Here you have the summary of the the geological eras and periodsand the billions of years before present when each of those theologicalperiods began. And this is information that you need to memorize. I may test you on all or part of this information on the midterm. Even though the animals that we'll be covering on the first midtermonly go up to the Devonian I might test you on the entireset of information on the midterm. HE IS REFERRING TO THE FIRSTILLUSTRATED NOTE FOR THIS DATE.
The standard information is presented in this conventionalmanner with the oldest geological periods at the bottom of thelist and the most recent geological periods at the top of thelist because that's the way in which they are found in the geologicalrecord, the rocks are laid down, the older rocks are laid downand then more younger rocks are laid down on top of those andso forth so that sort of represents the way they appear.
The period of time during which fossils of living organisms areknown starts with the Cambrian. There are some limitedfossils prior to that but the great diversity of living organismsstarted with the Cambrian and the 600 million years (that's whatMYBP stands for - Millions of Years Before Present). The 600million years are divided into 3 major geological eras. The Paleozoic,Mesozoic, and Cenozoic.
The Mesozoic is the period of time when the reptiles werethe dominant, terrestrial vertebrate although the firstreptiles appeared back in the Carboniferous. And the Cenozoicis -sometimes referred to as the "Age of Mammals"rather egocentric on the part of human beings to say that becausethere are lots of other animals around. But the great famousextinction of the dinosaurs occurred at the end of the Cretaceouswhich was the beginning of the Tertiary, somewherearound 63, 65 millions of years ago.
The animals that we are going to be talking about occurred duringthe Paleozoic era. And the very first fossils of vertebrateanimals are found in the late Cambrian. They are fragmentaryremains of the animals related to or similar to the one that isshown here. The sub-phylum Vertebrata includes the animals thatwe're primarily interested in this class is divided into 2 super-classes. One is the Superclass Aqnatha which means withoutjaws and that includes just a few animals, a couple of livingspecies and some fossil species. The majority of the vertebrateanimals that you're familiar with, perhaps all of the ones thatyou are familiar with, are placed in the other Superclass whichis the Superclass Gnathostomata (sp) - that is vertebrates withjaws.
There are some other sub-divisions within this Superclass Agnatha but for the sake of simplicity we will consider only animalsthat are in the Class Agnatha. There are 2 living orders andthere are a number of extinct orders and I'm taking this one extinctorder which includes the oldest fossil vertebrates as representativesof a group of fossil vertebrates. And this is the Order Osteostraci. And as I said the oldest fossils of vertebrates are found inthe late Cambrian and that means towards the end of the Cambrianor near the beginning of the Ordovician. So they are a littlebit more than 500 million years old. And we wouldn't really knowwhat these animals look like, were it not for the fact that wefind complete fossils of these animals during the Silurian orOrdovician. They can be identified as being, these fragmentaryremains can be identified as belonging to the same types of animalsthat are found intact in the Ordovician/Silurian on the basisof the microscopic anatomy of the fossils themselves.
The fossils are the broken up remains of this head and shouldershield here which is covered with a layer of dermal bone. That'_sthe bone that has the fundamentally different kind of microscopicstructure from the Haversian Bone that you looked at in lab. But nevertheless it has a distinct structure and these fragmentaryremains can be unmistakably identified as the remains of vertebrateanimals cause only vertebrates have bone. And they have the samebasic microscopic anatomy as the bones of these intact Osteostraciby the Ordovician and the Silurian.
The common term that many biologists use to refer to these veryfirst fossil vertebrates is Ostracoderms. Derm meaning skin -ostraco referring to bone. And it really refers to the fact thatalmost the entire outside of these animals' body was covered withbone. They were very heavily armored animals. The head and shouldershield I've already pointed out to you but the rest of the animal'sbody except for the very rays of the fins themselves, all therest of that animal's body is also covered with bony plates arrangedwith some sort of skin, flexible skin between them so that theanimal could beat it's tail from side to side, the way modernfish do. There's been quite a bit of speculation as to why theseanimals were so heavily armored. Probably the best explanationis that these are pretty small guys in the neighborhood of 6 to8 inches in length. And they were probably armored like thisto protect themselves from the dominant predators of the time. They had a skull and other things to qualify them to be vertebratesas well. An external protective covering with muscles underneathit and in more or less basic vertebrate anatomy underneath it.
