Why are dinosaurs considered reptiles? Something about the bone structure or something? Who made this determination and is there any disagreement among those who know (as there is in birds=dinosaurs)?
Well…they’re not, any more. Dinosauria is now considered a different class of animal than reptilia. (I’m not using Class in it’s taxanomic sense…I’m not sure exactly what grouping Dinosauria and Reptilia are…Orders?)
Why they were, I can’t, oddly, remember (these things usually stick with me.).
The teeth looked like lizard teeth (an observation preserved in the name of the Iguanodon, the first dino described). Mammals had teeth that were complex, with all those cuspids, incisors and suchlike. Birds had no teeth at all. Therefore the extinct things must be some type of awe-inspiring reptile or a terrific lizard. (Hmm, that could be a good name for the beasties, maybe if I say it in Greek…)
Reptiles have scales, a barrier to water. They are ectothermic, and they lay eggs on land. Dinosaurs fit all three characteristics. However, there is some debate whether they were ectothermic. Some believe they became endothermic and evolved into birds. Some believe they became alligators. Some believe anything.
Here is a page which gives an idea of the sort of characters used - http://phylogeny.arizona.edu/tree/eukaryotes/animals/chordata/amniote_lichen/Amniote_phylogeny.html
Essentially this remains a controversial topic. The traditional classification scheme lumped dinosaurs in with reptiles because they seemed to share certain gross morphological characteristics like scales and ( as DrFidelius mentioned ) relatively simple teeth. For the same reason birds were separated out because they had feathers and beaks. So you had dinosaurs as a subclass of Class Reptilia and birds elevated to a rank equivalent to the Reptilia as Class Aves.
Later, as researchers began studying morphology more closely and as the philosophical paradigm shifted to organizing animals according to evolutionary relationships, it became clear that this earlier classification was faulty ( despite the fact that it is the STILL the default scheme used for every basic biology course I have ever seen ). Based on detailed examination of various aspects of morphology it became apparent that birds were directly derived from a common ancestor with certain dinosaurs. Based on cladistic methodology ( where shared, derived characters, or synapomorphies, are the key units in establishing evolutionary relationships ), birds and dinosaurs were more closely related to each other than either is to, say, a snake. Specifically birds SEEM ( they’re still hashing this out, but the preponderance of evidence points in this direction ) to be offshoots from one of the three main recognized groups of dinosaurs - the Therapods, which includes those bipedal flesh-eating critters that Jurassic Park is so fond of.
Accordingly the Class Reptilia as it stood was paraphyletic. That is, it didn’t include ALL of the descendant groups in that lineage, because birds had been separated out based on their “uniqueness”. This is considered bad form in modern classification systems- Ideally all groups should be monophyletic, i.e. contain all the descendents from a common ancestor. This provides for greater interior logic and a more informative hiearchy. Of course many, if not most, biologists prefer the traditional classificationsthey originally learned, paraphyletic or not (However, I think most agree that polyphyletic taxa are a bad thing )…
At any rate quite a few new schemes have been proposed, depending on where you stand. But the most commonly accepted grouping seems to be to subsume the birds back within the Dinosauria, as a subgroup within a more inclusive Reptilia. But you’ll see a lot of variation out there, still.
Is there any controversy over whether dinosaurs should be classified with the reptiles? Some, I suppose. But most of that is hair-splitting over nomeclature, i.e. is Reptilia ( narrowly defined as the snakes/lizards/tuatara, turtles ) a name that should be used as a sister taxon to the Archosauria ( crocodiles, birds, dinosaurs ). By and large I don’t think their is a great deal of debate about whether or not dinosaurs are closely related to the “traditional” reptiles. They pretty clearly seem to be. See the above link and associated pages.
- Tamerlane
Yes, dinosaurs are reptiles. And birds are also reptiles.
Crocodilians appeared well before dinosaurs. I know of no scientific theory that has dinosaurs evolving into alligators or crocodiles.
Somewhat OT, but the great lesson I take away from cladistics is (to paraphrase Democritus), nothing exists but species and empty space; all else is opinion. Meaning a cladogram shows very clearly how individual species are related, at the expense of obscuring higher-level categories – indeed, revealing these categories for what they are: convenient inventions, rather than natural entities. Any time you draw a circle around a monphyletic group, you do so arbitrarily – you could always redraw the circle a little larger to include the next-closest ancestor, or a little smaller to exclude some species, and there are few rules to govern such decision.
Freedom, terrible freedom…
– Beruang
Beruang:
Quite right 
. My apologies if I seemed to imply otherwise. My point is just that many workers group the crocodilians in with the dinosaurs ( broadly speaking ) under the rubric of Archosauria. There’s no implication that one evolved from the other - Just that they are more closely related to each other than either is to the turtles or lepidosaurs.
As to your OT point - I agree again . And I’ll even add that some would argue that the concept of species is an artificial construct as well. Just look at the various dueling species concepts.
But artificial hiearchies have their uses for ordering the thoughts in our little brains, as long as we remember they are artificial. And the more logically consistent and informative the hiearchical framework, the more useful it is. At least theoretically. Which is why cladistics has become so popular in recent years. It’s philosophy is more objectively sound, IMHO, than the compeitition. Which doesn’t mean it is any more sound in practice, of course. Quite the contrary, on occasion. I’m certainly not a Hennigian Evangelical 
.
But there are SOME rules - I wouldn’t say that it’s COMPLETELY arbitrary. It’s all a matter of scale. A clade comprising humans and cnidarians may be monophyletic relative fungi. But not relative to treesloths.
It’s all just an incredibly tedious, extremely relative, shorthand 
.
- Tamerlane