Explain please? I thought there were two kinds of dinosaurs, bird like and reptile like? So I assumed birds descended from the bird like and reptiles descended from the reptile like, but I'm ready to be re-educated.
I want to point out that all of this is as I understand it and I may be off in some areas. I am not a biologist (yet).
It's complicated. And actually - to be fair - whether or not dinosaurs could have been considered reptiles depends on your definition of "dinosaur" and your definition of "reptile". Dinosaurs were certainly reptile-like in many respects and are often still considered to have in fact been reptiles - but what we consider reptiles today are so far removed from the dinosaurs, that much of the scientific community no longer believes this to be correct - and dinosaurs are instead often seen to be in a separate group. It's a bit like when there are two similar species that we believe to be in the same genus - but later discover that the link between them isn't as close as we thought and the species is reclassified. These days - we don't consider dinosaurs to be reptiles as often because our understanding of them has broadened and we now know how extraordinarily different they were from what we call a reptile.
When dinosaurs first evolved, we had the amphibians and animals that were reptile-like. Of the reptile-like animals there were the synapsid (which would evolve as mammals) and the diapsids. Within the diapsid branch we have Ichthyopterygia, Lepidosauromorpha and Archosauromorpha (as well as other orders that I'll not get into here). Ichthyopterygia contained ancestral reptiles that went back into the ocean - marine reptiles. Lepidosauromorpha is what evolved into what we consider reptiles today (except crocodilids - I'll get to that in a moment). It contains snakes, lizards, turtles and contained the creatures that would later evolve into these such as plesiosaurs. Then there's Archosauromorpha which is where other animals evolved. From Archosauromorpha we gained pterosaurs (such as pteranodon and quetzoalcoatlus), crocodylomorpha (crocodiles and other crocodylians), and dinosaura (which includes birds as well as the creatures we most typically think of as dinosaurs such as sauropods (Apatosaurus, brachiosaurus),theropods such as T-Rex and velociraptors, Ceratopsids like triceratops and other groups.))
So what I'm trying to say - is that the "dinosaurs" most like birds (pterosaurs) were separate from actual dinosaurs - and did not evolve into birds. Indeed pterosaurs would eventually die off. The group that birds -did- evolve from is the group containing Tyrannosaurus Rex and your standard children's playset of sharp-toothed killer dinosaurs.
What we call "reptiles" now among currently existing animals are crocodiles, turtles, lizards, snakes and so on. Crocodiles are complicated and would require their own lengthy explanation. What's important here is that a long, long time ago - literal dinosaurs and the group now representing reptiles branched off from the same group - but into two very separate group.
Thus, Dinosauria are not what we currently understand to be a "reptile". They are quite far separated from our snakes and lizards. When people say that there are living dinosaurs - expectant eyes always fall on what -looks- like a dinosuar, like a crocodile. But in fact - the living dinosaurs are birds. Aves (the class containing birds) is the only living member of Dinosauria to remain.
So the bird-like "dinosaurs" died out and what evolved into actual birds were the genuinely reptile-like dinosaurs.
Does that make sense? I'm sorry if it's a bit jumbled - and it's mostly from memory. If I'm not completely accurate - I at least hope that I'm close. x.x
