ZOO 138, Monday, February 17, 1997, 12:00 p.m.
Today I'm going to talk about the evolutionary trends in thesynapsida. It's the last subclass in the reptiles and one thatis particularly interesting to us because it is the group of reptilesthat evolved into the mammals.
The paleontologists who have looked at the fossils of this groupof animals has concocted a very reasonable, I think, synthesisof the ways in which -- or the factors that caused the animalsto undergo this set of evolutionary changes. And these are coveredin your Illustrated Notes.
It starts with a picture of a pelycosauria. This is the firstpage of the lecture notes on the evolutionary trends of the synapsida.
I'll give you information about the subclass of the synapsida,and these 2 orders, and showing how they go together and how paleontologistshave put them together in a coherent logical sequence of events.
Now, part of what you might want to consider doing for the lectureis to have a separate sheet of paper where you're going to sortof develop a diagram that looks like this. It's a flow diagram,a connection between evolutionary trends and how they relate toone another.
You might want to run that in parallel with the notes that youare taking. You can put on those your Illustrated Notes, if youare using those the way I intended for you to use them.
So we're really trying to understand it -- what caused a groupof initially very, very successful, very diverse, dominant groupof reptiles to be transformed into mammals. But not only is itimportant to understand that in that transformation they justbarely managed to avoid going extinct. And the very first mammalswere very small, not terribly diverse little animals, that livedon insects instead of seeds.
And those initial mammals were around for a very long time,during most of the Mesozoic when Archosaurs and all the dinosaursand the other dominant terrestrial reptiles were really rulingthe planet. And these group of mammals was very insignificant.If it hadn't have been for extinction of the dinosaurs at theend of Cretaceous, the mammals probably never would have amountedto anything important at all.
So this transformation from being a fairly large and successfulreptile into a small insignificant primitive mammal was somethingthat came about because of the fact that these animals were reallylosing out in the competition to the Archosaurs. And we don'tknow why.
But that's all we can say. These animals started to diminishin terms of diversity, and then through the sequence of evolutionarychanges that I'll be telling you about, they were transformedinto these mammals which ended up being important to us in hindsight.
So the first group of animals I'll talk about is the order "pelycosauria."This gets into -- the suborder includes synapsida. This is theorder that arose from the captorhinomorphs. Its ancestors werethe captorhinomorphs; its descendants were the Therapsids.
The first fossils of the pelycosaurs are found in the mid-carboniferous,which is shortly after the origin of the reptiles. And they werea very, very successful group. Remember the geological group thatfollows carboniferous is the Permian. In the early Permian, 80%of the all known reptile fossils, all the genre of reptile fossils,are a member of the pelycosauria.
So reptiles appear in the carboniferous. Shortly after the firstreptiles appear, we get the very first pelycosaurs. By the midcarboniferous, by the beginning of the next geological period,they are the dominant terrestrial animal, really not only themost numerous and diverse, but really the most numerous of allthe terrestrial animals.
There are several different lines of evolution. Several differentgroups within this order. I won't bother with the details. Therewere a couple of these groups that were herbivores. So there areseveral different lines of herbivores within the pelycosauria,and one group which is particularly important to us are the onesthat were the predators, the carnivores.
And they are the ones that evolved into the Therapsida. Thisis a group called the "sphenacodontia." It's a suborder.You don't need to worry about what level of classification itis. But the sphenacodonts were the group of pelycosaurs that evolvedinto the Therapsids, an animal that you have in the IllustratedNotes there which is the one that may be familiar to you.
If you have looked at very many books about Dinosaurs, it isplaced in the genus Dimetrodon.
And this animal is a sphenacodont. It's several meters in length,and it had this really bizarre set of very long neural spines,that that's the top part of the vertebrate, you know, above theneural arch, that projected way out of the animal's body.
These guys are sometimes called sail lizards. One of the littleDinosaur books that my kids used to look at actually showed onesailing off across the pond as they were actually using this structureas a form of locomotion, which is really quite imaginative.
It turns out that this thing was radiator. There is evidence-- upon close examination of the fossils there is evidence ofa very extensive blood supply to this structure. And it was beingused either to soak up or to give off heat, and we don't knowfor sure. The common assumption that is since this is a reptile,that they are ectotherms, that they might have been using thisto increase their surface area available, so they could soak upenergy from the sun.
