Reptiles: Because I'm out of good ideas

I've got some projects in the pipeline, but none of that is ready yet. So let's talk about reptiles...how does that sound?

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!

UPDATE:

Rattlerjen said...

Fantastic post on reptiles, and fund to read.
Turtles and tortoises do drink from their cloaca (yes, the bottom or aka anus). Many turtle species cannot "drink" unless they are sitting in water!
As for the breathing part, many aquatic turtles are able to hold their breath for a long time because they are able to absorb oxygen through their skin or cloaca. How weird is that!

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

Mosasaurs: Ancient Kansas Wildlife

There are few animals in real life that can capture our imagination the way some fossilized animals are able. Take for instance, the Tyrannosaurus rex, which has been fictionalized again and again to represent our more primal understanding of 'monster'. Take also, the beast that once roamed Kansas as the unquestioned top predator: The Mosasaur.

What? You've never heard of a Mosasaur? Imagine a crocodile with flippers and you're superficially there. See, way back around 100 million years ago the North American continent was split by a vast epeiric sea (a shallow, salty, inland sea) called the Western Interior Seaway (really creative science people).

The whole physics and biology of the Western Interior Seaway and other epeiric seas is pretty amazing, and we'll get into that later. For now, let's just focus on what is known: The sea was shallow (for a sea), probably extremely productive, and was fed by mini-continents on either side. In limnology we tend to think of shallow waters as being the most productive, because of the ability of rooted plants to grow. I think the same is true of shallow seas, but I don't think it is for the same reasons (any oceanographers care to enlighten me on this?).

Anyway! The top predator of this inland sea and the ocean at large became the mosasaur (pictured below). Although this is a reptile, this isn't a dinosaur, and it isn't a crocodile. Think lizard. In fact, think monitor lizard (Family Varanidae), which includes the still living and awesome Komodo dragons and the recently extinct Megalania (the feared giant lizard of Australia). Many modern monitor lizards are highly adapted for living in and around water (e.g., water monitors), and so it isn't hard to imagine a scenario wherein these amphibious lizards took the next evolutionary step and became aquatic.


And become aquatic they did! Unlike turtles, crocodiles, walruses, and quite like whales and dolphins, the Mosasaurs gave birth at sea, to live offspring. The arms and hands of this species developed into flippers and became somewhat detached from the backbone (meaning they could not support their own weight on land). Pull this lizard out of the water and it would asphixiate.

One of the many interesting things about this invasion of the seas by lizards is that they weren't exactly getting themselves into an unoccupied niche. The first real mosasaurs occurred in the Cretaceous, and by 90 million years ago (MYA), we had three big subfamilies with lots of known species. However, way back in the Jurassic reptiles had already colonized the oceans with the famous plesiosaurs (think the mythical Loch Ness Monster) and of course the everpresent sharks had managed to come up with a particularly devastating breed (the Ginsu sharks) and you had other random nastiness lurking out there (just don't go swimming in the Cretaceous). Nevertheless, the Mosasaurs got really big and are generally thought to have been the top predator.

The reason I started getting interested again in Mosasaurs was an interesting paper by Mike Everhart published in the latest issue of the Kansas Academy of Science. The paper explores the occurence of mosasaur on mosasaur violence. And for something that happened 90 MYA, we can actually figure out quite a bit. Everhart was able to, fairly convincingly, estimate the size and mass of the attacker and the way in which it bit down on the victim.

A plate depicting the first Mosasaur discovery. The name is from Latin Mosa meaning the 'Meuse river' in the Netherlands, and Greek sauros meaning 'lizard'. The first specimen was found in a Meuse limestone quarry.

The way Mosasaurs ate is interesting, and has lead to some debate about the origin of snakes. Mosasaurs, unlike sharks or dolphins or most other big predators we're familar with, didn't have teeth that cut, just teeth that crushed. As a result, Mosasaurs either had to bite their prey in half (although this might have been possible, the family lacks the heavy skull that is common in crocodilians, making this somewhat unlikely) or swallow it whole. In order to maximize the food consumption, the Mosasaurs apparently had a somewhat hinged jaw that has lead many people to think: SNAKES!

Indeed, the theory of a shared snake and mosasaur marine ancestory has recently been in vogue, although it was first proposed in the 1890s (it's called Pythonomorpha if you're curious). The idea of snakes evolving from water lizards sounds inherently unlikely to me (why evolve flippers if you are just as good off just getting rid of limbs altogether?). Luckily, I don't need to push my uninformed logic, because apparently more recent fossils have shoved this theory to the wayside (read the link on the Phythonomorpha to get a better understanding). You can rest comfortably, however, knowing that this issue will continue to be fought bitterly by individuals committed to one side or the other until they die, at which point the next generation of paleontologists will find some other issue to argue about. And you thought politics lasted forever.

Mosasaurs have been found with a variety of food items in their stomaches: Mostly fish, but also a plesiosaur, other mosasaurs, turtles, birds, and sharks. I know sharks are fish too, but at least some species of sharks appear to have gone extinct as the mosasaurs became more prominent, suggesting they might have out-competed them (via direct predation?). Some species appeared to specialize on clams, and at least one late evolving species (Leiodon) managed to evolve teeth that cut. All of this evolutionary achievement probably put the Mosasaurs at the top of the oceanic food chain for around 20 million years before the K-T event happened and the entire family was wiped out. According to Everhart:

"Mosasaurs ruled the oceans of the Late Cretaceous and were beginning to invade fresh water environments such as estuaries, swamps and rivers when the Age of Dinosaurs ended. Did they die suddenly due the catastrophic effects of an asteroid impact in the Yucatan, or was their extinction more gradual following the general collapse of the marine ecosystem? We may never know."

Everhart doesn't mention a third possibility, which is that they are still out there and we just don't know it. Seriously, don't go swimming in the ocean.