Today I'm going to be talking about the classification and characteristicsof an extensive group of reptiles that are all extinct and thismaterial is in your illustrated notes.
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CLASSIFICATION & CHARACTERISTICS OF REPTILES - As youcan see the Class Reptilia is divided into 5 subclasses: Anapsida, Synapsida, Euryapsida, Archosauria, and Lepidosauria.
One of the distinguishing features of these subclasses isthe condition of the openings in the side of the head. Theseare called the temporal fenestrae and there are 4 conditionsthat are found in reptiles. The primitive one is the Anapsidcondition - the one in which there is no opening. The bones thatform the temporal region of the skull in these animals are thepostorbital, squamosal, and jugal and from that initial primitiveAnapsid condition each of the other 3 appear to have evolved probablyindependently. Another one of these is the Synapsid conditionin which the opening is a little bit low on the side of the skull.Postorbital and squamosal meeting above it. The Euryapsid conditionis where the single opening is up high on the side of the skullwith the postorbital and squamosal meeting below. And the 4thcondition is one in which there are 2 openings. This is calledthe Diapsid condition and is found in 2 subclasses. The SubclassArchosauria and the Subclass Lepidosauria.
Many of these animals are extinct, that's what the littleplus out in front of the name of the group indicates. The turtlesare found in the Subclass Anapsida. The crocodiles are in theSubclass Archosauria and the rest of the living reptiles, thelizards and the snakes, are found in the Subclass Lepidosauria.There are other distinctive features which are used to distinguishbetween the Archosaurs and the Lepidosaurs that have a Diapsidcondition.
Remember again there is a general trend towards listing more primitivegroups first so the stem reptiles, the ones that form the baseof the family tree of the reptiles, are placed in the OrderCaptorhinomorpha within the Subclass Anapsida. Within theSubclass Archosauria, the most primitive group is the OrderThecodontia and they are the ancestors of all the others. The most advanced reptiles are the lizards and probably the snakesand so they are at the bottom. But the exact phylogenetic relationshipsagain are not communicated by this kind of written classification. What you're going to encounter very frequently is the graphicalrepresentation of the relationships between these animals. Oneof the major evolutionary advances made by the reptiles was theevolution of the amniotic egg and so the reptiles and their descendantgroups (the birds and mammals) are collectively referred to asthe Amniotes because they have this basic kind of egg structure.
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So this is a graphical representation of the phylogeny or theancestor/descendant relationships of the amniotes. You will seethese kinds of things used to represent the phylogeny of manydifferent groups of animals and then there are some conventionsabout this. For one thing, the time scale starts with the oldestat the bottom because that's the way the geological strata arediscovered. Groups that are thought to be descendant from othergroups. Each group is represented by one of these balloon-typestructures. Connection between this balloon for Pelycosauriato this balloon for Captorhinomorpha is the graphicalrepresentation of the idea that the Pelycosaurs are the descendantsof the Captorhinomorphs. The Pelycosaurs are the ancestors ofthe Therapsids that's why they are connected.
The place along the time scale that the connection is made isalso indicative of when they think that branching occurred. Thebottom of any given balloon represents when the oldest fossilsfrom that group are known, so the oldest fossil Thecodontsare from the very beginning of the TRIASSIC. The oldestfossil Captorinomorphs are from the upper part of the CARBONIFEROUS. The Pelycosaurs are thought to have come off from theCaptorhinomorpha somewhere around the upper CARBONIFEROUS. The dotted lines then represents the paleontologist's best guessabout who the ancestors were and roughly when they might havebranched off. This kind of graphical representation allows usto see some things. The birds for example are thought to be descendantfrom one of these 2 major groups of dinosaurs. And another conventionabout these little balloons is that the width of the balloon isa graphical representation of the diversity of the group. Thenumber of different types of fossils that are known. For example,if you look at the story of the mammals with this real thin balloon,you get up here to the end of the CRETACEOUS and the balloon forthe mammals suddenly gets very broad. That's where the end ofthe balloon for the dinosaurs is. So this is the famous extinctionof the dinosaurs, at the end of the CRETACEOUS. The balloonsfor Ornithischia, Saurischia, Pterosauria, and Euryapsida allstop there so that's when they all went extinct and it's onlyafter they go extinct that the bird and mammal balloons increasein size. So that's where suddenly there is what we call an adaptiveradiation. The sudden and dramatic increase in the diversityof a group of animals.