They are basking. They were a day-active basking type of animalthat sat out in the sun. The term we use for an animal that spendsa lot of time sitting in the sun soaking the sun's energy, it'sreally just sort of a subset ectotherm, is heliotherm. Heliosis the Greek term for the sun. Sun bathers. That's the interpretationof the sails, that these guys had evolved this sail-like structureto increase the surface area to soak up the sun's energy in orderto maintain a high and fairly stable body temperature. They werebehavioral thermo regulators.
So they were very successful early on, and they gave rise inthe next group, which is order Therpasida. By the late Permian,early pelycosaurs -- the late Permian, at least, according toone of the references that I saw, 84% of all reptilian genre werein the Therapsida. So the pelycosaurs were very rapidly replacedby their own descendants.
Now, if you look in the Illustrated Notes under the Therapsida,you a have phylogram, a family tree down there in the bottom left-handcorner, and you can see that I have whited-out the names of allthose groups because one time I had student who went about memorizingnames of all the little balloons, and I don't want anybody todo that.
Each of those little balloons represents a different familyof Therapsids. And only thing I want you to get out of that diagramis just the idea that there was a tremendous diversity of Therapsidsin evolution, and then at the end of the Permian they underwenta very rapid contraction, a lot of little balloons are all closedoff, and so there was a lot extinction that took place that, diversityof Therapsids was reduced.
And if you remember that the Triassic period had all the othergroups of Mesozoic reptiles, suddenly the appear the Thecodontsand all the different types of dinosaurs. And so at the same timethat the Therapsids are undergoing this tremendous decrease, theArchosaurs are undergoing an increase in their diversity. Andwhy that occurred, I haven't seen any good explanations for it.
It appears to be the case for some reason, the reason the Archosaurswere out competing with the Therapsids and causing a tremendousreduction in their numbers and in diversity. And it's really inresponse to that defeat that the Therapsids evolved into mammals.
Now, the big story, the big picture is basically the following,due to this loss of competition with the Archosaurs, the Therapsids,or at least some small group of Therapsids, started making a seriesof adaptations to become nocturnal, to become night-active animals.
Remember Dimetrodon has a big sail. There's no point in havinga great big radiator if you are a night-active animal. So it islosing out in terms of being a diurnal or day-active animal. Thatcaused a small group of Therapsids to expand their activity intothe nighttime.
When they can no longer use the sun or behavioral thermo regulationis not an option, that sets up a whole set of series of pressuresthat lead to the evolution of physiological temperature regulationthat caused animals to become endotherms, to change from beingsectotherms, sun basking to becoming endotherms. And remember thatthe difference, the biggest single difference between ectothermand endotherm is endotherm has a 7 times higher metabolic rate.
So if you are going to have a higher metabolic rate in orderto generate enough heat so that that heat can be your primarysource of heat, you have to have physiological temperature regulation.Then there are whole series of other things that you have to do.You can't just have a higher metabolic rate, you can't just burn7 times as much fuel.
If you are going to burn 7 times as much fuel -- if you haveto assimilate out of your digestive tract 7 times as much fuel-- you can't just assimilate 7 times as much fuel, you have tocatch 7 times as much fuel.
So these animals had to start catching a lot more food. Theyhad be to able to digest that food rapidly, and then they hadundergo a whole series of other changes so they could metabolizethat food rapidly.
And then that amount of heat enables that to be the primarysource of heat so they became endotherms, and they can have aconstant body temperature at night when the sun is not out touse as in the daytime. So that's the big picture.
Now, one of the keys is this business of digesting the foodso much more rapidly. In order to digest food more rapidly itturned out that mammals had to evolve the ability to chew thefood. That's called "mastication." It's the technicalterm for chewing food.
Now, why chew the food? Why does an animal -- how does thatcontribute to the evolution of endothermy? It contributes becauseit takes this big chunk of food that a reptile would have swallowedwhole and breaks it down into smaller pieces and increases thesurface area available for the attack by digestive enzymes.
