We need to learn some very important steps in vertebrate evolution.
Before we get on to that, I'd like to draw your attention tothe geological time scale, which is at the beginning of this setof Illustrated Notes.
These are the names that geologist and biologists use for referringto the various different geological time periods. They are traditionallylisted in this fashion with the oldest ones at the bottom. Thisis in your Illustrated Notes. You don't need to copy it down.
MYBP stands for Millions of Years Before Present, when the periodbegan. So most of the first fossils that I'll be talking aboutare from the Cambrian, and that period began 600 million yearsago.
Many of the groups of the fish that I'll be talking about appearin the Devonian, which began 400 years ago and ended 350 millionyears ago. You need to memorize this. At least, know the sequenceof the time periods. So that you know the Ordovician came beforeSilurian and after Cambrian.
So a few time marks along the way here, would be helpful, too.So just that you have a sense of what kind of time we're talkingabout. 400 million years is really an amount of time that's inconceivable.
The first signs in the fossil of vertebrate animals are theseanimals that are shown at the bottom of the page here. And rememberthat the phylum chordata has the group of animals that I'm talkingin this class is the sub-phylum vertebrata. And the sub-phylumvertebrata is divided into 2 superclasses. And the first superclass is the super class agnatha, that's without jaws.
And this is a pretty small group in terms either living orfossil forms, but it is first group to appear in the fossil record.
And there is a single class in that superclass, and that's somewhatunusual. And what's even more unusual is that it has the samename as the superclass, that is agnatha.
There are several living groups. There are 2 living groups I'llbe talking about that shortly. And there are several extinct groups,fossil groups. And I'm giving you one of these just to sort ofpoint out this important step in vertebrate revolution.
This is the order osteostraci. And there are, as I said, severalother classes -- those are all collectively referred to as Ostracoderms.
What does derm mean? Skin. This is related to osteo. Osteo.This is bony skin. Reference to the fact that these guys havevery heavy, what is called durable armor. They have plates ofbone within the skin.
If you look at the picture at the top here, you see all theselittle parallel curving lines right along here, those are separatebony plates that are connected to one another by little flexibleskin areas.
So the animal can swim by moving its tail from side to side,but all of this is covered with heavy durable armor. That eventuallyevolves into scales. So there is a tremendous reduction in theamount of that.
This part at the front, the head and shoulder region is alsocovered by huge solid bony plates. A ventral view of that bonyplat is shown here at the bottom of the page.
All of the stuff on the outside here I'm pointing at right nowis solid, and this part of the middle here is a like the mosaicof small bony plates, again, flexible, so that this part of themouth of the animal can pump up and down.
These little dark things around the perimeter then are gillopenings. So the animal was a bottom dweller. We know that becauseof the fact that this animal, if you look at in this dimension,is flatter, it's dorsoventrally, that is "dorso" top,and "ventral" bottom. dorsoventrally flat is kind ofpancake-shaped animal. Flatter in this dimension than it is inthis dimension.
These 2 pictures are not actually drawn to scale. These guysare flat. Bone is heavier than water, so these animals are weredenser than water and they tended to live on the bottom.
They did not have jaws as the name agnatha implies. They suckedup organic goo that was on the bottom the ocean. And these areyour most ancient ancestors, these bottom dwelling, scum suckingancestors. They are known as just fragmentary, almost microfossils from the Cambrian, but they are big enough that they canbe microscopically sectioned an examined. And they have the samemicroscopic structure as the bony plates do.
so it's clear that these fragmentary fossils, that are knownfrom the late Cambrian when they are found as fossils from Ordovicianand Silurian, the bony plates have the same microscopic structure.It's pretty clear what those fragmentary remains are remains of.
This tail with the larger sort of more massive dorsal lobe,is a hetero circle tail. And that is a characteristic that wesee in fish that are denser than water, that would tend to sinkto the bottom. The hetero circle tail generates some lifting forcesand allows the animal to get up off the bottom.
Now, the next group to appear in the fossil record after theOstracoderms are shown on the next page. And the class acanthodiiis the first group, the oldest group of vertebrates that had jaws.
