ZOO 138, Wednesday, February 5, 1997, 12:00 p.m.
So the last lecture was on the amphibians, and now we're going to talk about the reptiles.You have this covered in your Illustrated Notes, it starts with the classification of the reptiles.
This is the classification of the reptiles that you will be held responsible for on themidterm. What you can see is that the class Reptilia is divided into the number of subclasses.
Now, there were a number of different characteristics that are used as the basis forassigning animals to these subclasses, but one of the ones that's more obvious which is reflected inthe names that are given to these classes is the number and location of opening inside of the skull,which are these little crosshatched areas.
A single opening, as you have already learned in lab, is called a fenestrae. And these are inthe region of the skull that's called the temporal region. So the plural is temporal fenestrae.
So the characteristic is the number of the location. Temporal fenestrae, which is the pluralof fenestra. And, in fact, the name Anapsida, which is the condition of having no opening, notemporal fenestra, that's the condition that is found in animals placed in the subclass Anapsida.
And the Synapsid condition in which the temporal opening is somewhat low on the sideof the skull, and these 2 bones, the postorbital and squamosal bones meet above the fenestrae thatSynapsid condition is found in the subclass Synapsid.
The Euryapsid condition is kind of the opposite of the Synapsid condition. The temporalopening is higher up, the postorbital and squamosal bones meet below, and that's in the subclassEuryapsida. So far things are pretty simple, but you know that is not going to last for very long.
There is a condition in which there are 2 openings, the Diapsid, the "di" telling you 2. 2openings, postorbital and squamosal meet between them. And there are 2 subclasses that have theDiapsid condition, that's the subclass Archosauria, which includes the crocodiles and thedinosaurs, and the subclass Lepidosauria, which includes only one living order Squamata, butthat's where we find all the rest of the living reptiles.
So we have five subclasses Anapsida, Synapsida Euryapsida, Archosauria andLepidosauria, and 4 conditions of the temporal fenestrae. The turtles are found in the living orderwithin the subclass Anapsida. Synapsida is a very interesting group. I have a whole lectureplanned on that group, because that's the group that evolved into the mammals, and we'll betalking about how and why that occurred.
Today I'll be talking about the extinct groups, which are the ones that have the little plusin front of them, that's because this particular classification sheet was made up before I had aword processor that made crosses. That's the traditional symbol in zoology literature, is a cross,like a gravestone indicating an extinct group. So the ones that have little pluses in front of themare extinct, the ones do not have pluses in front of the are living groups.
You know, when we look at a classification like this, we know that in making up such aclassification, this is a very traditional kind of classification. There is whole other school ofbiosystematics that uses a very different kind of classification. And if you are interested in learningabout that, you should consider taking Dr. Baskin's course in ichthyology. He will be telling youabout that more recently developed theory of how to write classifications, which is calledcladistics .
This is a traditional classification. And when you write these traditional classifications,basically the groups that are considered to be more primitive, that is older in terms of their firstappearance in the fossil record, appear towards the top of the list. So this Captorhinomorpha, Ialready mentioned that group, that was the first group of reptiles to evolve, the ones that aredirectly descended from the Labyrinthodonts.
At the bottom of the list we have the snakes, that's the most recently evolved major groupof reptiles. So we tend to go from older to more recent.
But other than that, one thing which is definitely not included in this kind of aclassification, it's really important that you understand that this information is not here, and that is:Who is the ancestor, and who is the descendent? What group gave rise to what group? All right?
It would be a serious misunderstanding for you to think, "Well, the Anapsida gave rise toSynapsida, who gave rise to the Euryapsida, who gave rise the Archosauria, who gave rise to theLepidosauria. That is not even close to true, but I have biology students who thought that's whatit meant. Don't embarrass me.
So there is a general tendency for more primitive groups to be listed at the top of any list.For example, within the subclass Archosauria, the Thecodonts are the most ancient group withinthe Archosauria. And the more recently evolved groups, although which are still all extinct, theyare lower down.
