Biology: Diversity of Vertebrate Species

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If you think back to those first deuterostomes, they weren't very dramatic. What did they do? They just kind of laid around in the mud. They weren't very active. They certainly didn't seem to be like this group that was going to diverge into such a successful group as the vertebrates. Remember this word--invertebrates--vertebrates. They are generic descriptors. There is no such classification scheme as the vertebrates. It's simply referring to--well, we'll see what it's referring to.
Let's talk about these so-called vertebrates. Well, the urochordata and the cephalochordata had kind of like laid-back lives, and certainly there was an ancestry there for the rest of the chordates. But as the chordates continued to develop and more diversification happened, the key was that the diversification happened for lifestyle. Lifestyle meaning, well, what niches were available. There were niches available for predators, there were niches available for things to escape from predators, but the bottom line was that a more active lifestyle, if you will, was what was available to those not yet diversified chordates.
Let's take a look at what would be selected for if we're looking at this idea of a more active lifestyle. Let's start out with that whole deuterostomic idea of the neural tube and the neural fold. Remember this? If not, you may want to look back on some of those development lectures. We have the beginning of a neural fold with development is happening. The notochord, remember--these are chordata and there's going to be a notochord. That notochord is below what's eventually going to form the neural tube on the inside of it, so this is my neural fold. Well, it just turns out that there are specialized cells that are going to be to the side of this. This is going to form what is called the "neural crest," and this neural crest became a key in the development of the chordates, because what's happened is... Do you remember that with the neural fold the cells would grow together and close and it would form this circle, this hollow opening right here, so in essence, what you had was a closure and then underneath here would be the tube surrounded by cells. So here's what's going to be my neural tube. So that's right in there, my notochord below it, and all of these neural crest cells. The key here is that these neural crest cells would then do two things. Number one, they would stay here and form literally the backbone of the organism, the whole central axis of these organisms, and they would migrate, and they would migrate out of this region during embryogenesis and form into other bones. So this whole neural crest thing would form what we call the axial skeleton--that skeleton that goes along their axis--and the migration would form what we call the appendicular skeleton.
So now we get the growth of the skeleton. Why is that so great? That is so great because this is going to enhance movement, it's going to enhance support. Skeletons are kind of important things when you're a vertebrate, you know? And a third thing that's going to happen is a muscular digestive tract. As we pass through the chordates together and we see that the vertebrates, as they developed and became more and more diverse and more and more active, it was important to move food very rapidly through that digestive system. So we have a muscular digestive tract. That will be the third thing--muscular digestive tract.
So we have the neural crest, the skeletal arrangements would be number two, and then number three, the muscular digestive tract. All of that being said, what's a chordate? Or even more importantly, what's a vertebrate? Well, we're going to refer back and forth to this chart. We're going to work our way down through this chart. There are just some great things on here, including you and I, but we'll come to us later.
Again, just like anything else, our classification schemes are changing. This one is growing in leaps and bounds and we're really kind of reclassifying things, but generally speaking, we're going to divide this up by super classes. We're going to look at this first group right up in here where you see we have this thing called a hagfish and a lamprey. These are generally referred to as agnatha, and what's most important though isn't necessarily the words, but the concept behind what this is. These fish together--"fish" in quotes--be careful, because generically--okay, we're going to call this a hagfish, but if you use the word "fish" sometimes we refer to things like this as fish, or is that a fish? Or is that a fish? Or is that a fish? A generic word that means something that swims around in the water. That's why we've got to call them--it's an agnatha. These have cartilaginous skeletons, so generically speaking, in the agnatha, we have a cartilaginous skeleton--no bones. Hadn't evolved yet. So a skeleton, if you will, of cartilage. They're jawless. Now, this is important. The urochordata and cephalochordata are both jawless. These are no exceptions. In fact, what their mouths are like are almost circular discs surrounded with--this is going to look like something out of some kind of nightmare book--but surrounded with rasping structures that, in fact--let me tell you about how some of these things work. Some of these agnatha can actually cling on to a fish, pop its sucker-like mouth on there, and rasp out a hole in the fish's body, sucking up its viscera, as a pretty cool little predator/parasite. We'll get to that in a second.
Now, let's go through these a little bit at a time. Both of these lack some other things. So they're cartilaginous, they're jawless. Other things that they have--they really kind of lack fins. They don't have sphering fins, so they're not going to be the greatest swimmers. They're going to be squirmers more than they're going to be swimmers, so no fins on the side. Let's talk about the difference between a hagfish and a lamprey. Well, first of all, our picture--you'll see they're a little similar--and you can see over on the box next to me that there are some pictures of them, and in a real photograph you can see some real differences. One of the key things about hagfish is they're fairly primitive. Let me tell you that they have no cerebrum--hagfish. So you're not going to have much of a conversation with one. They have no cerebellum... Let's put it this way. Their nervous systems are not all that highly defined. But they can do some other things. Hagfish are famous for sneezing. This is just the coolest story. They have nostrils, and when their nostrils get clogged up with slime, because they live in like slime, they sneeze. So this is the sneezing hagfish, the infamous sneezing hagfish.