We can see who the ancestors and the descendants were but we canalso draw some conclusions about who the closest living relativesof a particular group of animals are. For example, knowing thatthe lizards and the snakes are in this group and the turtles arein this group, we could conclude that the lizards and the snakesare the closest living relatives of the turtles because of thefact that they all trace back to a common ancestor. And we couldconclude that the most common ancestor of the birds and mammalsis the Captorhinomorpha.
Represented in this graph are also some things that are littlebit less obvious. For example, who are the closest living relativesof the crocodiles according to this diagram? They are the birds. Not the other reptiles; lizards, snakes and turtles. The birdsare the closest living relatives of the crocodiles. Who is themost recent common ancestor of the crocodiles and the birds? That's the Thecodonts. So the crocodiles are more closely relatedto the birds than they are to the lizards.
I might ask a midterm question which essentially requires youto fill in the names on this kind of a balloon diagram. I wouldn'trequire you to produce the diagram but I could give you a midtermpage with this diagram on it with all these names missing, andyou would have to fill in the names. So you need to know theancestor descendant relationships of these animals.
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So the group that we need to start with then are the Order Captorhinomorpha. The stem reptiles. In the Subclass Anapsida we havethe Order Captorhinomorpha. Their ancestors are a group of extinctamphibians. The ancestors of the Captorhinomorpha are a groupof animals called Labyrinthodonts which are the major group ofextinct amphibians. In particular, a group of Labyrinthodontsthat are called Anthracosaurs.
The Anthracosaurs themselves were very reptile-like in many aspectsof their anatomy. They are very similar to the Captorhinomorphathat's shown on the page here. In fact, the translation of theword Anthracosaurs is "Coal Lizard". The fossils ofthese animals are found in coal deposits. Major deposits of coalin the world were laid down during the CARBONIFEROUS which isthe period of time when the reptiles appeared. The first fossilsof the Captorhinomorpha are found in the Carboniferous and soare the fossils of these Labyrinthodonts that are their ancestors. Pretty lizard-like critters probably feeding on insects thatwere quite abundant in the terrestrial environment at the time.
The descendants of the Captorhinomorphs are all the other 4 subclassesof reptiles. Two major advances shown by the Captorhinomorphsare: l) The amniotic egg and 2) Some improvement in the jawmechanism. The change in the jaw mechanism is shown here in thediagram. There is a downward extension on the bone called thePterygoid bone. The function of that Pterygoid flangeis to provide a place of attachment of jaw muscle. The jaw musclewhich is capable of generating it's maximum force when the jawis almost closed.
The difference between the jaws of the amphibians (Anthracosaurs)who did not have a pterygoid flange and the structure of the jawin these first reptiles probably is based upon the fact that theamphibians could close their mouth very rapidly. The musclesappear to have been attached in places that will allow them toclose their mouth very rapidly. Which is what you need to doif you're going to catch an insect. Studies of the fossils ofthe insects from the period indicate that during the early Carboniferousand prior to that, the insects were evolving a thicker and heavierexoskeleton, probably as a defense mechanism against these amphibiansthat were trying to feed on them. And so the pterygoid flangeevolved as a site for the attachment of a muscle that could generatea maximum force when the mouth was almost closed; which is whatthe reptile would need to be able to crunch through that exoskeletonof the insect. So what we have here is what we would call convergent-evolution. Sort of an arms race between the insects that are the prey speciesand these insect-eating vertebrates that are the predators. Theevolution of the pterygoid flange was the advancement that thereptiles made in order to overcome the defensive mechanism ofthe heavy exoskeleton of the insects.
So the pterygoid flange was the advancement of the reptiles. The pterygoid flange is used to distinguish reptiles from the amphibians. Other than that, the animals looked very similarto one another. We do not have any direct fossil evidence ofwhether a particular fossil animal was laying an amniotic eggwhich would indicate the animal to be a reptile or whether itwas laying a non-amniotic egg which would indicate the animalwas an amphibian. And it is not uncommon where we see major biologicaldifferences between a couple of living groups that's not representedin the fossil record. And the paleontologists are forced to identifysome relatively obscure characteristic of their anatomy that maynot have very much at all to do with the big biological differencesbut which is something that they can see in the fossil.
