Today's lecture is covered in the illustrated notes. Livingreptiles are found in 3 of the subclasses of the Class Reptiliaand the first of these are the turtles - Order Testudinata in theSubclass Anapsida.
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There are about 250 species of turtles. They are found intemperate and tropical regions of the world. And they cover thefull range of habitats from terrestrial to fresh water to marine.
The oldest fossil turtles are known from the Triassic althoughthey are thought to have diverged from the Captorhinomorphs earlyon in the evolution of the Captorhinomorphs back in theCarboniferous.
ANATOMY: All turtles are recognizable as turtles, not mistakenwith anything else and probably the presence of the shell, whichencloses most of the their bodies, is the most obvious thing thatmakes that distinction possible. The upper part of the shell iscalled the carapace and the lower part is the plastron and bothof these structures are composed of an outer layer of plates ofkeratin which is the same material that the horns of a cow aremade from so anything made of keratin is sometimes referred to asbeing a "horny substance". These are horny plates. Andthese plates are composed of dermal bone which can grow as theanimal enlarges. The growth occurs along the edges of the platesand the plates are laminated in such a way as to not have thesutures be in the same place. In other words, if you have thelower layer which has sutures where growth can occur, then theouter layer will have non-overlapping sutures. So these suturesbetween the bony plates do not occur in the plates as the suturesbetween the keratin plates.
In a few instances like a group of turtles called the "boxturtles", the plastron can actually fold up and has a hingeand the 2 layers do have their sutures in the same place.
The scutes which is what the individual box of keratin arecalled, form from the epidermis of the animal and the bony plates(inner plates) are formed within the dermis within the dermallayer of the bone. So they are like the dermal bone that we'veseen in the skull of the vertebrates.
Some of the bones of the internal skeleton are included in thesebony plates so for example the clavicle, a structure called aninter-clavicle and some abdominal ribs are included in some ofthe bony structures but other bones such as the ribs and thevertebrae are not included in that layer of bone. So in thecross-section of the diagram on this page, you can see thevertebra and the ribs which are not a part of the layer of bonyplates.
I think the most unusual thing about the anatomy of the turtlesis the fact that the girdles for the limbs are actually locatedinside the ribs. All other vertebrate animals have their shouldergirdles outside their ribs. And that kind of major change in therelative position of 2 major structures is extremely rare amongvertebrate animals.
Turtles of course are toothless, they have a keratin beak andmost but not all species of turtles have the ability to pulltheir heads and legs inside the shell. There are actually quite afew species that have secondarily lost that ability and it'susually associated with a significant reduction in the size ofthe shell which is usually associated with adaptations which makethe animal relatively more agile. So there are some kinds ofturtles that are fairly small but which are very active predatorsand in order to be active enough to chase and catch other fastmoving vertebrate animals, they have reduced their shells. Likethe snapping turtle, which does not have a big enough shell thathe can pull his head and legs inside for protection. He is soaggressive that he doesn't have to resort to hiding for defense.He just simply threatens any animal that tries to give him anytrouble.
There are some variation in the shape of the shells as well. Youcan see this tortoise that's at the top of the page here has ahigh domed shell, that's what you're going to see in aterrestrial turtle.
The sea-turtle there at the bottom has a much more stream-linedand flattened shell and that is an adaptation to locomotion inwater.
Then you have animals like the snapping turtle which the shell isreduced as an adaptation to increased agility which is associatedwith predation as well.
NATURAL HISTORY: The feeding habitats of turtles cover the usualrange. Some species of turtles are largely if not exclusivelycarnivores frequently feeding on fish. Some species areherbivores feeding virtually exclusively on plant material. Andsome are omnivores taking mixes of both plant, animal materialand sometimes invertebrates as well.
Courtship is fairly elaborate in the turtles and it evolves avariety of different kinds of mating calls. Turtles make noisesand they also produce pheromones which are used to signal betweenprospective mates.
