Eli5: How do we know that dinosaurs were likely reptilian/avian but not likely mammalian?

Question

Eli5: How do we know that dinosaurs were likely reptilian/avian but not likely mammalian?

In: Biology

Answer 1

I think they’ve found fossilized eggs of dinosaurs. Besides the platypus, I don’t think there are any mammals that lays eggs. And the platypus is a bit of a hybrid on all levels.