Acanthodii were not very large animals, they were maybe a halfa foot or so in length, but they are starting to look more likea real fish. They had jaws and teeth on the jaws. And they havea pretty good size eye. Notice that, they have a pretty good sizeeye.
And so they have the distinction, the important point of vertebratein evolution of being the oldest group fossil groups with jaws,the first ones to have evolved with jaws. And they appear in thelower Silurian, that is early Silurian right at the boundary withthe Ordovician.
And the next group to show up, which are towards the end ofthe Silurian and the upper Silurian, the way the geological rockswould be arranged, is the guy shown here. This is a class Placodermi.
And you can see the shape the stippled area there is still afairly large bony head and shoulder shield. And they have somewhatof a hetero circle tail, probably a little bit less armor alongthe sides.
Why do you think these guys have durable armor, so they willsink like a rock. There's probably somebody out there attackingthem, right? That's the reason that animals have armor.
Well, there are fossils of animals called water scorpions, whichwere like six foot long aquatic insects ancestral to terrestrialscorpions, that had big pinchers and must have been very, veryrespectable predators. That is why the earlier groups had fossilsit was protection from the Eurypterids.
So along come the acanthodii, they evolved jaws. It's probablethat there is some common ancestry between the acanthodii andthe Placodermi that is not particularly worked out.
The placoderms are also the second oldest group to have jaws.They appear later in the fossil record. They are second. But thepoint that's really interesting about them is that they reallytake advantage of having evolved jaws.
Because jaws allow you to not be a bottom dwelling, scum suckingliving off the organic goo, but to become a predator. And whenyou become a predator, because you can eat somebody big, thenyou can be bigger. And, in fact, the bigger you are, the biggerthings you can eat as a predator.
You need to have good eyes in order to spot that predator. Youdon't have to have good eyes to find scum at the bottom of theocean. But if you are trying to find another mobile animal, youneed to have eyes, you need to have a complex brain, you needto be able to have fairly complex behavior to anticipate the escapeof that prey species or to know where to go to look for the preyspecies.
In other words, the nature hallmarks of the vertebrate animals,complex sensory systems, complex brains, high levels of mobility,complex behavior, those are things that distinguish vertebrateanimals, as a group, from all the other animal kingdom.
This really comes about as the result of the evolution of thejaws and the ability to be predators.
This guy right down here at the bottom is a fossil flown thedinichthyes. "Nichthyes" means what? Fish.
"Din" where else do you hear that? What does Dinosaurmean? Terrible. So this is a terrible fish. This animal was 30feet long. This is just the head and shoulder region of a 30-foot-longfish. That guy could open his mouth four or five feet wide. Hehad some pretty sharp looking teeth there.
So relatively quickly after the evolution of jaws, some vertebratesbecome big terrible fish, ferocious predators. Now, the next pagein the Illustrated Notes is a part that I'm going to skip over.It deals with unfortunately some very interesting stuff abouthow jaws evolve.
STUDENT: What are the Eurypterids?
INSTRUCTOR: The Eurypterids were those big water scorpions thatwere the predators that favored the evolution of that shoulderstuff.
STUDENT: That's placoderms?
INSTRUCTOR: The Eurypterids were invertebrate animals that preyedupon the Ostracoderms. That's why the Ostracoderms had that durablearmor. They were also -- when the acanthodii and the placodermsevolved jaws, then they probably take over as the major predators,and you don't have to worry about it anymore.
So one of the first evolutionary advances in vertebrate evolutionwas the evolution of jaws. And the acanthodii are the first onesto do it, and it's important because -- why is it important? Whyare jaws important? Tell me something that comes about as a resultof the evolution of jaws.
STUDENT: You become a predator.
INSTRUCTOR: And what do you have to have in order to becomepredator?
STUDENT: Eyesight.
INSTRUCTOR: You do get ears also. Although, they don't looklike ears in the fish, they are just a way of sensing vibrationsthe water.
Mobility, complex behavior. And the senses in the complex behavior-- where do you --
STUDENT: Complex brain.
INSTRUCTOR: Complex brain.
So another important step in the evolution of the vertebratesis the evolution of the lungs. And you might not think that thatwould be true since the majority of living vertebrates are fishand adequate and do not use lungs to get oxygen. But, in fact,lungs evolved very early in the evolution of the fish. And duringthe Devonian time period, and in only a couple of, at least,originally fairly unimportant groups of fish, those lungs persistas respiratory organs, as a means of taking in oxygen.