Some truly ferocious looking invertebrates called Eurypterids. These were sometimes referred to as "Water Scorpions". They were really fierce looking animals. So these guys wereprobably the dominant predators of the time and the Ostracodermswere armored to protect themselves from the Eurypterids.
All of them were sort of pancake type animal shape. What we wouldsay dorsal ventrally flattened. Kind of flatter from top to bottom_than from side to side. Like a bat ray or something like thattoday. And that probably reflects the fact that they were a lotheavier than water. Bone has a density greater than 1 so anyanimal that has a lot of bone in it's body is likely to be heavierthan water and have a tendency to sink to the bottom and whatwe see among modern aquatic vertebrates is that dorsoventrallycompressed animals are almost always bottom dwellers and that'sprobably the best thing to do if you're going to sink like a stoneis just to go ahead and go with the flow. Spend most of theirlives sitting on the bottom.
If these guys were bottom dwellers, how did they feed? Well ifwe look at the fossils of the underside of the skull, what wesee is a mouth opening and a series of what can be demonstratedto the gill openings based on an anatomy of the internal structureof the skull region, it is possible to tell if there were gillarches and gills in there. But this oval shaped area posteriorto the mouth and between the gill openings was not a solid sheetof bone, rather it was a mosaic, a whole series of small bonyplates that once again seem reasonable to assume was probablyflexible. So these guys could probably pull water into theirmouth by using muscles to pull that structure down and then pumpthe water out through their gills. And their gills have a verydifferent anatomy from the bony structure of the gills from thegills of modern fish. And they were probably using them to befilter eaters. So these animals without jaws, no teeth, obviouslycouldn't have been particularly ferocious predators. Probablylived on the bottom and kind of little aquatic vacuums, suckingup the organic goo on the bottom of the ocean and there were lotsof other kinds of invertebrates and so forth in the oceans thatwere swimming around and producing this organic goo.
In fact it isn't really clear who the ancestors of the rest ofthe vertebrates are. These animals are so different from therest of the vertebrates that it's entirely possible that theyare one line of descent that gave rise to a couple of jawlessmodern vertebrates but that the actual ancestor of the rest ofthe vertebrates does not belong to this group but is completelyunknown because the fossils, there were no fossils that were producedor at least no fossils of them had been recognized as being vertebrateancestors.
NOW WE ARE ON THE 2ND ILLUSTRATED NOTE FOR THIS DATE. The firstvertebrates that are clearly related to the rest of the vertebratesincluding ourselves are shown here, these are the first vertebrateswith jaws. And as I told you the rest of the vertebrates theones with jaws are placed in the Superclass Gnathostomata anddivided into a number of different classes. Some of the classesyou are more or less familiar with that we'll be talking abouttowards the end of the quarter are Class Mammalia that includesourselves. The Class Aves, that includes the birds, Class Reptilia,Class Amphibia. And a number of groups of fish that we'll bestudying you may not be familiar with.
These 2 classes are both extinct. One of them has the distinctionof being the oldest known group of vertebrates that had jaws andthis is the Class Acanthodii. The very first fossils of the ClassAcanthodii are known from the early Silurian. That means towardsthe bottom of the Silurian period near the boundary between theSilurian and the Ordovician. The Acanthodiians are not very biganimals. Most of them under 10 inches in length but they didhave jaws and you can see from the picture of the fossil theyhad a fairly large eye, so vision was an important sensory modalityto these animals. They are not completely covered with a heavybony armor. They are not dorsal ventrally flattened. They arepretty much typical looking sort of fish. They had scales alongits flank and not a heavy dermal armor, and it was not a bottomdweller. This guy was an active predator. Maybe not a terriblyferocious one but nevertheless capable of fairly fast swimmingand capable of tracking down prey using vision and gobbling themup with the jaw and teeth. These animals are from the earlySilurian.
