Amniote origins and classification

Amniote origins and classification
The possession of a shelled egg unites the mammals, birds and reptiles into a monophyletic group the amniotes. The shelled egg freed the amniotes from the need to reproduce in water that hampered the amphibians ability to spread into harsh environments.

The Amniotic egg The amniotic egg is hard shelled and is called an amniotic egg because the embryo develops within a sac called the amnion. The embryo feeds on yolk from a yolk sac and deposits its waste into another sac called the allantois. The allantois and another membrane the chorion together lie against the shell, and being richly supplied with blood vessels, exchange gases with the outside through the pores in the shell.

The Amniotic egg Unlike amphibians amniotes lack a larval stage and after hatching develop directly into the adult form. The evolutionary origins of the amniotic egg are unclear because early amniote fossils are scarce and eggs especially so. The oldest known eggs are from the Early Permian and were probably laid by a Pelycosaur (early primitive synapsids e.g., Dimetrodon. This lineage ultimately gave rise to the mammals).

The Amniotic egg It has been suggested that the earliest amniotes were probably amphibious of semi-aquatic as were their immediate amphibian ancestors. They probably inhabited quite humid environments and eggs may have been laid out of water initially perhaps to reduce their risk of predation. Gradually eggs may evolved to have become less vulnerable to dessication.

There is considerable disagreement between cladistic and traditional classification of the amniotes. Traditional classification recognizes three classes: Reptilia: reptiles Aves: birds Mammalia: mammals

Because the class Reptilia does not include all the descendents of their most recent common ancestor (i.e., the birds) the reptiles are a paraphyletic group. Birds and crocodilians share a most recent common ancestor and thus form a monophyletic group (the Archosauria), which includes the extinct dinosaurs, but neither is more closely related than the other to the members of the Reptilia

Traditional classification considers birds because of their endothermy and feathers to be members of a different grade to the crocodilians and reptiles and so places them in their own class the Aves. Cladistic classification in contrast groups the amniotes on the basis of common ancestry.

One of the major characteristics used to classify the amniotes is the structure of the skull. The stem group of amniotes diverged into three lineages in the Carboniferous period (approximately 350 mya). These were the synapsids, anapsids and the diapsids.

Anapsids, synapsids and diapsids
These three groups are distinguished from each other by the number of openings in the temporal region of the skull. Anapsids (which include the turtles and their ancestors) have a solid skull with no openings.

Anapsids, synapsids and diapsids
Synapsids (which include the mammals and their ancestors) have one pair of openings in the skull associated with the attachment of jaw muscles. Diapsids (lizards, snakes, crocodilians, birds, and ancestors) have two pairs of openings in the skull roof.

Anapsids The anapsids are characterized by having no temporal opening behind the eye sockets. They are represented today by the turtles a group that has changed little since it evolved about 200 mya.

Synapsids The synapsids diverged from the Sauropsida (anapsids and diapsids) and radiated into a diverse group of herbivores and carnivores collectively named the “Pelycosaurs” (although that’s a paraphyletic group).

Synapsids Pelycosaurs looked lizard-like and include Dimetrodon (a predatory “dinosaur” you may be familiar with), which possessed a large sail on its back a characteristic of many pelycosaurs, which probably played a role in thermoregulation.

Edaphasaurus (left) an herbivorous pelycosaur Dimetrodon (below and below left) a carnivorous pelycosaur. About 11 feet long; mya)

Synapsids The pelycosaurs were the dominant group of the Permian period, but disappeared in the Great Permian extinction (approx 245 mya). During the Permian a synapsid lineage the therapsids diverged from the Pelycosaurs. This lineage is the one that gave rise to the mammals during the Triassic period

Fig 18.1

Therapsid to mammal transition
A series of evolutionary changes occurred in the therapsids that were passed on to their surviving descendants the mammals. These included: an efficient upright stance with the limbs positioned under the body rather than sprawled to the side. Homeothermy: there is fossil evidence that the therapsids evolved homeothermy. Cross sections of bones show Haversian canals, which are characteristic of fast-growing, warm blooded animals.

Additional evolutionary changes in the therapsids include: Diaphragm: there is indirect fossil evidence in the rib shape of therapsids that suggests they possessed a diaphragm another unique mammalian characteristic. Heterodont teeth: Differentiation of teeth into multiple specialized types. Secondary bony palate: separating nasal from oral cavities. Turbinate bones in nasal cavity: increase retention of body heat.

Additional evolutionary changes in the therapsids include: Three inner ear bones and a single jaw bone. An excellent series of fossils over about 40 million years documents the transition from the multi-boned jaw of pelycosaurs to the single dentary of mammals. During this transition therapsids evolved a double jointed jaw and eventually two bones from the original pelycosaur joint were incorporated into the inner ear.

