Reptiles are air-breathing, four-legged, cold-blooded amniotes that live everywhere but the polar regions. Everyone knows what a reptile is, but oddly enough, the taxonomy is a little more confusing that we might otherwise think. To understand why, I'm going to introduce two terms: Monophyletic and paraphyletic. Basically a group is considered monophyletic if it includes all the animals desended from a common ancestor. If you take a monophyletic group, and remove one of the decendents of that common ancestor, then you get a paraphyletic group.
Full of confusion? Ok, let's put it another way. Below are all the vertebrates. If we highlight a group that includes reptiles and birds, we've got a monophyletic group (sometimes called a clade).
This figure shows our best understanding of evolutionary connections. The earliest common ancestor was some kind of vertebrate, which split into tetrapods and pisces (fish). You can see tetrapods split into amphibians and amniotes.
On the other hand, the group 'reptiles' as we commonly understand it, isn't a monophyletic group. Take a look:
The term reptiles doesn't include all descendents from a common ancestor because the are believed to be part of that lineage as well. The reason this is a little weird has to do with the way humans percieve these groups of animals. Birds and mammals are 'good' animals. Reptiles and amphibians are "slimy", creepy or scary. Yet these are very different types of animals. As you can see in the figures above, they aren't even that closely related.
This perception problem is pervasive in the sciences as well. Take herpetologists. Herpetologists study reptiles and amphibians. The only real commonalities are that they are cold-blooded and generally smallish. Is this really a meaningful basis on which to lump these otherwise very different groups together? (...no)
Ok, but let's get back to reptiles. I've repeatedly used the term amniote without really explaining it. Maybe I should fix that? Amniotes are animals who's embryo is surrounded by protective membranes: They lay eggs that could survive on dry land! This improvement on eggs was the big development that separated the earliest reptiles and proto-mammals from the amphibians. At the time, amphibians were the masters of the terretrial terrain. However, amniotes were able to exploit a lot more land, and eventually began to displace amphibians as large, dominant herbivores and predators.
Reptiles, or at least, creatures that we would look at and think "reptile" predate mammals, and therefore you can safely consider mammals and birds specialized reptiles, if you feel like it. Those of you who think I'm crazy: Its right there in the family tree!
What did an early proto-reptile look like? Well, according to Wikipedia, Hylonomus is a good guess for one of the first reptiles:
Oh my gosh! It looks like a lizard!
Yeah, that was predictable huh? There are only 4 classes of reptiles left today (if you don't count mammals or birds). The crocodiles and the turtles are two of those classes. The third one is everything else you think of as a reptile: Snakes, lizards, mosasaurs (these are squamates). Unless you're amazing or a herpetologist, you probably have never even heard of the 4th group of reptiles. Why? Because (this is going to be a huge shock) even if you saw it, you'd just think it was a lizard.
I'm talking of course about the order Sphenodontia, a once hugely diverse order that now contains just a single species: Tuatara. And if you know anything at all about how the world works, you know this single remaining species of a once-great order is on the verge of extinction thanks to something mankind did. Fantastic. In this case, we brought rats to New Zealand.
Tuatara is a pretty ridiculously interesting animal. Like the last surviving member of any ancient lineage, people tend to refer to this species as a 'living fossil'. However, this has always been a hopelessly stupid thing to say. The implication is that the animal hasn't changed in the thousands or millions or tens of millions of years since our first fossil record. The likelihood of this being the case is incredibly small. Animals evolve to survive changing conditions, and there is basically no-where in the world that hasn't had changing conditions over the last 220 million years. Hence, this species had to have evolved. In fact, these guys (whose work I can't get anywhere) apparently found that tuatara is changing more rapidly than any other species tested.
Ok, aside from that tuatara has a third eye, incredibly primitive ears, and a fish-like spine (all unique or rare among reptiles). And now let's go back to reptiles.
There's a lot about reptiles is amazing, but I'm just going to talk about one more thing and leave it at that. Basically: How do they breath?
This seems like an obvious question. After all, we all instinctively know how mammals breath, and if you dig around the medical literature, you'll find talk about a diaphram and muscles causing your lungs to expand and contract.
Of course, this is how reptiles do it too, but in the case of the squamates, those muscles are also locomotion muscles. So when a lizard starts running, it isn't breathing. I haven't found anyone who says this, but I imagine this is why you see lizards making short bursts from hiding place to hiding place. The crocodilians breath differently. Like mammals, crocs have a diaphram (although it works a little differently). Turtles are where it gets really interesting. I don't know how many of you have seen a turtle, but they have a hard shell. Exactly how do you get your lungs to inflate and deflate if they are attached to a hard shell?
Turns out, different turtles do it differently. For the most part, there are two sets of muscles: One that pushes everything inside the shell out, and another set that pulls everything outside the shell in. Expand-contract. You get the idea. These muscles might interfere with locomotion, or they might not, depending on the species. You may also think: Aren't there a lot of aquatic turtles? Why yes, yes there are. And since you asked, some of them appear to breath from the butt.
Don't worry though. They aren't drinking from that oriface.
Well, that turned into a sufficiently bizarre post. Hope everyone enjoyed!
As Rattlerjen points out, yes, its really the cloaca that turtles are able to respire from (butt is a non-technical term in this case :), although this is limited to side-neck turtles. However, I can't find any actual evidence to back up Rattlerjen's other claims. The paper I linked to previously (the link doesn't seem to be working, so the citation is below) actually demonstrates that cloacal drinking does not occur in a species that is able to breath that way. Anyone have any citations to share on other species?
Charles C. Peterson and David Greenshields. 2001. Negative test for cloacal drinking in a semi-aquatic turtle (Trachemys scripta), with comments on the functions of cloacal bursae. Comparative Physiology and Biochemistry DOI:10.1002/jez.1055