Fertilization is internal by means of a single copulatory organ.That distinguishes the turtles from the largest group of livingreptiles in which the copulatory organ is a paired structure.They lay amniotic eggs as do all reptiles. All turtles areoviparous. In some species the egg has a very thincalcium-carbonate type of shell and in other species of turtlesit is a leathery covering for the egg. The eggs are usuallyburied in cavities in the sand on a beach in the case of a marineturtle and very frequently the eggs will all hatch simultaneouslyin a very short period of time and the combined digging activityof all the babies is necessary for them to be able to escape fromthis cavity that they are in, in the ground. The female has tocome up far enough up on the beach to lay the eggs above the hightide mark so that they don't become inundated with water.Amniotic eggs don't develop in water. And there are in somelocations predators that specialize in feeding on these babyturtles when they get out and so there's this kind of feedingfrenzy of seagulls and other kinds of animals that are standingaround at the right time of the year waiting for the baby turtlesto all hatch at the same time and they all make a break for thewater and a few of them manages to get through.
Another thing that's really bizarre about turtles is that thetemperature at which the egg is incubated during the 5th week ofincubation determines the sex of the turtle. What we are used toin human beings and in most vertebrates is that the animals havesex chromosomes and so the sex of the individual is determined bythe genetic constitution of the sperm that fertilizes the egg.But there are a number of different groups of vertebratesincluding some species of fish as well as these turtles in whichthere are no sex chromosomes. All the genetic informationnecessary to be either a male_ or a female is included in thegenetic make-up of the every individual. Some species of fish canactually change sex from one month to the next in order to beable to breed. But in turtles, sex is determined for anindividual by the incubation temperature during the 5th week ofincubation. They have an incubation period which is about 8 o 9weeks long and about half way through that time, if thetemperature is above 30 degrees the individual will be a male. Ifit's below 30 degrees, it'll be a female.
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Another familiar group of living reptiles are the members of theOrder Crocodilia. These include crocodiles, alligators and somesmall guys called the Caiman.
There are only a couple of species of alligators. The totalnumber of species in the order is 21. And they are tropical intheir distribution.
The 2 species of alligators are fresh-water animals. One of themfound in North America - southeastern part of the U.S. And theother one is found in China.
Most of the species are crocodiles and there are both fresh-waterand marine crocodiles.
The first fossil crocodiles are known from the Triassic so that'sabout the same time as their ancestors the Thecodonts appear. Andyou'll remember the Thecodonts were strongly bipedal and actuallyif you'll look at a crocodile you can see that it's hind legs area little bit larger than it's front legs and that may reflectthat bipedal ancestry.
ANATOMY: The tail of the animals are laterally compressed makingit more sort of fish-like in it's appearance and it's aneffective mechanism for swimming. They have an eardrum which iscovered by a scaly movable flap that keeps the water away fromthe eardrum when they are underwater. And the presence or absenceof an eardrum is another thing that we can look for when we'relooking at different species/groups of reptiles because snakesfor example don't have eardrums. But it's hard to see the eardrumon a crocodile because it's behind this movable flap.
The nostril which is located out at the tip of the nose has aring of muscle that can close the nostril off so that the nasalcavity does not fill up with water when it's underneath thesurface.
And they have evolved a secondary palate. The advanced Therapsidshad a secondary palate which is a plate of bone that separatesthe nasal cavity from the oral - mouth cavity - and allows theanimals to breathe while they are eating. And the same generalcause is assumed to be the factor leading to the evolution of thesecondary palate in the crocodiles. These animals don't chewtheir food the way the advanced Synapsids did but the way thatmany of these animals feed is by grabbing hold of an individualprey and then pulling it into the water and spinning very rapidlyholding onto that leg hoping to either drown the animal or tearthe leg off. So they do spin fairly long periods of time with thefood in their mouth and need to be able to breathe at the sametime.