But again ancestor/descendent relationships are not presented here. So when we want topresent that kind of information, we use something called a phylogram. This is a phylogram. Youhave probably seen them before. And if you study anything about biology, where we're talkingabout the evolutionary history and relationships of groups, are going to see phylograms again. SoI want to make sure that you understand how to interpret one of these.
A number of different kinds of information are included in this. An interesting insights canbe extracted from a phylogram. For one thing we have a time scale on the vertical axis here. Sothe more ancient things are towards the bottom. This is the carboniferous. This is the Paleozoicera. The Permian is the last period in the Paleozoic.
Then we go into the Mesozoic, composed of Triassic, Jurassic, and Cretaceous. Theseshould all be terms that you remember memorizing, remember millions of years before present.
Here we are, the Quaternary, that's where we find human beings appearing and so forth.
The famous extinction of the dinosaurs occurs at the end of the Cretaceous, the boundarybetween the Cretaceous. That's extinction of the dinosaurs, it occurs at that time. And it isrepresented on this graph. Because these 2 groups right here, the Ornithischians and Saurischians,these are the dinosaurs, the 2 major groups of animals.
And notice that the little balloon representing those groups, those are the balloons endright at that boundary, the boundary between the Cretaceous and the Tertiary. So one of thepieces of information that is included in these groups, is each taxon, each group, is represented bysome kind of a little balloon. And when that group first appears in the fossil record is shown,Ornithischia appear in the Triassic.
And when the group goes extinct, if it goes extinct, is up here at the end of theCretaceous.
The Captorhinomorpha appear in the upper carboniferous. It's hard to see on this thing,but you notice this line that has the mammals comes all the way back down to here, they also hadthe first mammals appear at the beginning of the Triassic.
Now, there is some information and that's ancestor descendent relationship. Those areterms that people tend to misuse. I'm not sure that people really don't understand them, butsometimes, you know, you have those neuronal malfunctions that you forget two things that areclosely related to one another.
Your parents are your ancestors. Your children are your descendants. This group has asits ancestor a Captorhinomorpha. And this group Pelycosauria , who are the descendants? TheTherapsids are the descendants.
It shows you by connecting the balloons which group is the ancestor and which group isthe descendants. Not all groups have descendent groups. Euryapsida, no living descendants. Howcan you tell that? Because there is nothing connected to this balloon that continues up here to thepresent which is at the top of the graph.
So ancestor descendent relationships are shown. Notice that in some cases the balloon isdirectly attached, and other cases it kind of a dotted line.
Here is some dotted lines over here. What does that tell us? Well, the oldest thetestudines or testudinada . Those are the turtles. This balloon over here, which continues up thepresent, that's the taxon that includes the turtles.
The oldest fossils turtles appear in the Triassic, that's why the balloon closes off at thebeginning of the Triassic. But the paleontologists who studied those fossils think that theyprobably arose from the Captorhinomorphs back in the upper carboniferous.
And that's what the dotted line is. It's a best guess about who are the ancestors and whendid that split occur. So that's what the dotted line tells you. They hope some day to find fossilsthat would allow them to document that hypothesized connection.
Oh, there is one more thing that's included in here. It has something to do with thediversity of the taxon, how many different genre are known from the entire fossil record. Of allthe fossils that have been dug up and described by paleontologists, how many different genre arethere? This is a very imprecise representation.
For example, you can see that the birds were -- the width, the difference between these 2lines right here, these were not a very diverse group. That means there is not a very extensivefossil record from the birds, until the end of the Cretaceous.
And suddenly we get this explosive increase in diversity. Same thing with the mammals.This is such a narrow line, that it just barely gets off to the side of the piece of paper here, butsuddenly up in this area it undergoes a tremendous increase of diversity.
That's what we call an "adaptive radiation." An adaptive radiation is when there is asudden increase in the diversity of a group.
Some people have interpreted the extinction of the dinosaurs as being that they werekilled off by the mammals. Well, if you look at the diversity of the fossil record, what you see isthat the mammals were insignificant little tiny mouse-like animals for hundreds of millions of yearsuntil the dinosaurs went extinct, and then the mammals inherited the earth and shared with thebirds in terms of diversity.
So mammals definitely did not kill out the dinosaurs. There is know way. Somethinghappened to the dinosaurs and the mammals took over.
