President Joe Biden (So-Called)

Yeah, but one is a bird skeleton. The other is from a diferent group of dinosaurs. Which one is the bird?

Alright. Just get it over with and smack me with the open hand of fact-based science.

Well, the one in the right is Archaeopteryx lithographica (the bird) and the one in the right is Zhenyuanlong suni (a dromeosaur). The last one has the characteristic sickle toe claw and the fingers are not as long due to the wing not being as large (as it didn't fly).

Wait so they're both dinosaurs

Yes, but one is a bird. Archaeopteryx is classified as an Aviala, which is the group of birds. It is generally described as the first bird.

This is fascinating. Thank you. My confusion then: dinosaurs, as a classification, what does it mean when we say 'dinosaurs'

Well, Dinosauria is an actual group of animals which includes everything that is usually regarded as a dinosaur like Tyrannosaurus or Triceratops. Its main diference with their relatives is that they maintain a "mammal-like" posture with their legs under their body.

To be clear. Pteranodon is not a dinosaur as it doesn't cualify for everything and neither are the marine reptiles of that era such as Mosasaurus (that one in particular was an actual lizard).