Now, if you think about it, if you I don't know -- if you haveever seen a fish or a reptile swallow a big chunk of food, theyswallow it down. All the teeth is just to hold onto it. They swallowit whole. So if you have a big bite of food -- you know, for thesake of simplicity it's a round bite of food. If you swallow thatwhole, the only area the digestive enzymes can get at this, isjust the outside the surface of that ball of food.
If you just bite that thing into 4 pieces. You have the samemass of food, but now you have tremendously more surface area.All this surface area has been added for the attack by the digestiveenzymes. If you bite that into 16 pieces you tremendously increasethe surface area, the total mass of food is the same but the surfacearea is tremendously increased.
And, therefore, the process of digestion, which starts by splittingup large food molecules into small ones is tremendously increased.We're going to see this innocuous act of chewing the food whichyou may not have thought of being the key to your very existenceas a mammal. It was one of the fundamental things that led tochanges that we see in mammals. There are others as well.
So we can go through this, look at the series of evolution characteristicsthat the paleontologist in particular have focused on as beingcharacteristics of mammals, and we can see where these things-- how these things relate to this business of becoming -- evolvinga higher metabolic rate.
One of the revolutionary trends is shown right here, and it'sone that is, again, it's an anatomical feature that the paleontologistsused to distinguish a mammal from a reptile, and that is the presenceof something called a zygomatic arch.
This is zygomatic arch is shown on the page here, it's thislittle -- what you have in sequence is going from a primitivepelycosaur through the primitive Therapsid to the mammal, thislittle structure right here, this little bar behind the eye. Youcan feel it on the side of your own head. If you go above yourjaw that little bone right there on the side is your zygomaticarch.
That is something that was found in mammals that is not inany other group. Why did this thing evolve? Where did come from?The result is from the enlargement of the temporal fenestrae.
Remember the temporal fenestrae or openings on the side of thehead that distinguish the different subclasses of reptiles? Thesynapsida have one that is low, on the side of the head. And thatthing enlarges, and as it enlarges it leaves behind just thislittle tiny arch of bone at the bottom, which we called the zygomaticarch.
So that is produced by the enlargement of the temporal fenestrae.Why did the temporal fenestrae enlarge? The best explanation isthat the muscles that are responsible for mastication are goingto be bulging through that opening in the temporal fenestrae.
If you put your fingers right above your zygomatic arch, andpretend you are chewing, you can feel the muscles of masticationthat are attached there. The enlargement of the temporal fenestraeis due to the evolution of mastication in a somewhat indirectmanner.
If we're going to try to develop this overall picture of theconnections between these different terms, mastication ends upbeing a very important one.
Remember, mastication leads to an increase -- that's what thelittle up arrow means, it leads to the increase in the rate ofdigestion. And that allows an animal to have or to satisfythe needs for higher energy demands. And that allows an animalto be an endotherm. And what I'm suggest is that something evolvedbecause they were becoming nocturnal.
And so the zygomatic arch relates to this whole evolutionarysequence because it is connected to the idea of mastication, it'spart of the evolution of mastication.
Another evolutionary trend that we see when we compare the skullsof these animals is in the complexity, diversity of the shapesof the teeth. If we look here at the bottom of the pelycosaur,you can see they have some simple peg-like teeth. They have onesort of big sharp one somewhat larger than others on either side.
By the time you get up the mammals, it isn't shown in the picturevery well, you have looked at a cat skull, you can see in yourown mouth, mammals have a very diverse assortment of teeth. Youhave incisors up at the front. You have little simple sort offlat shovel-like teeth that can used to bite things. You havecanines which are big sharp puncturing teeth, they can used grabonto a prey. You have premolars and molars along the side whichare used for grinding.
And that's the diversity of the tooth structure that we seein mammals, and there is nothing like that in other any othervertebrate animals. They have simple peg-like teeth, small versionsof your canine, but not the big grinding surface areas and sharpshearing surfaces. So that particular change is from the reptiliancondition, which is called "homodonty."
Homodonty is "same teeth." And the mammalian condition,which is called "heterodonty." And the translation ofthat is "different teeth." The presence of incisors,canines, premolars, molars, that's heterodonty. And heterodontyevolved because of mastication.
In other words, if you are going to chew the food, you needhave to teeth that are not only all just simple peg-like structures,you need to have some more that are for shearing and grinding.So in terms of relating to this other sequence, we can list heterodontyas relating to the evolution of mastication.