The crocodiles have another interesting feature and that is theyare the only group of living reptiles that have a 4-chamberedheart. All the other reptiles have a 3-chambered heart. And ofcourse the only other living descendants of the Thecodonts arethe birds and they have a 4-chambered heart as well which haslead some people to speculate that perhaps all of the Archosaurs(which are the dinosaurs) may also have had 4-chambered hearts.
The largest living member of the order is the Salt WaterCrocodile which is found living in the oceans in the Indo-Pacificregion. These animals reach a maximum body length of about 7meters.
NATURAL HISTORY: All crocodiles are carnivores and when I saycrocodiles, I mean all members of the Order Crocodilia. Not justthe family of crocodiles.
The 2 types of alligators have a broad snout. And this is a topview of a fish-eating crocodile with a narrow snout. But thatdifference is not consistent because there are members of thefamily that includes the crocodiles that have a broad snout andthose crocodiles that have a broad snout do not feed on fish,they feed on birds and mammals. So it appears that the shape ofthe snout is not something that's a consistent difference betweenthese 2 major groups. But it is something that reflects thefeeding adaptations of the animals. So the fish-eating crocodilesare the ones that have a long, narrow snout. And Caiman also havethis kind of long, narrow snouts and they are fish-eaters.
Courtship in crocodiles also involves loud mating calls andterritory defenses. The males will stake out a section of theriver and exclude other males from their area. Fertilization isinternal as is the case in all amniotes. They have a penis whichis a ventral outgrowth of their cloaca which is used to transferthe sperm. Mating takes place in the water.
Crocodiles produce amniotic eggs with a very heavycalcium-carbonate shell which has a different microscopicstructure than the shell of a bird. All species of this order areoviparous. They make a nest usually made up from rottingvegetation and lay a fairly large number of eggs into that nest.The significance of the fact that the vegetation is rotting isthat the bacterial decomposition produces heat which warms theeggs and accelerates their development. Parents will usuallydefend the nest, chase predators away from the nest and have beenobserved to help the young hatch. When the young are ready tohatch out of the egg (50 or 60 days) they start making littlescratching sounds and the female crocodiles have been observed toactually help them break out of the shell. And then they willprovide really remarkable and extensive parental care. They takethe hatched babies away from the nest which is usually on theriver bank some place, carry them out into the middle of a riverwhere they are going to be safe from many of the kinds ofpredators that might be able to get at them and in the case ofthe American Alligator, the young will stay with their mother for2 years. And the mother will help them avoid predators and findfood, share the food with them, protect them from predators andso they have a really fairly complex system of parental care.
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Now the rest of the reptiles are placed in a single order whichis in the Subclass Lepidosauria. There are a couple of extinctorders in that subclass but all the living ones are placed in theOrder Squamata. Squam refers to "having a flat plate orscale". So the name of this group simply reflects the factthat they are reptiles who have scales in their skin which isn'ta particularly unusual feature.
The most primitive member of the Order Squamata is a singlespecies which is placed in the Suborder Sphenodontia. Now thereare numerous extinct species from this same suborder but this issuch an unusual animal that it's the only living member of it'ssuborder. Common name of this animal is the Tuatara. It's foundoriginally on New Zealand. It's now restricted to a couple ofsmall islands off of New Zealand.
They are about half a meter or so in length. They basically looklike just about any other kind of lizard that you might seerunning around. But it has some very primitive features of it'sskull.
ANATOMY: Primitive features in skull. It has a pineal eye whichis a light sensitive structure on the top of the head. It doesn'thave the ability to form an image like the paired eyes but it issensitive to light and it's hooked up to the brain and it'sinvolved in their ability to measure the seasons and know whenit's winter and when it's summer and time to breed and so forth.
They are also active with a very, very low body temperature forreptiles. They are nocturnal which is not unusual among modernreptiles. But what is unusual is that when they are active, theyhave a body temperature as low as 8 degrees Celsius which is verylow even for a nocturnal reptile.
