This is covered in your illustrated notes.
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You can see here the classification and phylogeny of theamphibious animals. The Class Amphibia is divided into 2subclasses. One of these is an extinct subclass known as SubclassLabyrinthodontia which are first known from the Devonian and arethe first tetropods (first vertebrates with 4 legs). They haveevolved from the Rhipidistian-Crossopterygian fish and they arealso the ones that gave rise to the very first reptiles(Captorhinomorphs). There are all sorts of little pictures andnames here for which you are not responsible such as Sauropleura,Protobatrachus and Buettneria.
On the phylogram, the little branching lines are meant to be agraphical representation of the fact that they were a verydiverse group. The pictures and names are there to alert you tothe fact that there is a very extensive paleontological fossilrecord of these very diverse groups of animals.
2nd page of illustrated notes for this date:
Labyrinthodontia appear in the Late Devonian and the last fossilsare known from the very Early Jurassic. Their greatest abundancewas during the Carboniferous and the Permian.
The name of the group Labyrinthodontia comes from the structureof their teeth. The term Labyrinthodontia refers to theappearance of the highly folded outer layer of the tooth which isthe enamel of the tooth. If you cut one of these teeth incross-section, rather than getting a nice round cone as you wouldfor many animals and reptiles, what you would find is that theouter (enamel) layer is very highly folded which makes theoutside of the tooth look like it has a lot of grooves and somepaleontologist thought this looked like a labyrinth or maze-likestructure. It probably served to strengthen the teeth.
Three groups of vertebrate animals have labyrinthine dentition,which is the term we use for this tooth condition.
1) Rhipidistia
2) Labyrinthodontia
3) Captorhinomorpha
Anytime a paleontologist finds some feature in 2 or more groupsof animals, they have 2 possible explanations for it. Onepossible explanation for the presence of the same feature in 2 ormore groups would be convergent evolution such as the Ichthyosaurand the dolphin we talked about earlier having similar body shapeand tails. The other possible explanation is that the animalsshare this feature because they have a common ancestor or becausethey have an ancestor-descendant relationship to one another.This is really the simpler interpretation and so thisinterpretation is the one used in the case of Labyrinthinedentition. This is the strongest single piece of evidence whichshows that the Rhipidistia are the ancestors of theLabyrinthodonts and the Labyrinthodonts are the ancestors of theCaptorhinomorphs.
There were several major evolutionary events that occurred duringthe evolution of the Labyrinthodonts. One of them was theevolution of zygapophyses. Another was the enlargement of thebasal portion of the vertebrate.
The other subclass is the Subclass Lissamphibia and this includesall of the 3 types of living amphibians as well as some limitednumber of fossils of these living groups.
The characteristics that unite these 3 orders that are reallyvery different from one another are the following:
1) The term Lissamphibia means smooth amphibians and it refers tothe fact that these animals have very smooth skin without scales.Recognize that the 2 closest living relatives of the amphibia arebony fish that have scales on their skin and the reptiles thathave scales on their skin. So having smooth skin with no scalesis one of the characteristics that unite these 3 orders in asingle subclass.
2) They have an unusual feature of their teeth. They have what wecall Pedicellate teeth. What that means is if you look at thetooth of a reptile in side view, it will go down into a bony jawsocket and the tooth is one continuous structure. But in theLissamphibia, the tooth has a bony foundation-like piece that isseparated from the emergent part of the tooth by a layer ofconnective tissue. And so Pedicellate refers to the idea thatthese teeth are mounted on a little pedestal.
3) These animals have mucous glands in the skin which help keepthe skin moist.
4) And the absence of scales and the presence of mucous glandsare both functionally related to the fact that all these animalshave significant uptake of oxygen across their skin. They havethe capillary beds very close to the surface of the skin.
5) They are the most primitive group of vertebrates to have a3-chambered heart and double circulation.
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There are 3 orders of living amphibians in the SubclassLissamphibia and the first one is the one that is probably themost like the primitive amphibians and this is the Order Urodela(salamanders and newts).
There are about 300 species of living salamanders and while theyare found throughout the temperate and tropical regions of theworld, the overwhelming majority of them are found in North andSouth America. There is just a few species that are found in Asiaand in tropical other parts of the world.
The habitats of these animals are both fresh water andterrestrial environments. There are some salamanders that livetheir entire lives on land, usually in fairly moist tropicalhabitats which do not have to go into fresh water. There are somesalamanders that live their whole lives in fresh water lakes andstreams and then of course many species that spend part of theirlives in water and part of their lives on land.
In general most species will have 4 legs. You will see that thatdistinguishes them from the 3rd order. They usually have a tailbut they lack an eardrum - they do not have tympanic membranes,middle ears, middle ear ossicles, middle ear cavities and thosevarious mechanisms for sensation of airborne vibrations.