So now we have 2 separate things. Zygomatic arch and heterodontythat are both evolving and in support of mastication, the componentsof mastication.
Another evolutionary trend that we can really see pretty clearlyin the fossil record -- when we say, I mean the paleontologistswho study this stuff, can see, this is a little bit less obvious,and that is that when look at the study of what happens to theteeth in a reptile or in a fish, when those animals, if they breaka tooth or a tooth wears out, they just grow a new one.
And they sort of have a continual and sort of random replacementof teeth throughout their life. But as you know from yourselfand from the other mammals who you know, mammals don't operatethat way. Mammals have 2 sets of teeth. You're born with no teeth.You grow some juvenile teeth. And then somewhere after that thenyou very symmetrically and in a pre-programmed and regular fashion,you replace the juvenile teeth with adult teeth.
And then that set of teeth lasts you for the rest of your life.You do not have the option of growing a new tooth if one fallsout during adulthood; you're out of luck.
Now, that doesn't sound like much of an advance, and particularlywith some of your older relatives you can see they are missingsome teeth. It's a real distinct disadvantage. What seems to havehappened, this change from a reptilian condition, which is called"polyphyodonty." "Poly," many. "Phyo"groups or family. Phylogarm, phylogeny, many groups of teeth.The reptile condition is that they continually replace teeth.
And the mammalian condition, which the diphyodonty is 2 setsof teeth. First set of teeth are the juvenile teeth. The secondis set of teeth are the adult teeth. It's not so much the ideaof not having the option of replacing teeth when you get tobe older. It's not so much of the idea of only having 2 sets ofteeth that was selected here, this really was an improvement.
What you really want to emphasize is in terms of thinking aboutit, it's not included in the name, but what's important is youhave a regular pre-programmed replacement of teeth, so that whenyou are missing a -- say, you're missing your upper incisors,you are also missing your lower incisors.
When you are missing a pair of upper premolars on the rightand left side, you're missing the lower premolars that would normallyoperate against those teeth. And the reason for that is that inif you are going to chew your food, if you are going to be evolvingmastication, then you need to have -- the teeth on the top andbottom have to be there at the same time. And in order to maximizethe number of teeth that are available, involved in this processcalled occlusion, it's a process for the teeth that operate againstone another. You can maximize occlusion by replacing an uppertooth and lower tooth at the same time. Because if the upper toothis in the process of -- has been lost, and the new one is growingin, that's the ideal time to do the same thing on the bottom.There's no point in keeping the one on bottom.
It can't do you any good if it doesn't have something to occludeagainst it on at the bottom. The evolution of diphyodonty wasa means of maximizing occlusion and minimizing the length of time-- the animal's ability to chew the food was limited. That's whythe these Therapsids evolved diphyodonty.
Now, I want to stop for a second and look at this diagram. I'mtrying to develop here -- there is always some confusion aboutwhat this diagram is trying to say. I'd like to have you thinkabout the idea that there are some kinds of connections betweenthese ideas.
In other words, heterodonty relates to mastication. Zygomaticarch relates to mastication. They both relate to the increasein the digestive rate through mastication.
But let's take this issue of diphyodonty. Which one of the littlebubbles would you connect diphyodonty with? What is the closestconnection to it. It is related to this, to mastication. But itis more closely related to mastication or more closely relatedto heterodonty? Do you think that you need to have a pre-programmedregular replacement of teeth before you have teeth that have differentshapes and different functions?
How many people would put diphyodonty? I'll just abbreviateit. Now, how many people would connect it to diphyodonty there?How many people would connect that diphyodonty relates here? Evolutionof heterodonty, the different shaped teeth that have differentthings for chewing and grinding. How many people? Raise your handsup high. There is no extra credit. Now, how many people say itgoes over here?
So why do you think it goes better with mastication than throughheterodonty?
STUDENT: Because you have different types of teeth. But thenyou have evolved diphyodonty where you have 2 sets of teeth, thathelps mastication which is directly related to how one chews thefood, because it's related to how -- where an individual is asfar as the image and the environment that relates to the way organismschew the food, not to the other way around.
INSTRUCTOR: Well, I would argue that if you had -- if you didn'thave teeth with different shapes like incisors and canines andpremolars and molars that performed different functions, thenit would not be very important if you had, you know, lost onein one place or another.
