[MUSIC] The Diapsid lineage of reptiles is truly remarkable. It started off in the Pennsylvanian, began as relatively small organisms that had an active lifestyle that required they had a lot of protein in their diet. They ended up generating a fair amount of some of their body heat. And they were not successful in ruling the planet until at the end of the Permian, a meteor impact conveniently hit the planet, and the result of that was to knock their competitors, the Synapsids, off of the pedestal of being the top predator of the terrestrial environment on Earth. So after the Permian-Triassic boundary at 252 million years and then moving into the Triassic, we had a template that was open in terms of ecosystem space where the Diapsid lineage could radiate. And the previous dominant groups, the Synapsids did very poorly and they became dominated and preyed upon by the Diapsid reptiles that were evolving. So let's look for a moment about what some of the attributes are that make up the group of the Diapsids. Because we still have Diapsids in the modern day environment. And it's important to recognize what this are. So there are basically four major groups of Diapsid reptiles. The first one, we call them Thecodonts. And the Thecodonts were the ones that eventually evolved into the dinosaurs. And the Thecodonts included both herbivores and carnivores. And they were able to access both the marine environment and the terrestrial environment. The second of the great groups of the Diapsids if you will are the crocodiles. The great salt water crocs that we still have in places like Australia. These are serious reptilian predators, right? They're upwards of 15-16 meters in length. They're enormous organisms and they now evolved into basically two major groups that's the crocodiles and the alligators. The next distinct group of Diapsids we want to think about are the dinosaurs themselves. Now all dinosaurs went extinct at the time of the Cretaceus tertiary KT meteor impact. But the dinosaurs were very diverse and we want to visit what that diversity is composed of. Then the last of the major groups of the reptiles were the pterosaurs and they were the flying Diapsids. So this group of Diapsids that took to the air, and really mastered winged flight. So what we want to do is look at representatives of each of these. So, one of the groups that are relatively poorly known in terms of their exact phylogenetic relationship to the ongoing evolution towards dinosaurs is a group called the Ichthyosaurs. And the Ichthyosaurs are unique because they've undergone what we call convergent evolution. Convergent evolution is the notion that if two very different types of organisms, let's say a mammal or a reptile in the modern day. If they access the same environment that because of natural selection pressures because the survival is done by those who are fitted the best to the environment. That in the end they end up with a body that looks similar even though they're very different organisms. So the Ichthyosaur is a very good example of that because as you can see from this picture the Ichthyosaur looks like a modern day mammalian Porpoise. It has kind of a bottle shaped nose and has a sleek outline. It has a dorsal fin. It has fins laterally. But one of the things that it doesn't have is a fluke on the back that is horizontal. So what unifies the modern day mammals that have this general morphology is that they have a fluke that is horizontal. So I think most of us have seen pictures of dolphins. Or whales breaching out of the water and having their flipper come down and hit. Or even some of the modern day porpoises are so strong that they can elevate their entire body out of the water and then propel themselves, and more or less walk, or skim across the water surface by that horizontal fluke hitting the water. It's a very powerful way to make thrust and propulsion. But the Ichthyosaurs did not have that. Now remember the Ichthyosaurs were Diapsid reptiles that have branched off on the tree of life going towards the dinosaurs. So these are not dinosaurs but they're on the evolutionary branch going towards that. So we have a reptile that's looks like a porpoise. And not only did they have the general morphology of this except for they had a vertical fluke instead of a horizontal one is that, again, these were reptiles and one of the differences between reptiles and mammals is that the mammals have a womb and can incubate, if you will, the juveniles inside the womb to a relatively advanced stage before having them leave the womb. The reptiles have to rely on an amniotic egg and that egg has to hatch outside of the body of the animal. So again chickens and dinosaurs they both utilize nests to put their eggs into and hatch those eggs. Now what the Ichthyosaurs did was they kind of played the game both ways where they were definitely reptiles and they had their eggs hatch outside of their body but as soon as the eggs hatched, the juveniles swam immediately into a modified pouch that was on the lower back side of the Ichthyosaur. So it would swim to the pouch and then that juvenile then grew up to a very advanced stage within the pouch. Now these were air breathers, right? They were reptiles that had lungs and so once that organism, once the juvenile organism left the modified pouch, both the mother and the juvenile would have to swim up to the surface of the water for that juvenile to get its first big gulp of air. But there was a problem. We see in the fossil record that there are many female Ichthyosaurs that died in the process of having the juvenile leave the modified pouch. And you can say this, they died during childbirth but remember, it's not a womb. These are juvenile reptiles that are leaving a modified pouch that was up inside the body cavity of the mother. And for a variety of reasons, one of the most direct ways to evaluate this, is that they simply grew to be too large before they tried to escape from the pouch and so they only came out part way. So, as you can see from this diagram we have very good example of having Ichthyosaur fossils especially in the Jurassic time period. In which you see half a juvenile coming out the back of the modified pouch which went all the way up into the lower backside of the ribcage of these Ichthyosaurs. So the Ichthyosaurs are very famous, they were good hunters, they were very abundant, they were throughout