Now, as I was saying, there are some interesting things that we can determine when wehave this kind of a phylogram besides knowing when a group appears, and when a groupdisappeared, and who its ancestors were, and who its descendants were. We can also ask -- we'regoing to take 2 different groups, and we can ask what was the most recent common ancestorbetween the 2 groups?
A common ancestor is ancestral to both groups. What group, for example, is the commonancestor of the Ornithischia and Saurischia? Thecodontia. This group, Thecodontia, is a commonof Ornithischia and Saurischia.
Is Captorhinomorpha also a common ancestor of Ornithischia and Saurischia?
STUDENT: Yes.
INSTRUCTOR: Yes, it is common ancestor. And so are the Labyrinthodonts and so arethe rhipidistia. But the most recent common ancestor of Ornithischia and Saurischian would beThecodontia.
Now, that's an interesting thing in itself. But it also can allow us to answer the question,who are the closest living relatives of a particular group. Because the closest living relative, bydefinition, will be the group with the most recent common ancestor.
So we can say, "Who are the closest living relatives of the birds from this graph?" Well,we got to look for another living group that has an ancestor in common. Well, birds and mammalshave many similarities. They are the dominate groups on the earth. They have fur or feathers, highmetabolic rates. Well, the most recent common ancestor of the birds we keep tracking back hereto follow the mammal line back, and those two converge all the way back the Captorhinomorpha.The mammals and birds most recent common ancestor is Captorhinomorpha.
Let's look for the most recent common ancestor of crocodiles and the lizards. I mean, acrocodile just looks like a big lizard. So crocodiles come back down here like this toCaptorhinomorpha, and so do the lizards, which are included the lizards and snakes, which are included in the Lepidosauria.
So the most recent common ancestor of the crocodiles and of the lizards isCaptorhinomorpha.
Who is the closest living relative of the birds then? The crocodile. And, conversely, whois the closest living relative of the crocodiles? The birds. That's a relationship that has to be true.
Now, sit there and think about that for a second. The crocodiles are more closely relatedto the birds than they are to the lizards. Is that true? Yes, it's true.
That's why there is this whole other field of systematics, this area of cladistics . Oh,weight a second, if you are classifying the crocodiles and the birds in different classes, but they arethe closest relatives of one another, then there is something wrong. So throw out that wholeentire classification and make up a new classification, and that's what this other classification is it,recognizes a group called Archosauria.
And Archosauria includes the Thecodonts and the crocodiles and these 2 groups ofdinosaurs and the birds in a single group. And the birds are not in a separate class according tothat classification.
Now, if you are a traditional zoologist, you are going to say, "Wait a second, you aregoing to tell me that crocodiles and birds belong in a group together, even though crocodiles livein the water and low metabolic and low body temperatures, and birds fly and have fur and flythrough the air, and they are closer. Wait a second, there is something wrong."
There is huge hate-filled debates between proponents of this two different theories ofcladistics . That's the kind of information you can get out of this graph, and you need to be able tointerpret it. There a possibility that I might ask you to produce this graph on a midterm?
No. But I might give you this graph with all the words removed from it. I will not ask youto draw this graph from memory on a midterm, but I might give you this graph with all the wordsremoved from it and ask you to fill in the words that are here on this one right now, including thetime scale as well the names of the taxon.
So you don't have to practice drawing it. In under no circumstance would I ask to youdraw it. But I do want you to know this information. I'm going to go through this information insort of verbal manner starting right now.
So I'm going to go through these various groups and talk about some things about them.The first of the groups is Captorhinomorpha. I guess I don't have a overhead on that, but theyare covered in your Illustrated Notes.
So the kinds of information that I do want you to know about these groups of animals aretheir ancestors, who there descendants are if they have any, when they appear in the fossil record,and a little bit about their biology. The Captorhinomorpha are a very, very important group ofextinct reptiles. They are sometimes called the stem reptiles, because they form the stem on thefamily tree of all the reptiles.
Again, if you look at the cladeogram here -- I mean, look at this phylogeny, here are theCaptorhinomorpha. You consider all these others to be branches off. This is the stem of the treehere. It's actually more like a bush, like a family bush; not a family tree.
