So to continue our tour of our phylogenetic tree, we'll now be looking at the deuterostomes, so the echinoderms, and some of the basal chordates. So going back to this tree for our animal phyla, we've looked at the proteostomes, now we'll look at the deuterostomes. So the difference between proteostomes and deuterostomes, that name arises from during development, the first invagination within the embryo, that first kind of dimple.
that pokes in, does it develop into the mouth, which is what happens in proteostomes, right, first mouth, or does that first one develop into the anus and it's the second invagination on the other side that develops into the mouth, the deuterostome, right, stone for mouth. So one of the ways that we actually knew that these were very distinct groups before we had our molecular phylogenies was from the sequence of developmental events. And this is also interesting because If we want to learn about chordates, right, like us, and we're interested in studying their development, it actually makes a lot more sense to study the development of echinoderms before we would go and study the development of, say, worms or insects.
So while we know a lot about the development of, say, Drosophila, it's less likely that Drosophila will develop in the same ways that echinoderms do in terms of being comparable to humans, for example. So in fact sea urchins are one of the big developmental model organisms because they're actually more closely related to us than other more convenient organisms that we might wish to study instead. Okay so let's look at deuterostomes and first let's look at echinoderms and then chordates. So echinoderms are starfish, sea cucumbers, and all that.
So sea urchin, the sea cucumber kind of has the body stretched out with the mouth at one end the anus at the other end. See urchins, you have everything is ventral. Starfish, we're kind of familiar with. There's a wide range of different starfish.
A sea cucumber is kind of like a starfish that's been stretched out, right, with this end over there and then the bottom end over there. They appear to have five-fold symmetry, but when you actually analyze them, they have bilateral symmetry. So it's kind of a myth that a starfish is a pentagon sort of creature. It's really a bilateral creature, just superficially like a pentagon type creature.
So now moving on to chordates, the sister group of echinoderms. So this includes chordates and vertebrates. So basal within chordata we have tunicates, which are these guys here. They start off life living like we think a chordate should, but then they actually implant in the substrate and turn themselves into what looks like a little sponge. So they don't really live the lifestyle we would expect of a chordate or a member of chordata.
Amphioxus, the cephalochordate here, called cephalocordic because it actually has like a head region. You can see that they're quite small. And then these two groups are basal to craniata, which is hagfish and vertebrates, which we'll see next. So craniata includes hagfish and vertebrates. Again, if we're going to be naming with monophyletic groups, we would actually technically include all the vertebrates within craniata, but let's not worry about that right now.
So the most basal of these groups are hagfish. So a hagfish kind of looks like this. It does not have a lower jaw, it just has an upper jaw.
It doesn't really have eyes, it doesn't have limbs, right, it's not a very sophisticated looking organism. But hagfish, again, they've been evolving as long as everything else. They have their own superpower, they have slime glands.
So if you've ever seen this old movie Ghostbusters, they have the power to produce large amounts of mucus incredibly quickly in the same way that the ghost Slimer did in that movie. you actually bring these up on a boat you have to be very very careful because once they dry out if you accidentally get any water on them there's basically an explosion of mucus from these guys. So that's um hagfish there or hyperartia. This group hyperartia or lampreys. You can see they have better developed eyes, they have a oral disc that actually has teeth, they don't have this slime gland power but you can see they're starting to evolve fins.
for a bit more sophisticated swimming. So again, they're not more evolved than these guys, because both have been around for about the same amount of time. But they have evolved features that are a bit more like fish, which is going to be here in Nathostomata.
Now within Nathostomata, so this is now vertebrates that have jaws. Our hagfish and lampreys did not have lower jaws. Nathostomes, so nath means bite, stome means mouth, so these are biting mouths.
The basal group here are Chondrichthys, so these are fish that have cartilage, so that's sharks, rays, etc. So they don't have bone, but they don't really need bone because they're living in the water all the time. So you don't really need a super strong supportive structure if you're spending all your time within an aqueous environment where you don't need to really fight against gravity very much. And then we have two groups of fish that have bone.
We have ray-finned fish, Actinopterygii, and lobe-finned fish. sarcopterygii. So when you think of a fish you are probably thinking of an actinopterygii. So like tuna and salmon and all those things are raphen fish. This is what seahorses look like.
I had a student a few years ago who didn't realize the seahorse was a fish. I don't know really what what she thought it was but it was news to her when it was a fish. So they look kind of weird but yeah they're fish and they're raphen fish actinopterygii. So this is a very diverse group, lots and lots of species of fish within this group, but the Sarcopterygians are the interesting group because the lobe-finned fish are going to be the ones where initial lobe-finned fish individuals a long ways back, some of them remained fish, some of them evolved into terrestrial vertebrates.
So Sarcopterygii, lobe-finned fish, we have two major groups here. We have The dipnoi or lungfish, these are basically lobe finned fish that have remained fish from this ancestor here that was a lobe finned fish. And then that's a sister group to terrestrial vertebrates, which is mammals, reptiles, birds, and amphibians.
So within dipnoi, we have three major types of lungfish. We've got ones that live in Australia, and you can see that they actually have kind of muscular limbs, front and back. We have these ones in South America and these ones in Africa. Their limbs are not nearly as developed. so they don't really walk around in the same way.
So if we want to get a clue as to what this early terrestrial vertebrate or what this lobe finned fish that evolved into a terrestrial vertebrate was like, it's probably much more like this thing here. You can see they do use these limbs to walk around when they're underwater, but these are the only ones that are kind of big enough to walk around on land. And in particular they actually have bone in these fins, and they can support real weight with some muscle, which is different from ray-finned fish. which just have rays and they don't really have bone in there at all.
So now let's look at terrestrial vertebrates. When we think of terrestrial vertebrates, they've evolved from a sarcoptorygian. So this is actually a fossil of fish from the Devonian.
And when we look at the forelimbs and the hindlimbs, and we look at their lobe fin, right, and their fin that has bones in it, when we look at those bones, we see that there's a big bone here that kind of connects the body, and then there's a pair of bones here. Then there's some other bones here, and then there's kind of like five little protrusions here. And when you go to the hind limb, we have the same thing. We've got a bone there, a pair of bones here, some other bones, and then these protrusions. And this happened to be the arrangement of bones that this Sarcoptorygian ancestor of modern terrestrial vertebrates had.
And so this pattern of bones... in this ancestor all the way back in the Devonian, right, that's a really long time ago, that pattern of bones that happened to just make a fin that could be used for dragging itself around in the mud, that pattern of bones is what modern terrestrial vertebrates have inherited, right? So this homology of four limbs of one long, two long, some miscellaneous ones, and then five digits, that's inherited all the way back from this guy here who is a fish.