They are carnivores and insectivores feeding on small vertebrateprey as well as on insects. And in captivity, they_ have beenknown to live for as long as 70 years. That's really long for areptile of this size.
They are oviparous producing an amniotic egg which has anunusually long incubation period. It takes a year from the periodone of these eggs is laid until it hatches. Most reptileincubation periods are in the neighborhood of 45 to 60 days. sotaking a year to complete development is just unusual.
Fertilization must be internal but they have no copulatory organ.Apparently transferring a sperm by a cloacal kiss mechanism.
At the bottom of the page is another group of really unusualreptiles. This is the Suborder Amphisbaenia. Common name forthese animals is "worm-lizards". There are 130 speciesand are not very well known here in North America. Theirdistribution is tropical and sub-tropical. They are found inSouth America as well as in Africa. But they are not found intemperate regions so we don't usually encounter them.
These are very highly specialized fossorial animals spendingtheir lives burrowing through the soil.
ANATOMY: And as we frequently see with fossorial vertebrates,they have vestigial eyes covered with a continuous layer of skin.
They do not have a tympanic membrane but the stapes abuts againstthe jaw and so it appears that vibrations in the ground aretransmitted through their jaw through this stapes and into theinner ear structures. So they are sensitive to vibrations.
Most of them show no external signs of limbs at all but many ofthem do have vestigial limb girdles.
The skull is very highly specialized in it's shape and structureto produce a hard ram that they can use to drive themselvesthrough the soil. They feed on various kinds of invertebrates(worms) that they encounter in the soil.
Their reproduction is not very well known but they appear to beoviparous. So they are a lot like the Caecilians - they are thegroup of amphibians that share many of these same features andthose would be a good example of convergent evolution becausethey have ada_pted to a fossorial mode of existence and reallythe major thing that distinguishes them is reproduction. Thereptiles are going to produce amniotic eggs, this particulargroup is oviparous. Whereas with the amphibians some of those areviviparous.
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The overwhelming majority or at least the biggest single taxonamong living reptiles is the Suborder Lacertilia. These are thelizards. Very diverse group of animals. There are 3,000 livingspecies of lizards approximately.
They have a world-wide distribution and they inhabit a widevariety of different habitats.
HABITATS: The majority of them of course are terrestrial livingon the ground. But there are arboreal lizards that live in trees.Many iguanas live up in the trees. There is one that we can sayis aerial. It doesn't mean it's capable of powered flight but forexample the Draco has this very, very long ribs that extend outfrom the side of the body and it supports a membrane and it canfold those ribs back so that the membrane that is flat againstthe body can be opened and out. And the guy lives up in trees,jumps out of the tree, spreads his ribs out to make this greatbig flat disk thing and goes gliding through the air. There aresome of them that are fossorial which is a legless lizard and itburrows through the soil. And it has many of the same adaptationsas the worm-lizard. And then there also are some marine lizards.It feeds on seaweed on the Galapagos Island.
The oldest fossil lizards are known from the Triassic. And mostmodern lizards are small - 80% of all living species of lizardsweigh less than 20 grams which i s the size of a small mouse.
However the largest of all the living lizards is a good sizedcritter - it's called the Komodo Dragon. There is a family oflizards called the Varanid lizards and the Komodo Dragon is alsothe largest of all the Varanid lizards and this guy gets to be 3meters in length and weighs in at 75 kilograms (160 pounds). Theyare very active predators. All Varanids are active predators.They can run fast, they can sustain a long extended run. TheKomodo Dragons have been studied because they are suchinteresting large animals. They can kill goats, small humanbeings, they stalk and kill deer, they've even been known toattach water buffalo. They are very ambitious, intelligentpredators.
Snakes are evolved from lizards and probably from a group offossorial lizards and so there are a number of features which areused to distinguish snakes from lizards. Of course, the singlemost obvious thing is that no snakes have legs and most lizardsdo but there are some legless lizards and those are the oneswhere this distinction becomes important.
