system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How do we know that dinosaurs were likely reptilian/avian but not likely mammalian?
gpt
Species that are closely related to each other will share certain anatomical features in common, which they both inherited from a common ancestor. 

Other species that are not descendants of that common ancestor — and therefore are more distantly related — won’t have those same anatomical features.

For example, on the one hand, all mammals have three tiny bones in their middle-ears (called the malleus, incus, and stapes), and no mammals have hallow bones that connect to their respiratory systems. 

On the other hand, neither birds nor dinosaurs have those tell-tale three little bones in their middle-ears, but they both do have hallow bones that connect to their respiratory systems. 

From these and other similar anatomical comparisons, we conclude that birds and dinosaurs must be more closely related to each other than either of them are related to mammals.