjust the diet a bit to favor more primate-like faces, and avoid that nasty Great Rainforest Collapse, and it seems plausible that a branch of Drepanosaur might have taken on even more primate-like features.

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  • $\begingroup$ Chameleons have also been mentioned to have very mammalian-like limb girdles, and with their arboreal locomotion they are quite similar to primates. So it looks like there is nothing stopping a reptile from evolving a primate-like limb girdle. $\endgroup$ Jan 29, 2020 at 7:00
  • $\begingroup$ The main problem with them - they were (most probably) cold-blooded and had not-that-fast matabolism. So they were more like sloths, than primates. $\endgroup$
    – ksbes
    Jan 29, 2020 at 9:48
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Reptiles and mammals are families of life that are specific to Earth. Your planet wouldn't have reptiles, though it might have animals that through convergent evolution and happenstance might look or function similarly to them.

That reptiles ruled before mammals on Earth until an extinction event is pure happenstance, and not something you would expect to see on another planet.

The question then becomes "Can a clade of animals that are in some way like Earth reptiles evolve high intelligence and a body plan similar to Earth primates?". (Note - two separate things. You don't need the body of an ape to be intelligent.) I can't think of any reason why not. I don't think you even need a specific reason for this to happen, it's just how evolution happened to go on this world.

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