Lampreys, on the other hand, are adaptive. Their adaptations take them a little bit further. A lamprey was the one I described to you before with the rasping mouth that can actually attach to something. These things have been huge problems in the great lakes, because they've been introduced into the great lakes and they're attacking the salmon. There have been some real major fishery damage, ecological damage to the fishery of the great lakes because of lampreys. I had to tell you about those; those are great things.
Now, one branch that I want to tell you a little bit about because it's an important one, and it was a dead end. What we have here are the cartilaginous fishes, the bony fishes, etc., etc. But there's one other branch that went extinct called the placaderms. Even though they didn't make it to modern times, were very significant evolutionarily because they were the first organisms to show true jaws. I'll tell you about jaws in a minute, but these were the nightmare fish you see over to my right here. Look at this thing. Look at the teeth. It had these large plates on it. This was one nasty predator. These got up to ten meters long. Maybe good news--sad news for us fishermen--they're extinct, and they've been extinct for millions of years. But we still do have hagfishes, we still do have lampreys, and we have the next group, the cartilaginous fish. Again--fish--what's up with fish? What's up here--this is called the Chondrichthyes. I'll give you an example of what they are, and then we'll talk a little bit about what's so cool about them.
These and the placaderms and everything after it had the development of jaws. Now, sharks and rays lack bones, but they do have a jawbone. So they don't have a skeleton like you and I do, this endoskeleton made out of bone. It is made out of cartilage, except for their jawbone. You guys have seen shark jaws in museums. What's cool about this thing is how the jaws developed. Remember these? These are the pharyngeal slits that we talked about with the cephalochordata and the urochordata? There were primitive bone-like structures that held these pharyngeal slits open. Well, apparently what happened, through the course of change--because remember, evolution builds on something that exists that works really well, and may work even better another way, so it's always improving or changing what exists--some of these became--and we think that they weren't actually the forward ones, we think they were a few back--they became the beginning of support, so that what used to be a sucker-like mouth actually could be supported to form a jaw, which eventually gave rise to teeth. Now, sharks, the Chondrichthyes, don't have true teeth in the sense that there are bony teeth. They are actually scales that move forward and are constantly being replaced. My teeth, which are bone--the shark and the Chondrichthyes don't have true teeth like that, nor did the placaderms. Their plates came in from above. But nevertheless, they have these bones, and these teeth which are perfect for predation. So that's the Chondrichthyes, and that was key--the jaw. The jaw is the key thing here, because it brings us to our next group--the bony fish.
Now, these two things arose at fairly the same time, and they have ruled the ocean forever, the cartilaginous fish and the bony fish, and they're still there. And the bony fish, of course, had the full skeleton, the ability to take calcium salts out of the water and make what we call and endoskeleton. And so the osteichthyes, which means--oste is from Latin for bone, and ichthyes, as you probably know, is fish--osteichthyes--they have the toothed jaws--true teeth, and an endoskeleton. In addition to true teeth, we also have an operculum to cover the gills, and a swim bladder, which more primitive fish lack.
Well, what happened next? You know the rest of the story. During the age of Insects there were these kind of... Oh yeah, let me tell you a little bit about lobe-finned fish. The bony fish gave rise to two interesting groups--divergences. One of them was called the lobe-finned fish, and another was the lungfish. There's a little bit of debate as to which one of these eventually gave rise to the very first, what we called tetrapods, which you and I will know of in amphibians in about 30 seconds. But these things, the lobe-finned fish, were actually--the fins were attached to bones, almost like a primitive arm so that if they, we think, came out of water, could kind of like walk along and clump along the bottom and maybe clump along across a road if roads had existed, or the lungfish, which also had a primitive lung-like structure that allowed them to survive in drying conditions if a pond would dry up. Turns out that lungfish are still around. For years we thought the lobe fish had been extinct until we caught one--a coelocanth off the coast of Africa. They're still around apparently.
Well, that being said, we now have the opening to leave water and go up to land. So what happened was something "lobe-y" became something "leggy" and of course we have the beginning of the amphibians. The amphibians initially are part of the group called "tetrapods," and the tetrapods include everything that walks on land. Notice the word there was walk--four feet. So what are we talking about here? We're talking about amphibians, we're talking about reptiles, and we're talking about the mammals. So you're saying, "Wait a minute. I know some mammals that don't walk on land. Show me a whale walking on land." Oh, if I only had ten more minutes I'd tell you about how the mammals, some of them returned to the water... But we'd be here all day. And so you see... You thought you were impressed with the invertebrates. The vertebrates are just as cool.
The Evolution of Life on Earth
Deuterostomes
Diversity of Vertebrate Species Page [3 of 3]