If all your teeth were capable of doing the same thing, thenit wouldn't really matter if you were unable to have occlusionin one area. The necessity for having pre-programmed replacementof teeth really only comes about when these teeth have differentshapes and specialized functions. Diphyodonty relates to heterodontydirectly and to mastication via diphyodonty via heterodonty. Wecan argue more about it later.
Diphyodonty connects here, that relates to the whole processin that way.
Now, another thing that can be seen in these animals, whichis not as -- cannot really be observed in the fossil record --all these other things I have been talking about are things thatcan be observed. It's worth remembering when you are talking aboutany one of these. Those are things we can really observe. Andthere are things we can say, "You know, this is somethingwe can infer." We can guess that may have been happeningas well. Because it's important to know what you have hard evidencefor and what you have sort of circumstantial evidence for.
And this second one is a called "lactation," nourishingthe young with milk. This is achieved by mammals because theyhave specialized glands, mammary glands that produce milk. Thebehavior of suckling young through the mammary glands. And theyget nourishment in that way. Lactation is a product of mammaryglands which is not something that would be preserved in the fossil record. Soft tissues are never preserved in the fossil record.We cannot observe mammary glands. We can only infer that reptilesmay have evolved this anatomical feature, which the whole classmammalia is named for because of this idea of diphyodonty.
If you are born with no teeth, which seems to be a common feature,then you had better have some way of getting nutrition that doesn'trequire chewing. So the evolution of diphyodonty which can beobserved by seeing -- if you look at whole bunch of differentTherapsids skulls, and see teeth missing in symmetrical patterns.
That tells us they had diphyodonty. The inference is that theiryoung may have been born without teeth and therefore they mayhave needed to have lactation. We can put diphyodonty here, relateit to go heterodonty and put lactation connecting via diphyodontyto heterodonty as an inferred evolutionary trend. We don't havegood fossil evidence in favor of that.
Another thing that can be observed is the presence of what'scalled a "secondary palate." Secondary palate, I havealready described. That's the plate of bone that forms the roofof your mouth and separates the nasal passage so that you canbreathe at the same time that you are chewing your food. And that'sreally where the secondary palate fits into this picture. Secondarypalate comes along in support of mastication. So mammals can chewtheir food while they are breathing.
Now, another thing that relates to mastication are some changesin the structure of the jaw and jaw joint. A mammalian jaw, lowerjaw, is a composed of only a single bone. And that's not the casewith the reptilian ancestors of the mammals.
Remember, in looking at the lower jaw of the snapping turtlein lab you saw a whole series of different bones that made upthat lower jaw. You had angular and articular bones and a bunchof different bones like that. The bone here is the dentary bone,it has teeth on it. And if we look at the evolutionary trend,what we see is the lower jaw becomes eventually composed of justa dentary. The other bones are lost from the lower jaw.
And that also seems to relate to the evolution of mastication,because the dentary is the only bone in the lower jaw that refersto teeth and it is also the only bone in the lower jaw to whichthe muscles are attached, so we get this production of this bigupward extension that you can see that's sticking up the abovethe level of teeth. That's called the "coronoid process."That's the place where the muscles attach.
And so mastication led to the increase in the size of thedentary, that's the way I would phrase it. An increased size ofthe dentary. You get to the point in mammals where the lower jawis composed just of the dentary, and that's the point of thatevolutionary sequence that the increase in the size of the dentaryleads to lower jaw being composed of just a dentary.
And when the lower jaw is composed just of the dentary, thenthe jaw joint -- the hinges between the lower jaw and the skullmust evolve the dentary. And so we have the evolution of a newjaw joint.
And this jaw joint is between the dentary and another bone inthe skull called the "squamosal bone." So the evolutionof the dentary, squamosal jaw joint results from the increasedsize of dentary and that is occurring because animals are evolvingmastication.
Now, there is all sorts of really interesting homologies ofbones of the lower jaw that we don't have time to talk about.But you have heard quite a bit about it in lab. I won't hold youresponsible for homologies on the midterm, but I think you cansee how they relate to this as well.