And by the way, all of the animals shown on this group are descendants from theCaptorhinomorphs, and they all produce amniotic eggs, birds and mammals. Even the mammalslike ourselves that don't lay eggs, when you look at the structure of the viviparous egg, it has thesame futures as the egg of the reptiles and birds.
So those 3 major vertebrate classes can be collectively referred to as the amniotes. Sothese are amniotes, the reptiles, the birds and the mammals. And this is a phylogram of theamniotes.
So the Captorhinomorphs have a very important place in that entire phylogeny, they formthe family tree of all of these major terrestrial vertebrates, the birds, the reptiles and the mammals.Their ancestors were a group of Labyrinthodonts, as I have already told you. There ancestorswere a specific group of Labyrinthodonts called the "Anthracosaurs."
And the translation of Anthracosaurs, "saur" of course means lizard or reptile, and"anthraco" is a reference to a particular kind of coal, called anthracite coal. Translation of thisname is coal lizard because the fossils of this group of animals are found in coal deposits, coaldeposits laid down during the geological period called the carboniferous.
What's the translation of carboniferous? Carbon bearing. And what's coal? Carbon.
So the huge reptiles arose in the these huge swamps. And the Labyrinthodonts were livingin these swamps. And all those plants were subsequently fossilized and they were turned into coal,and that geological period is called the carboniferous, because of the huge amounts of coaldeposits that come from that period.
And these animals known as fossils from that coal were originally thought to be reptiles.They are very, very similar. The Anthracosaurs, remember this is a type of Labyrinthodonts. TheAnthracosaurs Labyrinthodonts are not reptiles; they are amphibians.
But their anatomy as was so similar to those of simple lizard-like reptiles, that they areoriginally classified as reptiles. Hence, they were given the name Anthracosaurs. Andpaleontologists always have to wrestle with this kind of problem where the things that distinguisha group of animals are soft tissue types of structures. They are not going to be preserved in thefossil record. You would not have any difficulty telling a frog from a lizard or a salamander from alizard.
But when all you have is the impression of the skeleton, it's harder to tell.
When we start talking about evolution of mammals, we're going to find that all of thethings that distinguish mammals, and really stand out in our minds as being the differencesbetween reptiles and mammals are soft tissue structures that are not preserved in the fossil record.
So the paleontologists had to come up with some way of saying is an animal an amphibianor is it a reptile. And they resorted to using the presence of a little flange on pterygoid bone as acharacteristic of reptiles. So if you are a reptile you have a pterygoid flange. If you look in yourIllustrated Notes, you have a picture of a skull that shows this flange on the pterygoid bone. Allthat flange is a point of attachment for some muscles that help these animals close their mouths.
Now, obviously amphibians can close their mouth, so what is the difference? Thedifference is that with an amphibian type of mouth that does not have a pterygoid flange, you canclose your mouth pretty quickly. So if you are trying to catch an incest, you can bite onto thatinsect pretty quickly and catch him between your teeth, in your upper and lower jaw.
But if that insect has evolved a thick heavy exoskeleton, you can't crunch through it. Soyou can hold on to it until you get bored, then he flies away.
What happened was there was kind of an arms race taking place between theLabyrinthodonts and the insects that they were eating. And the insects were evolving a defensemechanism, with was this thick heavy exoskeleton.
And then they finally got to a point where the exoskeleton was so thick that amphibianscouldn't crunch through it, so the amphibians evolved a new point of attachment for the jaw linewhich allowed them to generate a lot of force when the mouth was almost closed.
They were sitting there with an insect between your teeth, and if you have a pterygoidflange, crunch, you can eat the little sucker. So pterygoid flange is the reptiles answer to the thickexoskeleton of the insect. And that's why the paleontologists chose it as a characteristic thatindicated an advancement in the feeding mechanism of these animals. And so all reptiles have apterygoid flange; and amphibians do not.
That's the kind of length that sometimes a paleontologists have to go to in order todistinguish between 2 different groups of animals. And, clearly, it's important to be able todistinguish one class from another class. But as a biologists, to me, it's really interesting to seethat other differences that are preserved in the skeleton, between animals you can't tell thedifference between.