From the late Silurian comes the next class Placodermi, the upperSilurian or late Silurian, near the end of the Silurian periodand they are the second oldest group of vertebrates with jaws. And their paired appendages look more like the paired appendagesof the rest of the vertebrates so in some ways it looks like thePlacodermi may be closer to the ancestry of the rest of the vertebrates. You can see that this animal on the top, the Acanthodii had alot more ventral fins than you'd find in modern vertebrates andthe evolution of paired appendages is another issue that's important. The paired appendages of the Placodermi look more like what wesee in the rest of the vertebrates. I think the thing that makesthe Placodermi particularly memorable is the fact that they werevery definitely not bottom dwelling scum-sucking hide-in-the-cornerfrom the Eurypterids kind of animal. That picture at the bottomthere is the skull and head of a 30 foot long fish. That guyhad about a 4 or 5 foot gape. His name appropriately enoughis Dinichthyes. A terrible fish. Ichthyes means fish and Dinmeans terrible as in dinosaurs a terrible lizard and this is trulya terrible fish. And I think the thing to me is the most interestingabout this business of the evolution of jaws is that it can beargued that the evolution of jaws may have been the most importantsingle step in the entire evolution of the vertebrate animals. Because if we look at the things that distinguish vertebrateanimals from the rest of the animals, complex behavior, complexnervous systems, complex brains, these are the things that becamepossible when vertebrates became predators.
If you're a bottom dweller you don't have to have a complex nervoussystem, you don't have to have a very complex sensory systems. And you don't have to be very fast swimmers because the scumis not going to get away from you. But if you evolve jaws andteeth and then you can become a predator, then you need to havea good sensory system, then you need to have a complex brain thatcan figure out how to cut that guy off at the pass, you need todevelop a much more efficient locomotory system so that you canswim and catch that animal and all of these things really cameabout with the evolution of predation which was made possibleby the evolution of jaws. If it weren't for the evolution ofjaws vertebrates would probably not be standing here giving alecture right now.
NOW WE ARE ON THE 3RD ILLUSTRATED NOTE FOR THIS DATE. So theseare the 2 groups of animals that evolved jaws and since the evolutionof jaws was so important it's interesting to take a look at whatthe old biologists have figured out about how jaws came about. And on the next page you have a diagrammatic representation ofthe major theory about where jaws came from. Why they evolvedor how they evolved.
At the top is a hypothetical animal. This animal is not knownfrom the fossil record, this is an animal whose structure is deducedfrom studying the early embryology of sharks and fish. And thisis an animal that had no jaws but had a series of gill arches. The gill arches are jointed bony structures that have attachedto them the gills. And these jointed bony structures developfrom paired pockets of neural crest cells. Now you remember atleast identifying the location of neural crest cells when youwere looking at the neural tube stage in lab. Those cells arevery interesting because of they have a complex developmentalhistory. These neural crest cells have found that gill archesarise on both sides of the gut of the animal, in the throat regionin the pharynx, from paired little pockets of neural crest cells. And that's true of the gill arches that are behind the jaws invertebrates with jaws. So this animal right here, this primitiveGnathostome, that means an animal with a jawed mouth. This primitiveGnathostome is not unlike a shark. In terms of it's anatomy. And it has a series of gill arches, it has gill slits, the littledark things there are the openings in the side of the animal'sbody between the gill arches and the water that the animal isusing for respiration goes in it's mouth, flows over the gilllamellae that are attached to the gill arches and then comes outof the side of the animal's body through these separate externalgill slits. Well in addition, to having little paired pocketsof neural crest cells developing into gill arches, the jaws alsodeveloped in the same way. And that plus the fact that earlyin their development, jaws looked a lot like gill arches thatthey have a joint right in the middle of them the way gill archesdo is some of the best evidence for the theory that the jaws evolvedfrom gill arches of some primitive jawless vertebrate.
Now an interesting detail is that this theory suggests that thegill arches developed from the 3rd gill arch. Gill arches arenumbered from the front to back, and it looks like the 3rd gillarch is the one that developed into jaws. And the reason forthat is that when embryologists studied the development of thejaws in sharks and in primitive fish there are 2 little pairedpockets of neural crest cells in front of the ones that developedinto jaws and they developed into some of the bony structuresthat are located in the skull region. So there are 2 little pocketsof neural crest cells in front of the jaws and that's the reasonin front of the ones that developed into the jaws so that's thereason for suggesting that they developed from the 3rd pair ofgill arches.
There is another structure located behind the jaws in sharks andin other jawed vertebrates called the hyomandibular. Which attachesthe jaw to the skull. And it appears to have developed from the4th gill arch and there's another bone called the hyoid whichis attached to the hyomandibular and which supports the tongue,the muscles of the tongue are attached to the hyoid bone and italso appears to have developed from the lower part of that 4thgill arch.
