There are several Straight Dope columns discussing dinosaurs, but this one is the most relevant:
Pterosaurs aren’t considered true dinosaurs. Why not? (30-Jun-1995)
(and this article is of course mentioned in the link given by Darwin’s Finch in post # 15 of this thread)
Does the plumage of a kiwi bird fit the strict definition of a feather?
Kiwi feathers appear to be developmentally-retarded feathers (similar to those found in juvenile cassowaries). They may also well be, in many ways, similar to the feathers that the aforementioned Dilong appears to have possessed. Structurally, they do adhere to the standard pattern of feather anatomy, albeit they lack the vanes that typical feathers possess.
I think of it this way: birds are dinosaurs to the same extent that people are those little rat-shrews that took over after the big asteroid.
One way to pick out a dinosaur from a reptile is what’s called a sprawling gait. If you look at a typical lizard, their legs spread out horizontally, and their body is close to the ground. On dinosaurs, their legs point downward, supporting their body high off the ground. Compare a komodo dragon to a stegosaurus. This lends some support to the theory of dinosaurs being warm blooded, in that with their body far from the ground, they needed a way of generating heat internally.
Strictly speaking, just as birds are dinosaurs, dinosaurs are reptiles (and yes, that means that birds are reptiles, in a cladistic sense).
Note that the gait itself is not a diagnostic character for dinosaurs, but rather it is a consequence of traits that are diagnostic: the structure of the hip, the articulation of the femur and acetabulum, and the structure of the ankle, for example.
I believe there are certain groups (anklyosaurs?) whose legs emerge from the pelvis horizontally like crocodilians. I could be wrong.
It was once believed that this was the case (much as it was once believed that bipedal dinosaurs walked upright and dragged their tails behind them), but not any longer. The current view is that ankylosaurs held their legs under the body, just like all the other dinosaurs.
Yes, it would have been clearer. The group “apes” causes a problem for the group “monkeys” in the same way that “birds” create a problem for “dinosaurs”. That is, if you exclude the first from the second, then the second becomes paraphylitic. But I guess if you not into cladistics, then it isn’t much of a problem.
Most of my knowledge of dinosaurs is pre-cladistics, but I thought the thecodonts were the stem ancestors of the Saurischia and Ornithischia.
Best way to respond to this is to say that Order Thecodontia, among the Archosaurs, and Eosuchia, among the Lepidosaurs, were “catch-all” groups, paraphyletic in nature, including everything that wasn’t part of a major order-level grouping. They’ve since been broken down into smaller monophyletic groups.
And since Saurischia and Ornithischia have synapomorphies – that is, they share derived traits common to both groups but to no outgroups except the ancestral “thecodonts” – Dinosauria as currently recognized is a clade consisting of the two old Orders plus one or two families from the “thecodonts” that share those same derived traits and are either dinosaur ancestors or the close relatives of same – either “the dinosaurs’ grandfather” or “their great-uncle” if you will.
The “orangutans are not monkeys” lobby is still active, though.
“Ook!”
Dinosauria proper is limited to Ornithischia and Saurischia these days. “Wanna bes”, such as Lagerpeton and the lagosuchids are pre-node, and together with Dinosauria make up Dinosauromorpha. Then you’ve got Dinosauriformes, which is Dinosauromoprha sans Lagerpeton.