When I was a graduate student more decades ago than you remember, these animals wereconsidered to be reptiles. And it's only since that time that paleontologists have decided to classifythem as amphibians.
So the ancestors of the Captorhinomorphs are the Anthracosaurs Labyrinthodonts, andthere are some interesting aspects of the biology that go along with that statement.
Another major advance -- if you consider the pterygoid flange to be a major advance,another major advance, probably a lot more important one is that the Captorhinomorphs wereapparently the first animals to lay amniotic eggs.
And again, if it's hard to preserve soft tissue in a fossil record, it's really hard to preservebehavior. So what basis do I have for telling that the Captorhinomorphs produced amniotic eggs?There is 2 lines of evidence to support that.
One of them is that all of the living descendants of the Captorhinomorphs produce anamniotic egg. The turtles, the lizards, the snakes, the crocodiles, the birds and the mammals allproduce an amniotic egg.
And remember there is 2 ways of interpreting the presence of a feature in one or moregroups or 2 or more groups of animals. Either they got it by inheriting it from an ancestor or theyall evolved it by convergent evolution. That would be a whopping chunk of convergent evolutionto have five or six major groups of animals all evolve exactly the same thing.
A much simpler interpretation, Ocam's razor says that all these animals have an amnioticegg because they inherited it. So they must have inherited it from either the Captorhinomorphs orsomebody who came before the Captorhinomorphs.
Why do we choose the Captorhinomorphs and not the Anthracosaurs? If all these animalshave amniotic eggs, and if they all inherited it from an ancestor, with that much information, canyou rule out the Anthracosaurs as the group that evolved the amniotic egg? Can you?
I mean, I told you it was the Captorhinomorphs, but could you?
STUDENT: Well, you said amphibians came before, so we know today that manyamphibians don't have an amniotic egg.
INSTRUCTOR: Well, that's true except that the modern amphibians are not descendedfrom the Anthracosaurs. Modern amphibians are descended from more primitive Labyrinthodonts.So somewhere along the Labyrinthodonts you have could given rise to modern amphibians, andthen the Anthracosaurs could have evolved the amniotic egg and given it to theCaptorhinomorphs, who then diverged into all the other groups. The fact is you cannot.
But there is a second line of evidence, and that is that the oldest fossilized amniotic eggscome from the same age rocks as Captorhinomorphs. The oldest fossilized amniotic eggs do notcome from the rocks that are as old as the Anthracosaurs, who are somewhat older in time.
Now, tomorrow some paleontologists might find a fossilized amniotic egg from rocks asold as the Labyrinthodonts, and then all of a sudden they get credit, because in the oldestfossilized record -- oldest fossilized egg was older than the Captorhinomorphs, then clearly theCaptorhinomorphs themselves didn't develop the thing. The logic of that is pretty clear.
There is 2 lines of evidence.
2 reasons for believing that the Captorhinomorphs evolved the amniotic egg. All livingdescendants have one, and the oldest eggs are the same age.
Now, some other interesting old reptiles, subclass Euryapsida. If you like to see the olddinosaur books, you know these table top books of the artist's renditions of what these fascinatinganimals looked like, you'll see pictures of these guys.
There are 4 different orders in the subclass Euryapsida. They are all marine. They are allfound -- they all lived in the ocean. They are all found in rocky strata that were deposited at thebottom of salty water, the oceans.
One of the types is shown here, called a plesiosaur. You'll see the pictures of these guyswith their long necks and fish between their teeth, swimming around in these Mesozoic oceans.
There are a couple of other groups that you'll also see, that I won't bother with, buthere's another group that I think is just a really fascinating group. This is also one of those 4orders of Euryapsida, these are the ichthyosaurs, translation is fish lizard.
And who does this look like? This looks like flipper. So these guys look very much like amodern dolphin. But a dolphin is a mammal. A dolphin is a mammal that descended from anothermammal that was a little mouse-like animal. And this guy is an aquatic and a marine reptile thatdescended from a little lizard-like guy, the Captorhinomorphs.
