It's interesting that―while one group is coldblooded and the other warm-blooded―reptiles and birds share a common lineage, since they're all sauropsids. What a lot of people don't realize is that as mammals we also used to have an analogous cousin class. We're all synapsids, and the surviving synapsids (i.e. mammals) have warm blood. But there used to be a whole slew of coldblooded synapsids. In fact, they pretty much dominated the land before the End-Permian extinction. Which was some extinction.
If the non-mammalian synapsids were still around they'd be easily confused with reptiles, probably. They were put on the back foot during the Triassic period, and obviously never recovered. And here we are now. Life is full of odd reversals, some of which take place over a longer time than we can wrap our heads around.
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The species synapsids is definitely a new one to me. Do you suppose they had fur or hair, gave live birth, produced milk? Probably not but it's a consideration. After all it wasn't all that long ago we learned many dinosaurs sported feathers, or at least a type of feathers*. Makes one curious to see a live dinosaur or a synapsid for that matter - so long as we could drop by for a safe place to look, like a window, rather than going wholesale Jurassic Park.
When the Permian-Triassic extinction event came along (nobody knows why for sure but probably volcanoes as in the Siberian Traps or maybe an asteroid) taking out synapsids it also removed about 90% of all species. And at the end of the Triassic the Jurassic made room for dinosaurs. You're right that we can look at a chart of the timelines but not begin to understand just how much time the eras actually were. Even the extinction events took millions of years.
*
https://www.amnh.org/exhibitions/dinosaurs-among-us/feathers
It's pretty new to me as well. I knew that warmblooded animals--which is to say mammals and birds--evolved from coldblooded ones. Eventually I found out about the connection between dinosaurs and birds. But that there used to be something else that wasn't quite reptile, or reptiles as we know them, was a pretty weird finding. And by the way, it's funny how a predatory dinosaur covered in feathers looks so much like something from Where the Wild Things Are. It would be fun to see these things again, but we seem likely to see only artificial reconstructions if anything.
From what I understand the Permian-Triassic extinction was the closest Earth ever came to losing all of its lifeforms. Obviously we're all lucky that didn't wind up happening. But it must have had a huge effect on the kinds of life that would flourish later on. Of course since it was happening over a period of millions of years you wouldn't know how it would turn out, even if you were there.
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