And finally all gill arches have slits_ between them, have gillslits between them, and there is a structure called the spiraclethat is located along the side of the head of a shark. That spiracleis an opening to the surface of the animal body which is likea little tube that goes down and opens up into the throat region. And so it appears that the spiracle has developed from the3rd gill slit, the one that was behind the 3rd gill arch. Modernfish don't have a spiracle, so you might wonder why it would beinteresting for us to learn it. The answer is that the middleear cavity and your Eustachian tube, you know the tube that connectsyour middle ear cavity to your throat that when your ears popand you have a cold or you're going up an altitude, that cavityappears to have evolved from the spiracle. It's a little tubethat connects the throat region to the side of your head onlyit has evolved an ear drum over the end of it. So we have a numberof interesting evolutionary stories here that we're going to befollowing, that starts with the neural crest cells and the embryoand go through this early stage, the evolution of jaws in thesefirst vertebrates.
NOW WE'RE ON THE 4TH ILLUSTRATED NOTE OF THIS DATE. The evolutionof lungs. The paleontological evidence is that during the Devonianwhich was the geological period that followed the Silurian. TheDevonian began around 400 million years ago, and is sometimesreferred to as the "Age of Fishes" because the greatestdiversity of fish is known from that period. It appears on thebasis of the fossil record that there were 5 different groupsof Devonian fish that had lungs. Now we tend to think of a lungas being something that we find in a terrestrial vertebrate andnot something that we find in an aquatic vertebrate or a fish. But in fact the evidences that lungs evolved first. In aquaticvertebrates. Now the geologists who studied the same rocks thatthe paleontologists get the fossils out of can tell us that mostof the Devonian fish are found in fresh water habitats. The fossilswere laid down in streams and in lakes, not in the oceans. Andso these 5 groups of Devonian fish were fresh water fish and theyappeared to have all evolved lungs.
Two of these 5 groups I've already told you about, that is ClassAcanthodii and Class Placodermi, the 1st and 2nd groups of vertebratesto evolve jaws. They were probably subject to periodic drought. They were in an area of the world and the climate at the timewas such that there were periodic droughts in that fresh waterhabitat and so what happens if you have a stream and lake systemduring a drought when there isn't a normal amount of rain or snowon the mountains that's draining in there, is the river stopsto flow, the river gets smaller and the lakes start to dry up,they get smaller, the water in the stream in the lakes becomeswarmer and when the water in the streams and the lakes becomewarmer less oxygen can be dissolved and that's going to createa problem for a fish that's trying to live in that water. Andso it appears that lungs evolved as an adaptation to stagnantwater in the fresh water habitat during these drought plaguedtimes of the Devonian.
The 3rd group of fish that I've shown up here is the SuperorderDipnoi. In the Class Osteichthyes which are the bony fish andthat includes all 3 of these other groups below, are 2 major subclasses. The Subclass Sarcopterygii are the fish that have fleshy basedfins and you can sort of see that. It has several different bonesin the base of that fin to make it look kind of like a littlearm, it's not a ray fin like you see on modern fish. And theseanimals are 2 different superorders, the Superorder Dipnoi andthe Superorder Crossopterygii. Both of them were present in theDevonian and both of them had lungs. One group, the SuperorderDipnoi do have a few living descendants. A dozen or so speciesof lung fish. Fish that have both gill and lungs. That's whatdipnoi means, or refers to. Dipnoi means 2 (di) ways of breathing(pnoi). So these Devonian Dipnoi were lung fish and they gaverise to the modern lung fish which are not very important in anevolutionary sense but are fascinating to study because they arecurrently found in fresh water habitat subject to drought andso they are living in a manner that may be very similar to what all of these Devonian animals were that we're dealing with.
The second superorder within the subclass Sarcopterygii is theSuperorder Crossopterygii and within that is an order that wentextinct, the Order Rhipidistia but which has the dubious distinctionof being the group of fish that gave rise ultimately to all ofthe terrestrial vertebrates.
