So, what’s the deal, I’ve heard that dinosaurs are reptiles and then I’ve heard that they are not. Which is it and why?
Thanks.
Were.
There was a pretty good program on this a few years back.
Answer: Nobody knows.
Sure they do. And they are. And, cladistically speaking, so are birds.
In terms of nested groups, dinosaurs are typically classified thus:
Amniota
Sauropsida
Reptilia
Eureptilia
Diapsida
Sauria
Archosauromorpha
Archosauria
Ornithodira
Dinosauria
Somewhat simplified, of course.
Come to think about it, the program was actually about whether dinosaurs were warm or cold blooded. And it was several years ago.
I guess I read the question as “Are dinosaurs lizards”?
So that crow yakking it up in my backyard is a dinosaur? He wants my stale bread, so I guess I’d better give it to him.
Ever had a Bronto burger? Tastes like chicken.
Mangeorge is pretty much right; the jury’s still out on this one. Sure, depending on which scientist you talk to, birds are extant theropods (Gregory Paul) or not (Alan Feduccia), but generally speaking, dinosaurs are thought to be descended from reptiles but are something other than pure ‘reptile’. And depending on who is making the cladograms, dinosaurs are either at the base of all birds, or birds are just a highly-successful offshoot of maniraptorids, or birds and dinosaurs share a common reptillian ancestor, or aliens from the Crab Nebula made them or whatnot. Sometimes I think that scientists make this stuff up as they go along… 
I think the currently accepted theory is that birds are dinosaurs, so if dinosaurs are reptiles then birds must be reptiles too.
http://tolweb.org/tree?group=Dinosauria&contgroup=Archosauria
Then again, we all know I hope that apes are not monkeys, even though apes are catarrhines and there are both catarrhine and platyrrhine monkeys.
http://tolweb.org/tree?group=Primates&contgroup=Eutheria
So it could be interpreted as a matter of semantics. Monkeys are obviously paraphyletic so who says reptiles can’t be paraphyletic also?
One problem with the idea that dinosaurs are reptiles is that reptiles are cold-blooded, and so must absorb heat from their environment. That is why lizards spend so much time remaining still, basking in the sun.
Given the enormous size of many dinosaurs, it appears they would have had to lie still pretty much forever before expending the energy to move even a slight amount, or else the environment must have been much, much warmer in their time. If it was warm enough to permit them to soak up the energy to move as much as lizards do, however, they should have been baked to death.
Taxonomy is purely a human construct. Animals exist, and scientists develop methods for classifying them. Animals are not obliged to conform to the classifications which scientists devise.
So it is that from time to time animals are reclassified, or the definitions for various classifications are amended. It was once well-known that mammals do not lay eggs–and so scientists were able to conclude that the duckbilled platypus was a hoax.
Maybe the definition of “reptile” should be amended to include warm-blooded animals. Or maybe the dinosaurs should be classed as being “reptiles-sort-of-but-not-really”. Either way, the decision made will be a reflection on how scientists choose to classify things, rather than on the dinosaurs themselves.
While “cold-blooded” may be a problem for traditional Linnaean classifiatcions, it isn’t a problem for cladistic taxonomies. Cladisitics is based on groups which share derived characters. “Cold-bloodedness” (or, more appropriately, ectothermy) is not unique to reptiles; most fish, amphibians, insects, many reptiles, and so on all have this trait, so it doesn’t serve as a uniting feature for any one group, especially reptiles. Similarly, “warm-bloodedness” (or endothermy) is a poor trait to use to group organisms because it has also showed up in numerous groups (birds and mammals being the most obvious).
Also, Linnaean classification poses the problems with assigning dinosaurs that have been mentioned; do they go in this group, or that group?
In cladistics, however, all groups are nested, so there isn’t a problem with being a reptile AND a dinosaur, or even with being a bird AND a dinosaur AND a reptile. Reptiles are diagnosed by a number of characters which are presumed unique to that group (granted, determining just which characters qualify is a major area of debate among taxonomists). And based on those characters, dinosaurs, along with plesiosaurs, pterosaurs, crocodiles, turtles, snakes and birds, and many others qualify as “reptile”.