One of the features of both lizards and snakes is an unusualamount of flexibility within the skull itself. Rather than havingthe kind of very solidly fused skull that you have looked at inthe sea turtles and the snapping turtles, the skulls of lizardshave a number of extra joints between bones that are very solidlyfused to one another in the turtles. And that particular featureis called skull kinesis. Kinesis just means movement and lizardshave a modest amount of skull kinesis - the front end of thesnout can move relative to the brain case. Basically what thatdoes is allows them to open their mouth wider so they can swallowsomething that's fairly large.
Lizards also have movable eyelids just as we do. But snakes donot - they never blink.
Lizards have an obvious external auditory meatus (eardrum) andyou can see the eardrum in that opening at the base of which itis located in lizards. Snakes do not have eardrums and they donot have an external auditory meatus.
Many species of lizards have the ability to shed their tails andthe technical term for shedding a body part is autotomy. Theyshed that tail on purpose to avoid predation. They willregenerate the tail but it won't be the same as before.
Lizards have paired sensory structures located in the roof of themouth that are called Jacobson's Organs. And these Jacobson'sOrgans are accumulations of sensory cells that are chemo-sensorycells. They respond to chemicals and so it's like a sense oftaste only very highly localized. It's a very sensitive sense oftaste and the animal will stick it's tongue out which has 2separate tips, touches it on the ground, pulls the tongue backinto the mouth and sticks the 2 separate tips up into theseparate paired Jacobson's Organs, transferring molecules thatthey picked up on the tip of the tongue up there and they can usethis to track prey or to identify potential mates. Both snakesand lizards have forked tongues.
There are quite a few species of legless lizards and they can bedistinguished from snakes because of the fact that they have anexternal auditory meatus.
Another difference between legless lizards and snakes is thatthey both have greatly elongated bodies, but the legless lizardshave elongated their bodies by increasing the number of tailvertebrae or the length of the tail. In a legless lizard the tailis usually 2/3rds or more of the total length of the animal'sbody whereas in a snake the tail is usually fairly short - lessthan a quarter of the length of it's body and many times a lotless than that. So it appears that greatly increasing the lengthof the body is necessary if you're going to be legless and thequick solution of that in a evolutionary sense is to increase thelength of your tail. But given enough time, the ultimate solutionis to lengthen the part of the body in front of where your legswill be located and shorten the tail back up again.
Now why does leglessness evolve? It's associated with fossorialadaptations and many times it's just simply associated withliving in a very dense grass habitat. In fact there may even be anecessary preadaptation to becoming fossorial. Many species oflegless lizards are not fossorial, they just live in prairie-likehabitats where the grasses grow very thick.
NATURAL HISTORY: Natural history of lizards is very diverse. Mostof them are insectivores or carnivores. Probably most of them areinsectivores feeding on insects but there are a fair number ofmedium sized lizards that feed on other lizards. Some will feedon small mammals and birds and some of the larger ones areherbivores like the dessert lizards.
Courtship involves various forms of behavior. Color markings thatdevelop during the breeding season are frequently involved in thebehavior. There's frequently territoriality. The head-bobbing,movement that you can sometimes see is a threat against anothermale to tell him to stay away from the territory.
Fertilization is internal by means of paired copulatorystructures that are called hemi-penis.
They produce amniotic eggs and in the oviparous species, the eggshave a leathery covering although the full range of developmentalmodes is exhibited. There are a whole series of different speciesof lizards that are viviparous and ovoviviparous.
There is almost no known examples of any parental care even inthe viviparous species.
Let's start by finishing up the Suborder Ophidia. These are thesnakes. This is a suborder within the Order Squamata, within theSubclass Lepidosauria. There are almost as many snakes as thereare lizards. There are 2,700 species of snakes.