So dolphins and ichthyosaurs are a very good example of convergent evolution where thedescendants look more like one another than ancestors. Compare a lizard and a mouse, and thencompare the ichthyosaur with the dolphin. That's convergent evolution.
They have come to look alike because they were adapting to similar selected pressures.These guys are living in the ocean, feeding on fish, they have evolved a streamlined body, theyhave no neck, they have evolved flippers. They have evolved a dorsal fin like a fish, only this isnot a dorsal fin like a fish, this is an extension of structures out of that area.
They have a little vestigial hind limb, which dolphins don't have, and they have a tail. Soall of those things are adaptations, they are adaptations to the same set of selective pressures.You have to able to swim fast if you are going to catch, so your body becomes streamlined. Youneed flippers to control your movement through the water, and so that's convergent evolution.
Now, there is something interesting differences between the dolphins and the ichthyosaursas well. These guys have a vertical -- like a reverse hetero circle tail, right? They must have beenair-breathers. They didn't have gills. These are air-breathers just like dolphins are. So theyprobably were able to have neutral buoyancy.
But dolphins have a horizontal tail like a whale, that they beat up and down. We all knowthat. Whereas these guys have a vertical tail, that beat from side to side.
And it's interesting that, you know, that over the course of the last 60 or 70 yearsbiologists' appreciation of Darwinian evolution has waxed and weaned. It's increased, and then tosome extent it's even decreased. Whether natural selection is the mechanism of all of evolution is adebated issue. We're not debating whether evolution occurred; we're just debating mechanism.
But back in the 30s and 40s, biologists we're just totally in love with Darwinian evolution,which talks about adaptation. So there were old time zoologists and paleontologists who arguedthat this tail must be adaptively inferior to the horizontal tail of the dolphin because the dolphinsare still around, and these guys are extinct. And that's one of the dig differences between thegroups.
That's not an illogical argument. I would imagine that 30,000 species of fish were a littleworried about it. What we now know, and we have a better understanding of evolution, is thatyou can't -- that where you end up in an evolutionary sense, where you end up, is to some extent afunction of where you start.
Now, these guys are descended from little terrestrial lizard-like animals who still have thatside to side wiggle of the body. Think about a how a fish swims. Think about a lizard looks whenit walks. Think about how a crocodile looks when it walks. You take a lizard and put it in thewater, and its body is designed, its muscles are set up for wiggling its tail from side to side. Andso it evolved a vertical tail that it beats from side to side.
On the other hand, think about how a cat gallops or a dog. Mammals have taken theselimbs and they have rotated them underneath the body. And the muscles that used to wiggle itbody from side to side now flex the spine vertically. That dog galloping is flexing its spinevertically. A mouse galloping flexes its spine vertically.
And you take an animal that flexes its spine vertically, and you put it into the water and itdevelops a horizontal tail that it beats up and down. And, in fact, people have made mechanicalmodels and studied the hydrodynamics a vertical tail and a horizontal and there is no difference.They have the same hydrodynamic efficiency.
You can generate the same amount of thrust and energy input with a vertical tail as with ahorizontal. When you stop and think about it that's kind of intuitively obvious.
So this difference between the two groups of animals is an important manifest of thatgeneralization that in an evolutionary sense where you end up is -- to some extent you may bepredisposed to end up in one place because of where you started. If you start off with a sidewayswiggle you end up with a vertical tail. If you start off with an up and down wiggle, you end upwith horizontal tail.
Also notice these guys have a big eye, a much bigger eye than a dolphin. The reason forthat is that dolphins have evolved a system called "echolocation." It's like sonar. They use soundthey send out pulses of sound and they hear the echoes.
Echolocation is a very important sensory system in dolphins that depends upon the senseof hearing. And so dolphins don't have as big an eye. These guys have a huge eye. Obviously, theeye was a very important means of learning about the environment. And these animals, theyprobably did not have echolocation.
So those are the Euryapsids.
Now, we're getting into the group that is in the subclass Archosauria. And this group, theThecodonts are the most primitive group within the subclass Archosauria.