So birds are dinosaurs? Any other animals? And dinosaurs are not all extinct? I want the Kimodo Dragon to be a dino, but I know it’s a lizard. 
As always, there is some debate as to where, exactly, birds fall in the Grand Scheme of Things [sup]TM[/sup]. But, based on a number of characters, they do, indeed, appear to have descended directly from theropod dinosaurs, which makes them “guilty by association”, as it were. In cladistics, if you are a descendant of a group, you continue to be a member of that group, no matter what else you may also be. So, birds have many characteristics which make them unique, but they also bear numerous characteristics which are shared with theropod dinosaurs. They can thus considered to be dinosaurs (in the sense that they appear to properly belong in the Dinosauria clade). “Dinosaur”, as it is used in more common parlance, however, is paraphyletic (meaning it does not include all descendant groups) in that birds are often considered separately. Same with Reptilia, since most folks don’t include birds when talking of reptiles, either (and regardless whether one ascribes to the 'birds are dinosaur descendants" theory, there are very few who would argue that birds were not descended from some reptile ancestor).
Unfortunately nothing else that is currently around would be grouped with the dinosaurs. Komodos aren’t dinos, but they are darned fascinating themselves. They are thought to share a lineage with the extinct Mosasaurs, so they aren’t exactly slackers in the “had really cool extinct relatives” department, either.
So where do the mammals fall into this Grand Scheme of Things? We also have reptilian ancestors, don’t we?
Goodness I was just about to write on that subject!
Although there is no clade Reptilia, there is a clade Sauria which includes all extant reptiles and birds. Mammals, however, belong to the Synapsida along with our ancestors the mammal-like reptiles.
Therefore, the “Reptilia” not only includes saurians with the exclusion of avians, but it also includes synapsids with the exclusion of mammals. Since as far as we know all synapsid reptiles are extinct, we can ignore this odd piece of the puzzle when dealing with extant species.
So therefore mammals reptiles and birds (and dinosaurs) are descended from a common ancestor that was reptilian, and belong to the Amniota which excludes fish and amphibians.
I beg to differ. Apes just happen to be the word in the English language for a group of monkeys without tails.
Robert T. Bakker believes that Dinosauria should be a Class of it’s own. He doesn’t think Reptilia should even be a class…here’s what he says in the footnotes to ‘The Dinosaur Heresies’…
That’s a little bit harsh. Though taxonomy is a human construct, its mostly concerned with understanding lines of descent. But c’mon: faced with a duckbilled platypus, which basically is a totally unique mammal, would you have ever though it was related to other mammals in any close sense?
“Although there is no clade Reptilia, there is a clade Sauria which includes all extant reptiles and birds. Mammals, however, belong to the Synapsida along with our ancestors the mammal-like reptiles.”
Both are clades, and have different definitions. From tolweb.org,
Reptilia: The most recent common ancestor of extant turtles and saurians, and all of its descendants.
Sauria: All the descendants of the most recent common ancestor of birds, crocodiles, squamates, and Sphenodon.
However, some people now think turtles are saurians, so the clades would be synonymous.
I think slipster had a valid point. Outside of the species level, taxonomy is pretty fuzzy. This is not much different than the debate over Pluto’s status as planet. Nature cares nothing for our classifacations.
That is certainly true for more arbitrary classifications like Linnaean taxonomy. (Not to besmirch Linnaean taxonomy, it served a vital purpose in its time; it’s just out-dated now.)
Phylogenic systematics (aka cladistics) is not an arbitrary classification. Its goal is to show true evolutionary relationships between groups of animals. What we label as species or genus or class is arbitrary. But the historical relation between the groups of animals (whatever we happen to label them) is a discoverable fact. Explore the Tree of Life to see more. For fun, start at the top and try to drill down to humans (or your favorite species).
Is Bakker suggsting that the Saurischia is paraphyletic, and that the sauropods are closer relatives of ornithischians than of theropods? Is it true that I am not the only one to suspect that to be the case?