They are found in the tropic and temperate regions of the worldand they inhabit a very wide variety of habitats. The majority ofthem are terrestrial; they simply live on land and that means anyone of the wide variety of different habitats there. But thereare arboreal snakes - ones that live in trees. There arefossorial snakes that are burrowers. There are also quite a fewthat live in sand spending their whole life burrowing through itand never come out on the surface of the sand. There are severalfamilies of marine snakes - sea snakes - the most highly venomousof all the snakes. There is also a gliding snake which isdorsal-ventrally flattened that live in trees - they strike outand go gliding through the air which they do to get from one treeto the next.
The oldest fossil snakes are known from the Cretaceous, that'sright at the end of the Mesozoic era, so that's really the mostrecent of the sort of oldest fossil base that we've had so far.And that animal is the boa constrictor type of snake. So the veryfirst snakes to evolve are fairly recent and they are the type ofsnake that kills it's prey by strangulation - the constrictors.
Snakes range in size from about 20 cm. (smallest) up to thelongest (which is not the heaviest) which is 10 meters long (30foot).
Snakes of course are legless although some of the primitivespecies do have vestigial limbs. Most snakes don't have vestigiallimbs but quite a few of them have vestigial limb girdles - thebony structures within the axis of the body that formally attachto the limbs that are now completely missing.
And as an adaptation to this particular mode of locomotion thatwe frequently see is the significant increase in the length ofthe animal's body. Different species of snakes can have anywherefrom up to 160 to 400 vertebra so that's a tremendous increase inthe number of vertebra and the vertebra are much more complexthan the vertebra of other vertebrates in that they haveadditional articular processes in addition to the zygopopheses.They also have additional processes to which the muscles attachand the muscles of the snake are incredibly more complex in theirnumber and in their arrangement than are the muscles of othervertebrates.
They have a very highly kinetic skull - having segments withinthe skull that can move relative to one another - and you'reprobably aware of the fact that a snake can unhinge its jaw andswallow something that's quite a bit larger in diameter than thesnake is. The skulls of snakes have 8 separate segments that canmove relative to one another and that's as opposed to what you'veseen in the cat for example where they just have a skull and alower jaw. In fact there's only a joint between those 2 segments.
Some of the other characteristics that distinguish snakes fromthe lizards is that snakes do not have an eardrum (tympanicmembrane) and no eyelids - the eye is covered with a clear scalethat is called a spectacle. They do not have any of the musclesattached to the eye which allow most other vertebrate animals tomove their eyes around in the head. Their eyes cannot be moved atall.
Snakes have only a single lung which is very, very long andnarrow and very frequently the most posterior portions of thatlung, at least in sea-snakes, are not ventilated and are notcapable of exchanging respiratory gases. You don't haverespiratory epithelium in the back ends of this very long andnarrow lung. In the sea-snakes, it's serving as a hydrostaticorgan - like a swim bladder of a fish.
The glottis which is the slit-like opening at the front end ofthe trachea in a snake is protrusible so they can actually stickthat out of their own mouth so that when they are in the processof very slowly engulfing a large prey item, they can actuallystick this little tube-like structure out of the bottom of theirmouth and continue to breathe while they have the rest of theirmouth filled up with the body of the big prey. So that's kind ofa different solution from the secondary palate.
In spite of the fact that snakes have no tympanic membrane, theyare not deaf. Very limited range of frequencies can be heard -they can hear sounds better than frogs can. I'm not aware of whatis the basis for their ability to do that but it's possible toshow that their inner ears do detect some frequencies of soundvery well. They are also sensitive to and pick up vibrations inthe substrate that they are sitting on.