In one of those groups of fish, lungs stopped being used forrespiration and started being used as a hydrostatic organ.
"Hydro" meaning water. Hydrostatic organ. And it'sa device that allows the animal to adjust the density of its bodyso that it has the same density as the water surrounding it.
Now, let's do a little quick physics here. Here we have a bodyof water. And we're going to have something up here floating atthe surface, and something in the middle, and something at thebottom. And they all have exactly the same weight.
Why is it that with exactly the same weight one of them floats,and one sinks, and the other one floats right in the middle?
STUDENT: Different types of lungs.
INSTRUCTOR: Not different of lungs.
STUDENT: Density.
INSTRUCTOR: What is it about density that accounts for it? Andsince I have said these all of the same -- what is density definedas?
STUDENT: Mass.
INSTRUCTOR: Mass over volume.
So which one of -- if water has a density of one, what's thedensity of this one right here? That one is less than one. Thisone right here? This one right here equals one. And this one overhere is greater than one. Now, if they have all the same mass,what's different about these 3 things?
STUDENT: The volume.
INSTRUCTOR: Volume. So what these animals have evolved theirlung into was a device that changes the volume of their body sothat the density is equal to the density of water. Now, what happensif this fish -- okay, this is a fish with a body of a densityof one. Now, you tell me, here is a fish, he's swimming, he'ssitting right here, and he's very happy. His density equals thatof water.
What happens if he swims down here? If he has in his body agas filled chamber which has evolved from his lungs, he has agas-filled chamber in his body that has adjusted his volume sothat he has a density of one, what is going to happen to the volumeof that gas-filled chamber when he swims down 3 inches?
The volume is going to decrease. The pressure, the weight ofthat all that water pressing in on the body of the fish is goingto decrease the volume of gas in and that's called a Swim Bladder.What is that going to do to his density? Increase his density.
Now, does that make him likely to sink or likely to float backup to where he was?
STUDENT: Sink.
INSTRUCTOR: It makes him likely to sink. So fish live on a razor'sedge. They live on a razor's edge. He's fine as long as he staysright here in exactly this place in the water. If he swims downsix inches, the pressure compresses his Swim Bladder, makes himmore dense, he's now starting to sink. And the opposite happensif he swims up towards the surface.
There is less pressure, if that Swim Bladder can expand, increaseshis volume, makes him less dense, he's likely to pop up to thetop. So they live on a razor's edge. They have to be continuallyadjusting the volume of that Swim Bladder in order to keep theirdensity equal the density of the water around them.
And that marvelous hydrostatic organ, which is called a SwimBladder, evolved from the lungs of these early Devonian fish.When we look at fossils or paleontologists look at fossils, theycan identify five different groups of Devonian fish that all havelungs.
All of them.
They're all found in freshwater deposits. The geologist canlook at the chemical composition of the rocky matrix around thefossil fish, and they can tell you this was a fresh water deposit.They can also tell you that this was freshwater deposit that waslaid down at a time of drought.
They can tell, geologically, that the rocks were -- there weredroughts going on. Muds were being dried out and rehydrated, interms of a lot of other kinds of things like that which are beyondmy understanding of geology.
So the Devonian, based upon a number of different lines of evidence,was a time when freshwater habitats were subject to drought. Andfive different groups of Devonian fish can be seen to have lungs.The class acanthodii -- what is important about the acanthodii?
STUDENT: Eyes.
INSTRUCTOR: I did point that out because they were predators.The thing that really --
STUDENT: Jaws.
INSTRUCTOR: The first one is the jaws.
The class Placodermi is another one of these five groups thatappears to have had lungs. What do remember about the class Placodermi?Second group, and they get big.
Now, another group that in the fossil record that has jaws are-- in a way I have constructed the graphic here. This is a fossilanimal in the superorder dipnoi, these are lung fish. This isfossil lung fish here.
And this is a living lung fish. There are about six or sevenspecies of animals are that are in superorder dipnoi, I'll talkabout them a little bit more in a little bit, who had lungs.