I think I forgot to give you some information on the Euryapsida. Their ancestors with theCaptorhinomorphs. They have no descendants. And their first fossils are from the Permian.
Now we are looking at the order Thecodontia.
Their ancestors were the Captorhinomorphs. Their descendants are all the otherArchosaurs. If you look at the classification, there are pterosaurs, and 2 groups of dinosaurs, andthe crocodiles. They are all descended from the Thecodontia.
The first fossil Thecodontia are known from the Triassic. And they were basically sort oflarge lizard-like animals, but they have this one interesting thing is that they were bipedal. Theytended to stand on their hind legs. And what we will see when we look at all the descendentgroups, there is -- in many cases there is some remnant of this bipedal ancestor. Once again,where you start from tends to predispose where you end up.
It may not absolutely determine it, but it frequently carries on.
Those are the Thecodonts. More interesting from the standpoint of who they gave rise tothan who they were.
One of more interesting groups of Archosaurs that came from the Thecodonts is the orderPterosauria. They have no living descendants. They are not the ancestors of the birds, eventhough they were able to fly. They were contemporary with the first birds, but much more diverse.They appear in the Jurassic, same time as the first bird fossils. But they were a very diverse group.
But they went extinct at the end of Cretaceous at the same time the dinosaurs did. Andthey were very diverse in terms of sizes. And there is a thing up on the wall in the lab that showssome pictures of what different kinds of pterosaurs looked like.
I think the most amazing thing about these guys is that this wing, which is a membrane --I mean, it's like 2 layers of skin. It's a like a web of skin between your fingers or your toes. It'sjust a double layer of skin, a membranous wing. This is this guy's hand. That's his hand.
The distal path of the entire length of the wing is supported by one finger. This is next tothe last finger. Think about that. Think about if your forefinger there were as long as the wholerest of your arm, and you had this huge web of skin that extended from the end of that finger allthe way back along the entire side of your body and down your leg.
That would be a pretty strong finger to support all that weight, because the guy is goingto flap that wing up and down and fly through the air.
They range in size from something about the size of a sparrow. Wouldn't that be cool tohave a little sort of sparrow pterosaur that you keep in your pocket and fly around the room.
They get to be pretty big. But there is a lot of interesting arguments about how big theygot to be. Probably they got to big as big as the biggest birds are today. The biggest birds that canfly, like big vultures and hawks and owls and eagles. That's probably about as big as they got.
There is one pterosaur that's known only from the fossilized humorous. And there wasone scientist who sort of made a big name that ended up being a bad name for himself, becausesince he only had was the humorous of this huge ichthyosaur, he looked at some fossils where hehad the entire fossil and he knew exactly what the wingspan for these little guy. And he measuredthe ratio of the total wingspan and the length of the humorous, and then he applied that same ratioto the humorous that he had.
And he calculated that these things were about these size of piper-cub airplane. Hereported a pterosaur with 26-foot wingspan. And the only problem is that no bird could fly if itwas that big. The aerodynamics -- the cost of flight goes up so fast that you can't generate enoughpower if you are that big.
So another guy went back and he looked at the relationship between wingspan andhumorous length in a bunch of different sized pterosaurs, and he found out that that ratio is notconstant and it changes dramatically with size. So that in really big pterosaurs, the humorous is alarger fraction of the total wingspan than it is in the little tiny pterosaurs, and that's why he madethe mistake.
So you have to be careful about what you do with your data. You can really make a badname for yourself if you aren't careful.
Now, the 2 groups of dinosaurs are the Saurischia and the Ornithischia, and I don't wantto spend a lot of time developing this. There are some differences in the arrangement of the pelvicgirdle, which are shown -- you have a slightly different picture in your Illustrated Notes.
The Ornithischian pelvis should look to you like -- I guess you haven't looked at the birdskeleton. When you look at the bird skeleton, what you will see is that in a bird skeleton the pubislies right next to the ischium and looks like this. This part down here looks like a bird skeletonwith the ischium -- with the pubis lying next to the ischium.
So this is a bird-like pelvis, the Ornithischian pelvis, only really with respect to that. TheSaurischian pelvis is a lizard-like pelvis. In that one the pubis goes forward. That's the differencebetween the two.
