Gnathostomes are jawed vertebrates and represent a large evolutionary step with the development of jaws. Now, gnathostomes include fish, amphibians, reptiles, and mammals. Though the early gnathostomes were basically just jawed fish. Common features among gnathostomes include jaws, larger forebrains, and paired appendages. So, if you think about us, for example, we don't just have one arm, we have two paired arms. Likewise, a lot of fish, such as the shark right here, don't have just one fin on one side, they have two fins, one on each side. Those are paired appendages.
An interesting system that we'll see in some of these organisms is called the lateral line system, which is a sensory system that detects movements and vibrations in the water. You can see it highlighted here in this shark in red, and you can also see a real photograph of a fish here where the lateral line system has been highlighted and appears as all these white dots along the fish's base. As you can imagine, that's probably pretty useful for an organism that lives in water, especially a predator.
Living relatives of early gnathostomes are the cartilaginous fish, or chondrichthyans. That's the jargony name but, cartilaginous fish is just fine. These organisms include sharks, rays, skates, and what are called chimeras, which are these little kind of cute fish that you see right here. The defining feature for these organisms is that they have a cartilaginous skeleton, meaning a skeleton made of cartilage, and they have those paired fins. Which are, again, kind of hard to see in these pictures because they are profile shots, but, you know, you get the idea. This fin right here has a pair, or a partner, that's on the other side of this fish that we can't see.
It should be noted that sharks actually use internal fertilization, and include species that are oviparous, oviparous, and viviparous. Remember that means, they lay eggs or they keep their eggs internally, and the young hatch from the eggs and leave the mother right at birth, or they give birth to live young. Right? Viviparous. If you don't remember those terms, you can go back and check out our earlier videos on animals where we covered them. Also, it should be noted that skates are oviparous, meaning they lay eggs, and rays are viviparous, meaning they give birth to live organisms. You've probably seen, if you've been to a beach before, something like this wash up on the shore. Those are actually chondrichthian eggs. You can see, this one is all dried out here in this image, but you can see in the image behind me, a nice little shark egg with the shark developing inside there.
Alright. With that, let's flip the page and talk about some other vertebrates.
