Why are birds often referred to as dinosaurs, or dinosaurs often called non-avian dinosaurs? Haven't birds changed enough since splitting off from dinosaurs that they can be considered distinct, in the same way that even though mammals evolved from reptiles, we don't refer to them as reptiles?
See here, where this question has been discussed before:
Basically, we do refer to mammals as reptiles in a strict, phylogenetic sense. But in common language it seems rather silly to do so.
So birds are dinosaurs, but the term 'bird' is used in common tounge for a distint subset.
We use the term non-avian dinosaur to distinguish between those dinosaurs that are very close (or belong to) birds, and those which followed a different evolutionary branch.
One minor point of disagreement with Peter's post. The most basal split within amniotes (the group that includes living birds, mammals, and reptiles in the everyday sense of the word) is between a lineage leading to mammals and a lineage leading to reptiles (again in the vernacular sense) and birds. As far as I know, the term Reptilia is applied in current classifications only to the reptile/bird lineage, not to the mammalian lineage.
The implication is that while birds are reptiles, in the same way that birds are dinosaurs, mammals and early members of the mammalian lineage are non-reptilian amniotes. This makes the traditional term "mammal-like reptile" somewhat misleading, and it seems to be falling out of favour among palaeontologists.
Corwin is of course right... Apologies for the error in my post... generalising a bit much!
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