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The Subclass Euryapsida is composed of 4 groups of marine reptiles. Their ancestors, of course, were the Captorhinomorphs. Theyall went extinct at the end of the Cretaceous so there are nodescendants of the Euryapsids. The first fossil of the Euryapsidsare found in the Permian. There are 4 different orders of marineanimals. One of the them shown here is known as the Plesiosaur. This was a large animal, 30 feet long, probably feeding on fish. It's hard to say whether these animals were able to get out ofthe water. They do have a fairly large paddle-like structure. Couple of the other types are also ones that might have beenable to get out of the water. Another type of Euryapsid reptileshown here is called the Ichthyosaur. This animal almost certainlycould not get out of the water. And one of the reasons that that'san interesting thing is that among living amniotes, none of themlay their eggs in water. All modern amniotic eggs are laid onland. Even sea-turtles for example, have to crawl up on the beachin order to lay their eggs. And assuming that this animal wasincapable of laying it's eggs in the water leads to the conclusionthat this animal must have been viviparous.
What do they look like? They look like dolphins and theyare considered by some to be a good example of what's called ConvergentEvolution. Convergent evolution is a situation inwhich 2 unrelated groups of animals come to look more like eachother than their ancestors did as a result of adapting to a similarset of conditions.
These guys were almost certainly fish eaters. They have kindof a long narrow beak that you see in many kinds of fish eatingmodern reptiles. They have modified their front limbs and theirhind limbs into little flippers. They have a little dorsal finthat's made up from an extension of neural spines.
What are the differences between Ichthyosaurs and dolphins basedon what you could see here? The tail. This guy has a verticaltail whereas the mammal has a horizontal tail. Studies show bothtails work well for the animals. So one tail evolves not becauseit's better but because it starts in a different place in an evolutionarysense. And I think that's an important general thing to rememberabout evolution is that where it ends up to some extent is goingto be determined by where it starts. Another difference betweenthem is that the Ichthyosaur has a very large eye and dolphinshave relatively small eyes. Dolphins have a very important sensorymechanism called echolocation which is like sonar which bats have.
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Now the Subclass Archosauria is the one that includes theanimals that are properly called dinosaurs although you'll seepictures of the other Euryapsid type animals in the dinosaur books.Technically, the ones that are really dinosaurs belong in theSubclass Archosauria.
The most primitive group of Archosaurs is the Order Thecodontiashown here. Their ancestors were the Captorhinomorphs and theirdescendants are all the other groups of Archosaurs.
The first Thecodonts appear in the Triassic and as indicated inthis rendition, many of these animals appear to have been bipedalwhich means that they stood up on their hind legs running aroundon 2's. And signs of that bipedalism will be seen in many oftheir descendant groups. These animals did not get to be terriblylarge but some of their descendants are very big.
One of the more interesting groups of Archosaurs to descend uponthe Thecodonts is the Order Pterosauria. Animals like the Pterosaurare a very readily recognizable and identifiable group of animals. Their ancestors were the Thecodonts. They have no living descendants. These animals are not the ancestors of birds or bats even thoughthey were able to fly.
The first fossil Pterosaurs are known from the Jurassic whichis about the same geological period from which we find the firstfossil birds. It's possible that these animals may even haveevolved after the birds evolved.
The smallest Pterosaurs that are known from the fossil recordare about the size of a sparrow, a little bit smaller than a robin. The largest probably was about 15 feet.
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The dinosaurs are placed in 2 different orders and the basicdifference between these 2 orders has to do with the structureof the pelvis; the pelvic girdle.
The primitive reptilian Thecodont type of pelvis from which theseanimals evolved is shown here in the middle. There are 3 centersof ossification. You learned these names in looking at the mammalianpelvis in lab, the pubis - which is anterior and ventral, ischium- which is posterior and ventral, and the ilium - which is upabove.