So in these animals, the fossil guys evolved lungs and theyhave gills, too. See this flat plate here behind the eye, that'sthe operculum, that covers the gill region.
These guys have gills and lungs. In fact, that's what dipnoimeans. "Pnoi" is the same root as in pneumonia, refersto lungs breathing. And a "di" means two. These animalsdipnoi, have two ways of breathing; they have lungs and they havegills, two ways of getting oxygen into their blood.
And they stay as fish. There are species of fish today thatare lung fish. They have lungs. You can monitor functions of theirlungs, just like you would in a frog or a lizard, but they alsohave gills.
So that's 1, 2, 3.
We have 2 more groups. The 2 other groups are the ones thatare evolutionarily the most interesting. Why did I not createthis graphic the same way for acanthodii and Placodermi? Why didI not give you 2 pictures? It's extinct. There is no second group.
But the other 3 groups all have fossil representatives in theDevonian and living descendants that are extant today.
So the forth group is a group of -- again, in the class Osteichthyes,which are the bony fish, in the subclass Sarcopterygii, but adifferent superorder. This is the superorder crossopterygii. Andthen the order Rhipidistia.
And this is a fossil group -- the order Rhipidistia is a fossilgroup. The reason they are particularly interesting in an evolutionarysense, is that their descendants are all of the terrestrial vertebrates.I have used a chimpanzee here, one of our closer relatives, asa representative, sort of in a humanly-ego-centric manner. But,in fact, all terrestrial, all vertebrates, that means all mammals,dogs, cats, rats, squirrels, all birds, all reptiles, turtles,snakes, all amphibians, salamanders, frogs and some others, allterrestrial vertebrates are descended from the order Rhipidistia.
So the lungs in the Rhipidistia continue to function as lungs.
But did the Rhipidistia evolve lungs in order to invade land?No? Because dipnoi never invade land. And this last group downhere, that infraclass chondrostei, gave rise to 30,000 speciesof living fish, they never invaded land.
In fact, lungs did not evolve for the purpose of invading land.Lungs evolved for the purpose of allowing fish to continue tolive in water as fish.
And the reason for that was because of this business of thedrought. There were droughts. What happens in a drought? Whathappens to lakes and streams in a drought? Less water. Water leveldrops. Streams stop flowing and become isolated ponds.
Lakes get smaller. What happens to the temperature of the water?Water temperature goes up. what happens to the amount of oxygendissolved in water when the water temperature goes up? It decreases.
The solubility of gas in water decreases when the temperatureof the water goes up. It's the opposite of what happens -- youcan dissolve more sugar in water if you needed a solid. Solubilitygoes up with temperature. Gas solubility goes down with temperature.
So in a drought, we have smaller bodies of water, higher temperatures,lower solubility of oxygen, and the animals living in those waterare in trouble.
And they are in danger of asphyxiating. They are in danger ofsuffocating for lack of oxygen. And so they have evolved the abilityto get oxygen out of air as a means of surviving as a fish inwater.
The dipnoi, the lung fish, still do that. The descendants ofthe class chondrostei, down hear at the bottom, are skill aquaticvertebrates. They have evolved their lungs in order to continueto live in water. And then they subsequently converted it intoa Swim Bladder which allows them to adjust the body density.
But in the case of the Rhipidistia, in retrospect, the evolutionof a lung in order to continue to live in water and be a fish,the evolution of the lung was very fortuitous for these animalsbecause even though they evolved lungs in order to continue tobe fish and live in water, having evolved lungs some of theirdescendants subsequently evolved the ability to live on land.
And so in the Rhipidistia, and in that group only, the lungis what evolutionary biologists, some would argue, would calla pre-adaptation to land. It's only a sort of judgment that youcan make in hindsight.
In other words, the lungs were necessary for survival on land,and having evolved lungs in order to be able remain in water asfish, this particular group, the Rhipidistia, invaded land. Inhindsight, we can say, well, their lungs were a pre-adaptationfor land.
STUDENT: So the evolved lungs were for the low amounts of oxygenin the water.
INSTRUCTOR: They have to surface in pull air into the lungsin order to get the oxygen out.
So there are five different groups of Devonian freshwater fish,all living in freshwater habitats. Why it is important that theylive in freshwater?