Another thing that we see in the skeletons of these Therapsidsis the dramatic rearrangement of their limbs. A pelycosaur, likemost reptiles, or a crocodile have what we call a sprawling gate,that upper segment of the leg is running sort of parallel tothe ground. They are going around doing this permanent pushup.
When we look at the Therpasids, those limbs have now rotatedunderneath the body. The limb bones are vertical and they go backand forth. Think about the way legs of horses are different fromthe way a reptile or crocodile operates. And that rotation ofthe limbs underneath the body probably does not make the animalsrun faster. It probably does not make the animals run with greaterenergetic efficiency, but it makes it more agile. And a mammalchange directions faster than a lizard. They can run fast. Theycan change direction faster than a lizard.
So they think the rotation of limbs under the body, which isanother one of these trends is the rotation limbs underneath thebody leads to the increase in agility, that is the ability tochange directions. And how do you suppose that relates to allof this? Does that relate to evolution of the -- I mean, doesthat directly relate the evolution of mastication? Why would ananimal need to have greater agility? To catch the food.
So of the things that I have up here, what would you relateincreased agility to? Meeting higher energy demands. I mean, youcatch the food, as well as you have the ability to digest it morerapidly. So they evolved that.
There is another evolutionary trend that appears to be substantiatedby the fossil record that these guys evolved fur. Now, normallyyou might expect that fur like any other soft tissue would notbe preserved in the fossil record, so we don't actually have fossilfur from the Therapsids.
But what some paleontologist have found in a couple of casesthat in the rocks which would have been underneath the body ofthe Therpasids is the impression of fur, just like a foot printcan be fossilized, the impression of fur has been found in therocks immediately underneath the body of some of Therapsid fossils.
So these we think mammals may have evolved fur. Now, do youthink that they evolved fur before or after they became nocturnal?If you were a sun basking lizard, would you want to have fur?So fur is related to the process of endothermy. That it's insulationthat helps you keep this heat, but it's only something that youevolve after you have abandoned the day-active way of life.
But it's something that we can say would contribute to endothermyand a means of having a high and stable body temperature.
Now, one last thing, and I have to go through it quicklybut it's really a fascinating thing. That is when we look at thiscast -- in other words, not only do we get a fossilized skull,but you also get a skull that is going to have inside of it whatis called a cranial cast, which is pretty much the same shapeas the brain of the animal.
Again, the brain itself is soft tissue. But the brain fits prettytightly inside the skull cavity. What's in there if you removethe fossilized skull bones in the cranial cast, they will tellyou something about the size and shape of the brain of the animal.And the brains of these Therapsids or the mammals have a new areaof the brain that is called the "neopallium."
The neopallium is a new area of the brain in the Therapsidsthat's not found in reptiles. Mammals have a neopallium. It isan enlarged area of the brain in the same location.
Now, clearly we cannot find out what the neopallium of Therapsidsdid, but when the neurophysiologists studied the function of theneopallium in primitive animals they find that area of the brainintegrates information from 2 sensory systems, from the senseof smell and from the sense of hearing.
Now, what major vertebrate sensory system is not included inthat list?
STUDENT: Vision.
INSTRUCTOR: What kind of animal would have evolved a tremendouslyincreased area for the integration of information from its sensorysystems and leave out vision?
STUDENT: A nocturnal animal.
INSTRUCTOR: A nocturnal animal. This whole story is predicatedthat you should have said, "How in the heck can we tell byfossils that this was a night active animal?" How can youtell that from a fossil that's been dead for 180 million yearswhether it was running around in the nighttime instead of thedaytime?
Well, this is the reason why, because these fossils have a neopallium.And a neopallium in modern mammals is a structure which is depictedin a nocturnal animal.
And since the neopallium of modern mammals has evolved fromthe neopallium of these Therapsids, we can say that a group ofreptiles, the Therapsids had a mammalian brain. They had fur.They had mammary glands. They suckled their young.
They had limb structures and the orientation of mammals. Theywere very agile. They probably had high digestive rates. Theychewed their food. They had mastication, diphyodonty, heterodonty,almost everything that it takes to be a mammal. They had the mammalianjaw joint. At least, the more advanced ones.
So they made that transition to being like mammals as adaptationsto being night active and when they could not use behavioral bodytemperature regulation because they lost out to the Archosaurs.