The 2 types of pelvis that evolved are referred to as the OrnithischianPelvis which means the bird-like pelvis and a Saurischian Pelviswhich means the reptile-like pelvis. Now if you think about whatthe pelvic girdle of a bird looks like, you'll recognize thatthey have a big synsacrum and the ischium runs along parallelto that synsacrum forming a large boundary. Then the pubis isa little tiny thin bone that's right on the front of that ischium. So this is definitely a bird-like structure. The major differencebetween the 2 pelvis' is in the orientation of the pubis bonebecause in the Saurischian pelvis, the pubis goes forward andin the Ornithischian pelvis, the pubis goes backwards. These2 different pelvic structures appear to be just different solutionsto the same basic problem. In both of the groups of dinosaurswhat happened was 1) they went from having a sprawling reptiliantype of gait with their hind limbs out to the side like an alligatoror crocodile having hind limbs oriented vertically. This is associatedwith their tremendous increase in size. It's probably necessaryif you're going to be really big to have your limbs oriented verticallybecause vertebrate bone is pretty strong in compression. It'smuch stronger in compression when you apply force on the end ofit than it is when you're applying force on either end. If theanimal has a sprawling gait, then the femur is going to be orientedhorizontally. If you orient the leg vertically, you have compressionalforces applied to that bone and it's much less likely to break. So these guys were evolving an erect posture with verticallyoriented leg bones and in order to be able to run you have tohave a new place to attach the muscles that are responsible forpulling the leg forward and backwards. And that's what you couldsee in your illustrated notes is the orientation of the femurand the sites of attachment for the muscles that pull it forwardand backwards.
In the Ornithischian Pelvis the femur is here and there are musclesattached to this forward extending ilium to pull it forward. There are muscles attached to the pubis that can pull it backwards. Whereas in the Saurischian Pelvis the femur is right here andthe muscles that pull it forward are attached to the pubis andfor the backward extension, it is attached to the ischium. Sothere are 2 different solutions to the same problem. Just differentways of going about doing it.
The first order of the dinosaurs that I'll talk about is a fairlyhomogeneous group. This is the Order Ornithischia. Their ancestorsare the Thecodonts. They have no living descendants. The firstfossil Ornithischians are found later in the Triassic so theybranched off fairly early from the Thecodonts and all of theseanimals were herbivores - plant eater types. A variety of differentanimals that you may or may not be familiar with is the Stegosaurthat's shown here. In the Stegosaurs, you can see that this guyhas much larger hind limbs than his front limbs and that seemsto reflect the bipedalism that was found in the Thecodonts. Speculationis that these guys probably could rear up on their hind legs inorder to feed on plant material that was up off the ground. Theplates on the Stegosaurs was not for defense, it acted as a radiator. They have branching grooves on the surfaces that look like branchingblood vessels. We don't know if it was used mostly to absorbheat and warm up this huge massive body. I mean Stegosaurs are18 feet long and it takes a long time to warm up that much meat. Robert Backer would have us believe, these guys were all endothermichot-blooded dinosaurs and they needed these plates to get ridof their heat. It could have actually been used for both functions. If the animal wanted to warm up, it can turn sideways to thesun. Due to the large surface area, it could soak up lots ofheat. That way if it wants to cool, it faces into the sun andradiates the heat out.
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The other order in the class of dinosaurs is the Order Saurischia. These are the ones with the reptile-like pelvis. They are alsodescendant from the Thecodonts. But they do have a descendantgroup being the birds which we place in a separate class, theClass Aves. There are people that would argue with that classification.First fossil Saurischians are from the Triassic and looks likethe guy in the upper left hand corner of the page. This is theCoelurosaur. Only about 9 feet tall. Strongly bipedal. Theorder has 2 major subdivisions within it. The 2 were the verylarge carnivores of which Tyrannosaurus rex is the best knownexample. But this Carnosaur is another example. They use theirteeth as their primary weapons. Huge heads, huge 6 inch longknife-like teeth. Pretty small looking front limbs on this animal. Studies suggest that in fact these front limbs may have beenpretty strong, they may have been able to hold up to a coupleof hundred kilograms of weight with these little tiny front limbs.
The other major group of Saurischian dinosaurs were the herbivoresand the Apatosaurus (Brontosaurus) is an example of one of theseanimal. These were really giant animals. In the neighborhoodof 20,000 to 50,000 kilograms compare that with an elephant thatweighs 5,000 kilograms. The longest one is 30 meters long - that'sa 100 foot long animal.