The largest species of salamanders is the Giant JapaneseSalamander which is over a meter in length. It is a fresh wateranimal.
Different species are specialized for feeding on differentthings. Some larval salamanders are insectivores (feeding oninsects and other invertebrates) and some are carnivores (feedingon other vertebrate animals). Some of larvae are scavengersfeeding on dead animal parts. Adult salamanders are carnivoresand insectivores as well.
Courtship involves various kinds of swimming behavior.Frequently, the animals have very brightly colored areas on theirskin that they display in a very stereotyped fashion during thiscourtship. The courtship usually involves the productions ofpheromones - a chemical produced by one member of the species andused to signal something to another member of the species. Notonly sexual information is transmitted by pheromones amongvertebrate animals. The pheromones are produced by glands on themale salamanders that are called hedonic glands.
Fertilization in most species is internal. Some of the realprimitive ones have external fertilization but most have internalfertilization which is achieved without an intromittent organ.The male produces a little structure called a spermatophore andthe male swims through the water displaying in front of thefemale releasing the pheromones from his hedonic glands. When sheis interested, she will start to follow him around and then hestops, settles to the bottom of the pond and deposits this littlespermatophore on the bottom. He swims forward just the rightdistance and stops as she settles over it with her cloacasettling over the spermatophore and the sperm is transferred intoher reproductive tract.
Embryonic development is usually oviparous so after the eggs arefertilized, either internally or externally, they are shed by thefemale and developed externally. There are some advanced speciesof salamanders which exhibit viviparity, so they have uterinemilk and secretions that are ingested by the developing embryoswithin the female's uterus.
As I stated, in most cases the development is external. There isa larval stage which is aquatic and the larva feed on whateverthey can find in the environment and grow up to be largeranimals. When they get to an appropriate size, they undergo ametamorphosis into a sexually functioning adult and in most casesthat adult then is going to be a terrestrial animal. So in mostspecies you start off with an aquatic larval stage, you gothrough metamorphosis and then you have a terrestrial adultstage.
There are stereotypes for a large fraction of the species andthere are lots of variability but for the ones that follow thatsort of typical lifestyle, the larval stage has a number ofcharacteristics which are appropriate to their aquatic life. Someof these characteristics are lost during metamorphosis as thelarva is making adjustments to becoming a terrestrial livingadult and these are:
1) Very weak limbs - tiny limbs that are not strong enough tosupport the body of the animal but of course in the water, thebuoyant forces of the water are taking care of that.
2) Fish-like tail - they have a long tail that has vertebratesand muscles, etc., but it also has fin-like extensions that go upand down from the tail to increase the area making it a betterorgan for propulsion.
3) The larval forms have no eyelids - but the adults do. When youbecome a terrestrial animal, you have to have eyelids that cankeep the cornea and other parts of the eye moist.
4) The larval forms have a spiracle - a little trapped gill slitthat opens up on the side of the head.
5) And the larval forms have external gills - these gills are notyour usual fish gills. These are a new evolutionary development.
All of these features are converted into corresponding adultfeatures or lost. Weak limbs become a little bit stronger. Theylose the little extensions from the tail but they can redevelopthose when they go back into the water to breed. They developeyelids. They lose the spiracle. And they lose their externalgills.
Some species of salamanders as adults are still aquatic animals.Some of them are very strongly aquatic and virtually never getout on land. And there are some adult salamanders that canactually spend a significant fraction of their life in water anda small fraction of it on land. And so in some species ofsalamanders, some of these larval features, not all, are going tobe lost because the adult is going to be fairly highly aquatic.And the term that we use for the retention of some larvalfeatures in a sexually mature adult individual is Paedomorphosis.
The most extreme version of this would be an animal that is soexclusively aquatic that it is going to live all of it's life inthe water as an adult. It isn't going to give up any of thoseaquatic larval features and the only thing that's going to happento the animal during metamorphosis is that it's going to developfunctional gonads which is called Neoteny. So neoteny can beeither defined as the retention of all larval features as opposedto some in a sexually functional adult. Or it can be defined inanother way which is the development of sexual capability in alarval form.
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These are the Order Anura which are the frogs and toads. Anurameans without a tail. This the largest group of livingamphibians. There are 2,000 species roughly. They are found intropical and temperate regions of the world. There are somethinglike 6 or 7 different families within this order and some ofthose families are called frogs and some of those are calledtoads.
Frogs = In general, they are more aquatic, the have smootherskin, and relatively longer hind limbs.
Toads = In general, they are relatively more terrestrial in theirlifestyles. Some of them may be virtually completely terrestrialliving their whole entire lives on land. They generally havedrier rougher skin and they frequently have, while the hind legsare usually larger than the front legs, the 2 sets of legs arecloser to each other in terms of their size.
