Eli5: why are birds believed to be the descendants of the dinosaurs?
In: Earth Science
Based on dna evidence and fossils of similar animals that show obvious features from both animals. You can see the fossils go from raptor, to raptor with wings, to bird with teeth, to bird. [This image](https://ars.els-cdn.com/content/image/1-s2.0-S0960982215009458-gr1.jpg) shows the lineage and different features.
Because when looking back at therapods (the upright walking, mostly carnivorous dinosaurs) you can see a very clear line of decent in which more and more birdlike features are gained in some lineages. Including a wide variety of proto-feathers leading up to quite complicated and advanced feathers by the time you get into the raptors.
Even as late as the 70s or 80s there was still some debate about whether or not birds were dinosaurs or just close cousins to the dinosaurs, but at this point the fossil record is pretty overwhelming and we can say with a good bit of certainty that birds are therapod dinosaurs.
We know from DNA analyses that birds are most closely related to lizards and reptiles (particularly crocodiles and turtles), and we know many of those lizards and reptiles are essentially “cousins” of the dinosaurs that split off from the species that predated the dinosaurs.
Further, we know from the fossil record that therapod dinosaurs have a huge amount of physical features (particularly feathers, special bone structures, joints, etc.) that don’t appear in any other living creatures *except* for birds. Further, the closest structures to bird wings in the fossil record are the bone structures of the upper limbs of the therapods. And of course therapods incubated their eggs in a brooding behavior very similar to modern birds.
We also have fossils of creatures that seem to bridge the gap between the therapods and modern birds.
QED; birds are the descendants of the therapods.
-Nowadays we have a pretty extensive repository of fossils documenting the early evolution of birds.
-Genetic analysis places birds as being closely related to modern crocodiles and turtles, not at all close to mammals; with that knowledge, dinosaurs are the only “kinda-birdish” transition that would make sense, unless you’re proposing all of the differences of birds spontaneously erupted into existence.
The largest physical similarities that have been argued over the decades even before we had decisive proof:
-Many dinosaurs were feathered and small, like birds.
-Many dinosaurs have hollowed bones, like birds.
-Dinosaurs have wishbones, like birds.
-Dinosaurs were warm-blooded, like birds.
-Many dinosaurs have ankle, wrist, and hip joints similar to those in birds.
-An assortment of other evidence regarding fleshy tissue and behavior that isn’t really proven, for example there’s some evidence from fossil imprints that dinosaur lung shape and structure could have been like birds.
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