Lecture: Tetrapods: Narrator: Listen to part of a lecture in a biology class. Professor: Today, we're going to consider the evolution of vertebrate – animals with backbone. Remember, evolution involves changes in body structure. As behavior changes, form – the body – changes to fit that behavior, eventually resulting in a new creature. But the new question we're asking today is rounded water-dwelling vertebrate, like fish, when did they used to water and crab up onto land? That is when did fish evolve into tetrapods? Here's the rough idea of what an early tetrapod might have looked like and how it would have moved. Tetrapod is determined used for land-dwelling vertebrates with four appendages and early tetrapod with the ancestors of all modern land-dwelling vertebrates, from mice to elephants. Anyway, that article I gave you last week discussed tetrapods and who can summarize it? Female Student: A paleontologist in Poland, found tetrapod footprints. Well, tracks, there are mud during some 395,000,000 years ago. They're significant because they thought the first tetrapod appear, much later than that, and I think ... Professor: 375,000,000 years ago. Before this, we all thought we had a pretty clear understanding of the timing of the evolution of vertebrate. You know what had occurred when. But this discovery pushes the evolution from six tetrapods 20,000,000 years further back into the past. Throughout it all happened a lot longer ago than we thought, and this is forcing us to reexamine some of our other assumptions as well. For example, we thought that tetrapods had evolved from a family of fish called elpistostegids. Now, as I said, elpistostegids which was a type of fish, but it had features of both fish and tetrapods. So naturally we assumed that elpistostegids was an evolutionary link between the two. For example. Well, some elpistostegids had gills like a fish, but also had lungs. So why is that important? Well, fish, true fish, have a small bone structure that coordinates the functions of breathing and eating underwater, your book goes into it in detail with all the terminology. Anyway, in elpistostegids, this structure is greatly reduced. So perhaps these creatures were spending less time underwater and more time on land breathing with lungs. Elpistostegids also had four appendages like a tetrapod, and while other elpistostegids still had gills, they didn't have a gill cover. That's a protective bone covering between the head and shoulders. So, since it lacks this type of rigid cover the elpistostegids had extra flexibility which made it easier for it to turn its head out of the water. You see in water, animals that they want to see behind them can use their entire body easily. But in shallow water and on land moving entire body requires more time and effort. That is not free of a gill cover, the output oxygen to turn head from side to side without having to maneuver its entire body. So as I said, we'd assume that elpistostegids had evolved into tetrapods. But there's a problem with timing – the tetrapods tracks they found. They're 10,000,000 years older than the oldest elpistostegid fossil we have on record. So tetrapods are well before elpistostegids. Yes, John. John: But the timing, is that the definite proof? I mean, maybe there are elpistostegid fossils we just haven't found yet, fossils from an elpistostegid that are older than the tetrapods. And you just said that some elpistostegids had leg-like appendages. Right? Well, maybe what they found in Poland was left by an elpistostegid we just haven't found fossils for you. Professor: That's a good point John. And we're constantly finding fossils that challenge our theories. But as for the tracks, well, maybe certain they weren't rushed by elpistostegids. You see, the tracks that were found were very well-preserved. So that kind of paleontologists were able to gather lots of information about the vertebrates left them. In oreder to move forward, the vertebrate that left them walked with coordinating opposing leg-like appendages. That means that while the left front leg move forward, the right back leg did as well. And vice versa, this is the way baby crawls and you see what I mean. Ask your question. Well, let's look at an early species of elpistostegids we want to call it. Like tetrapods, elpistostegids had four appendages and those appendages are strong enough to support to the elpistostegid's weight. But the position of the bones of elpistostegids is very different from an early tetrapods. Rendering elpistostegids incapable of performing the walk that coordinates opposite leg-walk, seeing in the tacks.