What is the other choice? Salt water. Can you imagine many situationsin which the ocean would be subject to drought, stagnation, lowoxygen contents? No. So it's really critical that these animalswere all living in freshwater, because only a fresh water habitatcan be subject to brought and stagnation and low oxygen content,and create a situation where a lung would evolve.
It's really an accessory, a back-up respiratory organ in theseanimals. And in 2 of the groups go extinct for some other reason.One of the group king of hangs around and are really interestingto look at, but they are still doing what all these guys we'redoing back in Devonian. And they are even found in freshwaterhabitats subject to drought in some cases.
One group, in the Rhipidistia, the lung is a pre-adaptationfor land. And some of their descendants crawl out on land as thefirst amphibians, who then evolve into the reptiles, who thenevolve into birds and the mammals, and ultimately leads to yourcousin here.
And one group, the infraclass chondrostei, these guys stay inwater and they go back into the ocean, where there is no longerany need for a long. But now they have gas-filled chamber in theirbody. And they evolve the ability to adjust the volume of thatgas-filled chamber which becomes the Swim Bladder and leads tothe evolution of tremendous diversity.
There are more species of bony fish even in this most advancedgroup, there are more species than all other groups of vertebratesadded together.
There are 30,000 species represented by this one little tinyguy right here.
There are 5,000 species of mammals, 8,000 species of birds,6,000 species of reptiles. That adds up to 20,000.
So there are 30,000, these guys are highly successful very,very diverse.
So lungs evolved, and I don't have time to tell you more abouthow they evolve because we're running late.
Let's go on to the lecture I was supposed to start a half anhour ago, which is on the classification of natural history ofthe living fish.
Any questions about that?
STUDENT: Wasn't the Swim Bladder supposed to evolve first inorder for the lungs to evolve, because they needed to change thedensity in order to surface to get the air from the lung? So didthe Swim Bladder evolve first or the lung?
INSTRUCTOR: Yeah, in a sense, it sort of intuitively seems like,well, terrestrial vertebrates have lungs, fish have Swim Bladders,fish are more primitive than terrestrial vertebrates, thereforeyou might expect Swim Bladders to have evolved first.
But you are sort of hypothesizing that they needed to have SwimBladders in order to get up to the surface to gulp air. Becausethey became big, and because they evolved jaws and were able tobecome predators, they didn't have to have their body totallyencased in all that bone.
So when we look at the those Devonian fish, they skill havescales, but they don't have the massive durable armor of the Ostracoderms.So the jaws kind of got them away from that. And they are no longerdorsoventrally flattened pancake-shaped animals the way bottomdwelling fish are today; they are more torpedo shaped.
Dinichthyes, obviously, didn't make a living on catching otheranimals by laying on the bottom. He's out there chasing them.
So these guys without Swim Bladders or without lungs had theability to get up to the surface to gulp the air. And that behaviorof going up to the surface to gulp the air and the anatomy andthe physiology of the lung come about to solve the problem ofthe stagnation and the drought.
And there is evidence.
Now, as soon as you have an air-filled chamber in your body,you have the potential for neutral buoyancy it's not an either/or;it's both and more.
You can have -- if you have a lung and you have evolved in orderto get oxygen, it also helps you to keep your density close tothat of the water. So it's not like they had to choose. It's justin the terms of thinking about it logically, given the drought,given the stagnation, it seems like the original reason for evolutionof lungs was for respiration.
And in the bony fish, all these other guys, the 30,000 speciesthat are in a group called teleost (spelling), the Swim Bladderis sealed off. It's not -- its no longer attached to the digestivetrack, the way our lungs are. It has special little areas, thatyou'll see these lab when you dissect the fish, that can secreteand soak up gas out of a swim bladder.
So the first thing to evolve was an accessory respiratory organor a lung. And it probably had some beneficial effects in termsdensity. But why it evolved to begin with was to get oxygen.
And then in one group, it becomes a highly modified and very,very effective hydrostatic organ. In one group, it stays as alung and becomes the only respiratory organ after the animalsinvade land, that's Rhipidistia.
In a third group, in the dipnoi, they continue to have both.Some species actually can use both to an equal extent.