no seriously at what point do we stop saying dinosaurs and start saying prehistoric birds?? is it a matter of time?? a matter of species??? SCIENCE SIDE OF TUMBLR PLEASE HELP
We don’t actually know for certain!
It’s all a gradual series of transitional forms, and where we stick the
arbitrary point of “birds start here” depends on on what you consider to
be their defining traits.
Do we define them by having feathers? If we do that, then fuzzy feather-like structures actually seem to go all the way back to the common ancestor of all dinosaurs and pterosaurs – which would therefore make every single member of those groups birds too.
If you’re not into the idea of calling things like Pteranodon and Triceratops “birds”, then maybe we should go more specific and use the presence of pennaceous feathers instead. The kind with quills and barbs that we normally associate with birds’ wings.
Those are at least found only in theropod dinosaurs, except… they’ve been found in ornithomimosaurs, a group well before anything we’d usually say was a “bird”. (And they might go even further back, since there are possible quill knobs known from the carnosaur Concavenator. And then Tyrannosaurus also gets to be a bird!)
Okay, that’s still a bit weird. What about being able to fly?
That at least seems to have happened in Pennaraptora, a group of theropods that includes oviraptorosaurs, dromaeosaurs, Archaeopteryx, and everything else leading to modern birds. So we’re definitely getting much more bird-like in here.
But we don’t know exactly when flight originated. Opinions on whether Archaeopteryx could actually fly go back and forth constantly (the newest research suggests yes) – but some earlier small dromaeosaurs might also have been capable of powered flight. And there’s even the possibility that bird-like flight independently evolved multiple times in different branches of this particular family tree.
It’s a bit of a mess.
Then there’s always Archaeopteryx as the traditional option for “first bird”, but we’ve found so many other similar birdy things by now that’s it’s not actually unique anymore. Pick out any defining “bird” feature in Archie and there’s an older dinosaur with the same trait which then also has to be a bird too.
And so we end up with the most conservative option: Neornithes
(or Aves), the group that contains all modern
birds going back to their last common ancestor. Those are all definitely
unquestionably birds, but…
…Then things like Confuciusornis and enantiornitheans and Ichthyornis don’t get to be birds. And they’re very birdy-looking. If you saw one alive you’d call it a bird.
But if we keep backtracking through the evolutionary tree along everything that still looks like a bird we just end up right back in the pennaraptoran mess again. So that’s not much help either.
A lot of paleontologists still tend to consider “birds” starting at a point just before Archaeopteryx, in a group called Avialae, but there’s also a lot of inconsistency and disagreements. Really you can pick whatever definition you personally like best and just roll with that, because there’s unlikely to be a Definite Official Bird Definition anytime soon.