the oceans of the world at that time period. So the Ichthyosaurs is one important example of the lineages that were evolving off of the mainline that was moving towards the dinosaurs. Another one that's really important to us is a group called the Plesiosaurs. These long necked reptiles that were very fast swimmers because they had large flippers that were on the order of meters in length. And these were large organisms, upwards of ten,15 meters in total. And they were great swimmers and fishers, and they actually had such a strong jaw that they were able to feed upon one of the other classic invertebrate fossils of the Mesozoic. And those are called Ammonoids. And the Ammonoids, as you can see in this picture, was a squid-like organism that lived in a shell that spiraled. And what it did was it produced a gas and it floated in the water column. And so the Plesiosaurs had such a strong jaw that they could come in and just grab these floating restaurants, if you will, that were in the water column of the Mesozoic seas and they were strong enough to snap that shell and then get the squid like Ammonoid out of it and eat it. The Plesiosaurs' a great example of the Diapsid lineage than evolving towards dinosaurs, but again, this is a very distinct phylogenetic diversion off of the main line that evolved toward the dinosaurs. They're related to dinosaurs, but they are not dinosaurs themselves. And you remember there are some of these thought possibly to be living in the modern day environment. We've all heard of the Loch Ness monster. That's a Plesiosaur. So whether or not Plesiosaurs still live in the modern environment that's up to you, but we have an extensive and beautiful fossil record of the Plesiosaurs. Another one of the Thecodonts were the crocodiles. And we have the Crocodilia still in the modern day environment in the form of both alligators and crocodiles themselves. The last group we want to think about are the true dinosaurs. Now the dinosaurs started off, the true dinosaurs were in the mid to late Triassic, but even at the very beginning of the radiation of the dinosaurs we saw that two major families of dinosaurs came up. The first of these was called the Ornithischians. And in this diagram, which branches upward, so immediately in your mind's eye you should say to yourself, okay, I'm looking at geological time going from older at the bottom to younger at the top, and I'm seeing the tree of life from the side. So in this diagram we see that there's a root stock at the bottom and then it's diversifying upward. And the way to think about this in the over all Karl Woesian tree of life is that we're now zooming in on some of the branches that are high level within the eukarya. So these are eukarya branches in the tree of life and we're zooming in in one of those major branching points which is the radiation and diversification of the dinosaurs. So on the right, we see that the first lineage of the dinosaurs, and those are called the Ornithischians. Now, all of us as children, we learned about dinosaurs to some degree. And, the Ornithischians, have always been chosen to be the, the more or less snuggly, happy, warm type of dinosaur. Right they're the dinosaurs that when we learn about them as children they're the ones always made out to be relatively friendly when people make movies about them. So things like the triceratops, the three horn very, very large dinosaurs. The Stegosaurs, they're the ones with the plates across the back. The hornbilled Hadrosaurs, the ones that could stampede in herds and make all these trumpet types of sounds. All those were part of the Ornithischians. And the Ornithischians were unified to one major degree because of the fact that they were all herbivores. There were no carnivores within the Ornithischians. And I think that's why people have always related to them a little more warmly than the other branche that we're going to talk about now and that's called the Saurischians the branch on the left included two major types of Saurischians. One group that were herbivores again and we call them the Sauropods. And the herbivorous Sauropod Saurischian dinosaurs were the long necks, the ones that were extremely large with the very, very long necks, the long tail. And we're going to be talking about some of their evolutionary adaptations as we go forward. The other group within the Saurischians were, the showtime ones, right? The ones that were the big carnivores. And these are definitely organisms to be scared of, right? These were predators at a level that are hard for us to comprehend in the modern day environment and they're also the ones that have been chosen therefore to be the ones to terrify us in some of the big action movies. So the other branch then, we call those the Theropods. And the Theropod branch were the Saurischian dinosaurs that were carnivores. And some of the stars of the show for this one are the T-Rex, the Allosaurs, and the Raptors. Now importantly, that last group, the Theropods that we just described, that's also the group that evolved eventually into the birds. So we have good evidence that modern day birds have evolved from the Theropod dinosaurs. But interestingly enough, that diversification, it didn't wait until the Cretaceous. It actually started diversifying, and we have good evidence of the transition through a fossil called Archaeopteryx. To go from Saurischian dinosaurs into birds as early as the late Triassic, and into the early Jurassic. So when you look at a phylogenetic tree, one of the first questions that should rise in our mind as we look at evolutionary biology through time is, where does time actually start and stop? Where does geological time bifurcate? Which events took place when and it's a very, very complicated puzzle. So the phylogenetic tree that I'm showing you is a generalization. And remember how I said, well you were looking at the tree from the side because you see branching going upward. But remember in this case a lot of that diversification, it happened really fast at the end off the Triassic into the early Jurassic. It didn't wait until the Cretaceous for all these radiations to take place. So this is the template of the dinosaurs and its cousins that rose into especially the marine environment. The Plesiosaurs, the Ichthyosaurs, the crocodiles and the other groups that were manifesting themselves at that time period. [MUSIC]
