The next major evolutionary step we mentioned was the development of a bony endoskeleton, and we're going to see the living relatives of those early pioneers today: the bony fish, or, if you want to get that fancy jargony name, it's Actinopterygii. These organisms have an internal skeleton mostly made of bone, which makes them much heavier than chondrichthyans or cartilaginous fish. As a result, they've had to evolve a mechanism to keep from sinking called a swim bladder. This is basically just a gas-filled sack that allows bony fish to remain suspended at a particular depth, and believe it or not, this evolved from the lungs. You can see a picture of a swim bladder right there, and you, that one obviously has been removed from a fish, but you can see it right here, in this internal diagram of the fish as well.
So, you might be wondering, if these bony fish need a swim bladder to prevent themselves from sinking, what about sharks? Guess what? Sharks are actually denser than water and have to keep moving in order to stop themselves from sinking. So, even though these bony fish are heavier, they actually don't have to expend as much energy to float. Another rather important feature of these bony fish is called the operculum, and this is basically bony flaps that protect the gills. What we think of as fish gills, usually, when we look at a fish and we see those flaps on the outside, those are actually these bony structures. The gills are going to be those fleshy internal structures.
Two groups of bony fish that we should talk about are, first, the ray-finned fish. These guys are named for the structure of their fins. You can see it very clearly in this diagram right here. Basically, you have these parallel bones, and then these webs of skin in between them. That's how the fins are created. This is actually the most diverse group of vertebrates. Amongst all fish, these guys are the single most diverse group. And that's because the other guys we're going to see, lobe-finned fish, are not as species-rich as ray-finned fish, which are found in most marine and freshwater environments. They're pretty much ubiquitous. Let me jump out of the picture here for a second, so I can just quickly point out the swim bladder right there. And here we have the gills. So again, that's the fleshy internal structure, whereas the operculum is going to be this bony flap covering it.
Moving on to lobe-finned fish. The most well-known example of a lobe-finned fish is an organism called a coelacanth, which was long thought to be extinct. They had found fossils of them, and then, within recent history, living coelacanths were found, completely overturning that assumption. You can see a preserved, or, a preserved organism there. And here, you can see a drawing of one. The reason I've included the drawing is because it's a little easier to see these defining features, these muscular lobe fins, as they're called, which actually are what allowed these guys to be our terrestrial vertebrate ancestors. These muscular fins that you see, and let me hop out of the image here, are what are eventually going to become limbs, and it's a little easier to conceive of them as precursors to limbs in the drawing.
With that, let's flip the page and talk about when life finally left the water for land.