But there was a 5th group of Devonian fish water fish that hada lung and this is in the other major subclass of bony fish, theActinopterygii which means the ray finned fish and the SubclassActinopterygii is divided into 3 major groups and the most primitiveof these is the Infraclass Chondrostei represented by this fossilanimal and the Chondrostians through a series of evolutionarysteps gave rise to the majority of the bony fish that we can finaround us in the world today and those bony fish includes troutand mackerel and tuna, etc., except for maybe the little shark,all of those bony fish of which there are something in the neighborhoodof 30,000 living species are descended from the Devonian Chondrostiansand those animals have lungs. And their lung evolved into a veryimportant structure in the rest of the bony fish, a structurecalled the Swim Bladder. When you dissect the fish in lab afteryour practical you're going to see the swim bladder. It's a gasfilled chamber located right at the top of the body cavity immediatelybelow the spinal cord, immediately below the vertebral columnof the animal and it serves to allow the animal to adjust thedensity of it's body so that it's body is equal to the densityof the water surrounding it. And when an object is in the waterand it has the same density as the water around it, then it haswhat we call a "neutral buoyancy" and it will neithersink nor float. The swim bladder evolved from lungs and so thelungs of these 5 groups of Devonian fish are very important, notonly to the animals that have them because they allow them tocontinue to live in fresh water but because ultimately one ofthose groups of animals for another set of reasons crawled outon land and when they crawled out on land having already evolvedthe lung they were able to continue to survive and in anothergroup of animals that lung evolved into the swim bladder whichis one of the major adaptations of the bony fish.
So 5 groups of Devonian fish, one became a lung of a terrestrialvertebrate and one became the swim bladder in the bony fish. And one is still represented by the lungs of the lung fish.
NOW WE'RE ON THE 5TH PAGE OF THE ILLUSTRATED NOTES FOR THIS DATE. These animals were found in fresh water habitats subject to periodicdrought where the availability of oxygen would be reduced andso it appears that the lungs evolved as an adaptation permittingthese animals to survive in the aquatic environment. How didthey evolve. Summary at the bottom of the page. There is moredetail on the bottom of the page more detail then I expect youto remember. Lungs in Terrestrial Vertebrate that still havelungs and in the Lung fish we study the embryology of Lung fish,the living Dipnoi that have lungs. Lungs developed as pairedevaginations of the gut. Two little pouches grow out of the bottom,the ventral aspect of the gut in embryo of a Terrestrial Vertebrateor in the embryo of a Lung fish. This condition is seen as shownon the bottom of the diagram as representing what is assumed tobe the ancestral condition. That is the way lungs looked and theway lungs developed in these primitive fish that have lungs. It points out that this same condition, with the lungs as pairedventral out pocketings which come to sort of lie along side ofthe gut (that is what this horizontal tube is, the gut ). Thatcondition is also found in the Australian Lung fish and in ananimal called the Polypterus (will be shown in lab). Now it iseasy to see how you can go from that condition, paired ventralout pocketing, to this modern amphibian and on the left you havea cross section. In the center that little balloon is the gutand the one on the side is the lung. In this case it has a lotof jagged lines around the inside of it, which is there to indicateit as a respiratory organ, this a respiratory epithelium, hasalveoli around the walls and it serves as a respiratory organin the lungs in the amphibians. And it is also easy to imagehow you can get from this paired ventral out pocketing to thecondition seen in the South American and African Lung fish, inwhich their lungs are no longer paired, but still develops asa ventral out pocket__, it is still attached at the bottom, onlythe lung has moved all the way around the gut and is now up abovethe gut. Why would natural selection favor a fish that has alung above its gut rather than below its gut? So he doesn't swimupside down. I mean the density of his body is going to be arranged,if you have your lung on the bottom you're going to tend to wantto float belly up so if a fish wants to maintain his normal orientationin the water, natural selection is going to favor having the lungmove around to the dorsal side of the gut which is what it does.
So this is a condition in the African lung fish.
The condition that is seen in the carp which is a fairly advancedbony fish is that it has a swim bladder, a normal jiggly linesthere tells you that it's a lung, but in the carp the swim bladderis attached by an open tube to the side of the gut so we can interpretthis as being an intermediate stage in the evolution of the conditionthat is seen in the most advanced bony fish which is the conditionshown at the top of the page where the swim bladder is above thegut and it's not attached by any kind of a tube. So this isthe condition seen in most Teleosts that's the most advanced groupof fish there's a swim bladder and a little thing tells you thatthere's no pneumatic duct, there's no connection between the gutand the swim bladder. There is in some Teleosts and Sturgeons,the swim bladder is on top and is attached by the pneumatic duct,there are several types of sort of fairly primitive bony fish,the Gar and the Bowfin in which the lung, it is a lung now, ithas some respiratory function, it's still attached by a tube butit's now completely up on the top of the gut and the connectionis to the top of the gut.