Of course the anatomy of frogs is very highly specialized andmost of those specializations are adaptations for their mode oflocomotion which is called saltatorial which just means hopping.
They also have a larval stage but in this order, the larvae areall herbivores. The adults are usually insectivores althoughthere are some really big frogs that are carnivores as well.
Now reproduction in the frog involves a variety of differentkinds of calls. The sort of stereotype of reproduction in a frogwould be something like the following: During the springtime whenthere are puddles and streams and lots of water around, the maleswill go to these puddles and since the puddles are usuallylimited in their availability, you may find very large numbers ofmales aggregating in these puddles. They emit calls and they maybe for any one of 3 different purposes:
1) Advertisement calls = this is the most obvious one. Thesecalls are intended to be heard by the females of the samespecies. These are calls that the males are generally saying,"Here I am, a reproductively confident male sitting on alovely puddle of water. Why don't you come and we'll have a goodtime".
2) Territorial calls = These are intended to be heard by othermales of the same species. The males want to guard a section ofthat pond as being their territory so that any females that comeinto that territory are going to breed with them. So the maleswill set up their territories within the pond using theseterritorial calls and the intended recipients of those signalsare the other males.
3) Release calls = Tells other males to get off of him. Duringreproduction, the male is on the top hugging the female by hisfront legs and this is a process called amplexus. So basically,if another frog goes through this guy's territory and it's aboutthe same size, then he jumps on the back of it and grabs a holdof it. The problem is there are other males frogs that wonderinto this guy's territory and so the guy on the bottom has thiscall which essentially translates as, "Get off my back,there's no future in this relationship".
So those are the 3 different types of calls that frogs emit.
Now in most species of frogs, fertilization is external followingthis amplexus but there are a couple of species in whichfertilization is internal where the male actually has anintromittent organ and deposits the sperm inside the female'sreproductive tract. In most species of frogs, development itselfis external; that is, most species of frogs are oviparous butthere's quite a bit of variation and there are again severalspecies of frogs in which viviparity exists and nutrient transferis achieved by uterine milk. There's a frog in the Northwesternpart of the U.S. that's called the "Tailed Frog" whichhas this tail-like thing which is in fact an intromittent organwhich he uses to transfer the sperm into the female'sreproductive tract.
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The last order is the Order Apoda. There are 160 species. Theseanimals are also known as Caecilians. They are very unfamiliar tomost of us. They are found in tropical regions of the world. Thetranslation of Apoda means no feet and it reflects probably totheir most obvious anatomical characteristic which is thecomplete absence of any limbs. These animals have a habitat whichis technically described as fossorial and it refers to the factthat these animals live their lives buried within the soil. Theyactually burrow through the soil wedging their way along. Theyare worm-like animals although the largest of them get to be apretty good size of up to 4½ feet in length and an inch indiameter.
Anatomy: They have very long ribs, no limbs, and they have notail. The very back end of this animal's body is where we findthe opening of the digestive tract which technically is called aterminal anus; the anus which is the opening of the digestivetract at the terminus or extreme back end of the animal's body.They are also blind with vestigial eyes; they have just a smallbasically non-functional remnant of an eye that is completelynon-functional. They can detect light and dark but they cannotform images. They have a continuous layer of skin over the eye.
This general set of characteristics are what we tend to find whenwe're looking at fossorial vertebrates. The elongation of thebody, shortening of the tail (or complete loss of the tail as isthe case here), the loss of limbs, and the eyes become vestigial.
They feed on worms and other small invertebrates that theyencounter in the soil. Their reproduction is quite varied. For afairly small group, there's quite a bit of variability. There aresome species that are oviparous and they have externalfertilization. There are some which have internal fertilizationand are viviparous.
The critter at the bottom of this page looks like an animal witha strange head-dress. This is a member of the Genus Typhlonecteswhich is the best studied of the viviparous species and isactually an embryonic typhlonectes. What was found was a 50centimeter long female and just prior to giving birth, she wouldhave nine 20cm. babies inside of her uterus and their combinedbody weight would be 60% of her total body weight counting them.So the babies actually outweigh her just prior to birth. Thesebabies have these strange head-dresses which are to increase thesurface area available for the exchange of respiratory gases.Nutrients are transferred in the form of uterine milk. And whenthe egg leaves the ovary, there's only enough nutrients to fuelthe development to about 3 cm. in length and all the rest of thegrowth from 3cm. to 20cm. in length is fueled by nutrientstransferred in the form of the uterine milk. So even though thereare very unfamiliar to most of us, this is a really veryinteresting group of animals that have been studied fairlyextensively.