The best hypothesis as to the evolution of the snakes is thatthey evolved from a species of fossorial lizards. There's quite abit of evidence that's consistent with this idea. And many ofthose lines of evidence have to do with various aspects of theanatomy and embryology of the eye. Usually fossorial animals havevestigial eyes. The eyes of snakes are more unlike the eyes ofany other group of vertebrates than just about anything elseknown among all of the vertebrates. The eyes of snakes are reallystrange in terms of details of their embryology and details oftheir anatomy. How they are formed is different - embryologicaldevelopment of the eye is different. Not having an eyelid, notbeing able to move the eyes are a couple of things that I'vealready mentioned which are peculiar to the eyes of snakes.
Another thing is the way in which a snake focuses on things thatare near and far away, that process is called accommodation. Theability of the eye to change it's focus from something that'sclose to something that's far away. What's happening in all othervertebrates is that the lens is changing it's shape, at least inreptiles, birds and mammals. When you focus on something that'sclose to you, the lens gets more rounded. When you focus onsomething that's far away, the lens which is pliable becomesflattened out. The actual mechanism of that change in shape isdifferent in mammals and birds but in snakes, accommodation isachieved by moving the lens forwards and backwards. The lensitself is not flexible, it cannot change shape but snakeaccommodate by moving the lens within the eye itself to bringthings into focus on the retina. They also have a completelyunique structure to the cones which are the structures within theretina that are responsible for color vision and so it seemslogical to assume that the reason that the eyes of snakes are sodifferent is that earlier in the evolutionary history of thegroup they had a vestigial eye. And that's an interesting andimportant sort of evolutionary insight. In this case, what we'resaying is that if an organ becomes vestigial, a re-evolution ofthat organ might occur but it's not going to exactly retrace thesequence of events that lead to the eye becoming vestigial and soit's not going to end up with exactly the same embryology andanatomy that the animal's ancestors had back when they had eyes.It comes up with a functional eye but it follows a differentsequence of developmental steps and different solutions to theold problems.
NATURAL HISTORY: Of course they are all carnivores. Feeding on avariety of different kinds of vertebrate food. Usually snakes arefairly specialized in the kinds of food they take. There aresnakes that feed on other snakes, snakes that feed on lizards,snakes that feed on birds or mammals or fish. Even in the samehabitat you'll have one species of snakes that feed on one kindof vertebrate prey and another that feeds on another kind ofprey.
The most primitive mechanism of killing prey in snakes isconstriction. That method requires the snake to put itself atrisk of a biting vertebrate like the rat. So what seems to haveevolved is the evolution of various kinds of poisons (toxins)that are used by the snakes and in various kinds of toxininjection mechanisms.
Now the poisons are basically broken down into 2 classes, 2basically different types of toxins:
1) Hemotoxins - They are proteolytic enzymes, that is they arevery closely related to the digestive enzymes that break downprotein. In fact it is thought that these hemotoxins may haveoriginally evolved not so much to kill the prey but to speed theprocess of digestion because snakes don't chew their food. Snakesare going to swallow that whole food item down and the digestiveenzymes can only attack from the outside so the process ofbreaking down that big single chunk of food is going to be veryslow. If on the other hand, the animal could inject the kinds ofenzymes, digestive enzymes, that breakdown protein into the bloodof the animal before the animal's heart stops beating, then itwould circulate these digestive enzymes throughout the entirebody of the prey species and those digestive enzymes would start"eating" and breaking down the protein from the insideout and that would greatly accelerate the process of digestion.This also causes the prey to die so the snake doesn't have tohold on for so long. This toxin is very painful and is not verylethal to humans but it can produce permanent tissue damage.
2) Neurotoxins - These toxins block the transmission of actionpotentials from nerve to muscle. And what that does to the animalbesides keeping it from running away is that it also keeps itfrom breathing and so neurotoxins can act very quickly and theyare usually painless. This toxin is very lethal and the mostvenomous of all snakes are the sea snakes and they produce thistype of toxins.
Reproduction in snakes covers the full range of modes ofreproduction that we see in all the lizards from oviparous toovoviviparous. The most advanced snakes are the rattlesnakes andthe sea snakes and they are viviparous.