And so dinosaurs that have an Ornithischian pelvis are placed in the order Ornithischiaand the dinosaurs that have a pubis going forward are placed in the order Saurischia.
The only thing I want to alert you to here is birds evolved from Saurischian dinosaurs. Sothere is an example of convergent evolution in a sense. The birds obviously have a bird-like pelvis,but they did not inherit it from the Ornithischian dinosaurs. They modified it from theSaurischian dinosaurs from whom they evolved.
So among the dinosaurs we have the 2 major orders, one shown at the bottom of yourpage here is Ornithischia. Their ancestors were the Thecodonts. They have no living descendants.The first fossils are found in the Triassic. And all of the Ornithischian dinosaurs were herbivores,plant eaters.
One of the types that you may be familiar with if you have ever paid any attention todinosaurs is shown here, this is stegosaur. The guy with the spikes on his tail and big plates on hisback.
If you look at the kiddy dinosaur books, you'll probably see the idea that this guy hadthese plates for defense. And there is these traditional pictures that show the guy with his platesfighting against the tyrannosaurus rex and the plates are his armor.
The problem is that when you look at fossilized plates, what you see is an extensiveradiating partner of grooves that looks like blood vessels.
These plates were almost certainly highly vascularized. They had an extensive bloodsupply. And you do not put an extensive blood supply in your armor. You put an extensive bloodsupply in your radiator. These plates were heat exchange surfaces, they were not armor.
And the subclass Archosauria, the other order that is -- technically should be calleddinosaurs, is the order Saurischia. Their ancestors were the Thecodonts, and their descendants arethe birds. First fossils Saurischians are from the Triassic. And there are several major groups inhere. A couple of lines of herbivores, like at the guy shown at the bottom of the page, whichincludes the largest terrestrial vertebrates to have ever lived were these very, very large dinosaurs.
This guy is technically known as a Apatosaurus. In some of the kiddy dinosaur books hewas called Brontosaurus. That's another good example of a scientist that made a mistake. Thisguy found a huge fossil skeleton. He got a big grant, and spent a couple of summers out in theblazing heat in Wyoming digging up this fossil, when he got done he didn't have a skull.
And they were sending all these bones back to Chicago to make this huge impressivedisplay in the museum and had to come up with a skull. So he found a skull that was about theright size and shape in the same fossil deposit. He dug it out and sent it back. And he put it onthere and called it Brontosaurus. And then a couple of decades later, somebody found an intactBrontosaurus and it had different skull on it.
And turned out that it was a skull that had been previously described as Apatosaurus, butit was only described as a skull when it was first found.
So take your chances when you fudge the data. And so Brontosaurus is defunct scientificname.
Now, the guy in the middle is technically a Carnosaur, same group as tyrannosaurus rex.But I did not want to use Tyrannosaurus rex, because I wanted you to know that there were awhole bunch of carnivorous huge ferocious bipedal tyrannosaurus types of animals.
And this Carnosaur is a relative of tyrannosaurus rex. And they were big carnivores. Theyhad these little vestigial front limbs. At least, we assumed they were vestigial. There has beensome calculations recently that these little tiny wimpy arms could hold 500 kilograms of meat.That's not a vestigial limb anymore if it can hold that.
In fact, it's very interesting, why would a big carnivore like that want to carry around 500kilograms of meat? Well, maybe because she has some offspring back somewhere else that she'staken it back to feed. And there is accumulating evidence that animals have complicated,sophisticated social systems, traveled in herds, took care of their offspring.
There are even arguments that they have been warm blooded and had high metabolic ratesand high body temperatures. Although, that's very highly controversial and there is not aconsensus of opinion on that in spite of Lucas and things that people say about those.
Did you see Jurassic Park? There is not a consensus among scientists about all that stuffbeing true about them being warm-blooded.
But anyway, they all went extinct, and there is all kinds of arguments about why theywent extinct. Probably the best current theory is that there was a meteor that struck the earth andput up such a huge cloud of dust that it caused winter-like conditions to last for a couple of yearsand caused massive extinction of terrestrial and aquatic animals.