So that's a series of conditions that are found in living animalsthat represents the sequence that it is thought this ventral pairedevagination went through to end up in the condition that you seein the most advanced bony fish where the swim bladder is locatedabove the gut _and it's not connected by any kind of a duct tothe gut. A good question would be well how do you get this tobegin with? In other words, this is a kind of a nice evolutionarysequence. It sort of makes sense. But the question is how doyou get this to begin with. And of course we don't have verymuch in the way of evidence that gives us an answer to that questionbut we have a little bit. It turns out there are some modernbony fish that live in waters that is subject to drought and stagnationand these animals when the oxygen content of the water gets tobe very low, will come up to the surface and grab a mouthful ofair and hold it in their mouth cavity where it's exposed to thegills but it's also exposed to the epithelia lining the mouthand physiologists have been able to demonstrate that they areable to absorb some oxygen from that bubble of air in their mouth,directly across the lining of the mouth and so one theory is thatthat holding a bubble of air in your mouth is a very, very earlystep in the evolution of the lung. If you have an animal that'sdoing that, then you can imagine that there might be some selectiveadvantage to having an area of the mouth where the epitheliumis very thin because the thinner the epithelium the more oxygenthe animal can absorb across there. And the you might imaginethat there could be another evolutionary stage where there isa pouch like outgrowth that has a thin epithelium lining becausethat would be better than having thin epithelium in your mouthcause it's subject to having food go over it and damage it. Sowe can sort of imagine an evolutionary sequence that might gosomething like this. If this is a side view of a fish, here hehas his mouth open and here's the inside of his mouth here, andthere's his the beginning of his esophagus and it's going to comedown here and we can imagine that this animal takes up air intohis mouth through bubbles and fish do that when they are feedingat the surface so that could be something that happens fairlyfrequently in the life of a fish but if a fish happened to feedon an insect that was floating on the surface and the water wasstagnant and when he went back down below he had a big bubbleof air in his mouth and he is able to get a little oxygen acrossthen he might survive and that habit of taking that bubble ofair into his mouth might be favored. And then we say well okayso now you have some thin epithelium here that increases yourability to take it up and then you have a little tiny pouch likearea of thin epithelium that increases the surface area availablefor the absorption of oxygen out of the air and then all of asudden you know with a few more million yeas, you have a pairedventral evagination and that's the beginning of the lung. Itstays paired and ventral in terrestrial vertebrates and in thefish, it moves around to the top of the gut and it's attachmentmoves along around the side and becomes up off the top of thegut as well.
There's an experiment you can do that's really kind of amusing. To demonstrate in the carp that it's swim bladder is still attachedto it's gut by this little pneumatic tube, if you take a jar andyou fill it up with water, most of the way with water and youput a stopper in there and you hook this stopper up to a tubeto a vacuum pump, and the carp is sitting here on the bottom ofthe water, he's sitting there motionlessly, very happy, and thenyou turn the vacuum pump on and you suck, you decrease the atmosphericpressure on the water, what happens is that this animal the pressureon his swim bladder is decreased and his swim bladder starts toexpand and he starts to float up here to the top and then whathappens is he kind of aims down and beats his tail and he's swimmingdownward and trying to keep from floating up to the top and thenfinally he burps and you can see it, you see a little tiny coupleof little bubbles of air come out, in fact that's the only timeI've ever seen a bubble of air come out a fish's mouth, but itwill happen in a carp under these experimental conditions. Heburps out just enough air to get his swim bladder back down tothe correct size and then he's sitting there in the water veryhappy, neutral buoyancy and then you open up the valve and letthe atmospheric pressure come back in, the pressure increasesthe swim bladder collapses and he goes straight down to the bottomof the container and then he has an organ inside that swim bladderthat will secrete gas from his blood back into the swim bladderand re-inflate it and then he'll be just fine in a couple of hours. But that shows you the importance of neutral buoyancy and theevolution of lungs and swim bladders in fish.