First mammals The earliest mammals first appear in the mid-Triassic (about 210 mya) and most were small mouse-sized animals. For about 150 million years they lived in a world dominated by the dinosaurs and underwent large scale diversification only late in the reign and rapid evolution of large body size only after the disappearance of the dinosaurs in the Great Cretaceous extinction 65 mya. Morganucudon legacy/college/levin/ /chap_tutorial/ch12/images/le12_60.jpg

Diapsids The third lineage derived from the stem amniotes was the diapsids. The diapsids split into two major lineages the Lepidosauria (which includes the Tuatara, modern snakes and lizards) and the Archosauria (which includes the extinct dinosaur lineages, crocodilians and birds).

Differences between reptiles and amphibians
Reptilian skin is dry and scaly, which limits water loss. The reptiles’ amniotic egg frees reptiles from the need to lay eggs in water. Thus they can occupy much drier habitats.

Differences between reptiles and amphibians: Reptilian jaws
Reptilian jaws are more powerful and can apply a crushing grip. The openings in the skull provide additional surface area for muscle attachment allowing greater pressure to be exerted. In snakes, skull and jaw flexibility allows very large prey to be swallowed.

Differences between reptiles and amphibians: Dentition
With the exception of turtles which have a horny beak (sometime serrated) all reptiles possess teeth and many have them on both the palate and the jaws.

Python teeth

Most reptiles have homodont dentition, but partial heterodonty occurs in snakes and a number of lizards. Monitor lizards have incisors, canine-like teeth and molars.

Komodo Dragon http://www. tropicalisland
%20Komodo%20dragon%20gargantuan%20monitor%20lizard%20%209%203008x2000.jpg

Differences between reptiles and amphibians: Orientation of limbs
In amphibians, such as salamanders, the orientation of the limbs is outward from the main axis of the body. As a result salamanders sprawl. In most reptiles, in contrast, the appendages are rotated towards the body and the long axis of the limbs lies more parallel to the body’s main axis.

Differences between reptiles and amphibians: Orientation of limbs
In addition, the angle between the upper and lower limbs is reduced so the limbs are overall straighter. In the forelimb the elbow is oriented towards the tail. In combination, these modifications provide better support for the weight of the body and raise it higher off the ground. Together these changes make greater agility and speed possible.

Differences between reptiles and amphibians
Reptiles have internal fertilization and so males have a copulatory organ either a penis or hemipenes. Reptiles also have a more efficient nervous system and a more efficient circulatory system.

Differences between reptiles and amphibians: circulation
Reptiles are the first truly terrestrial vertebrates and the cardiovascular system reflects the loss of gills and the need for efficient pulmonary circulation to bring blood to and from the lungs. In contrast to the situation in amphibians, the ventricle in reptiles has developed a septum that partially divides the ventricle into separate left and right chambers. In crocodilians (and birds) the separation of the ventricles is complete. This greatly reduces the mixing of oxygenated and deoxygenated blood.

REPTILES (EXCEPT BIRDS) Pulmocutaneous circuit
Vertebrate circulatory systems FISHES AMPHIBIANS REPTILES (EXCEPT BIRDS) MAMMALS AND BIRDS Systemic capillaries Lung capillaries Lung and skin capillaries Gill capillaries Right Left Systemic circuit Pulmocutaneous circuit Pulmonary circuit Systemic circulation Vein Atrium (A) Heart: ventricle (V) Artery Gill circulation A V Systemic aorta Right systemic aorta

Differences between reptiles and amphibians: respiration
Reptiles depend almost entirely on lungs to oxygenate their blood and reptilian lungs are more developed than those of amphibians. In amphibians the lungs are simple sacs, but in reptiles they have divided into chambers and subchambers (called faveoli), which increases the surface area for gas exchange.

Most reptiles breathe by expanding and compressing the pleurpoperitoneal cavity by movements of the ribs produced by contracting the intercostal muscles. Turtles cannot move their ribs and instead use specialized sheets of muscle to expand and contract the lungs.

Although reptilian respiration primarily depends on lungs, some gas exchange takes place across the skin, the inside of the mouth and in the cloaca particularly in various turtles. In soft-shelled turtles up to 70% of gas exchange may take place across the leathery skin that covers the shell

Softshell turtle

Modern reptiles The modern reptiles being a paraphyletic group include anapsids and diaspids. The anapsid representatives are the turtles (Order Testudines). Turtles have changed little from the oldest known fossil forms 210 mya. Turtle fossils from 210 mya are known from across the globe so the group clearly originated some time before this.

Turtles Turtles have a shell that consists of a dorsal carapace and a ventral plastron. Ribs and vertebrae are fused to the shell and the head and limbs can be withdrawn into it.

Turtles The carapace and plastron are both made of dermal bone overlain by horny scutes. In the carapace a series of 8 bony plates run along the dorsal midline and are attached to the neural arches of the vertebrae. On either side of the midline are pairs of costal bones that are fused to the ribs and 11 pairs of peripheral bones lie outside these.

Bones of the turtle carapace

Turtles Flexible areas called hinges are found in the shells of many turtles. In box turtles the anterior and posterior ends of the plastron can be raised to close off the front and rear openings of the shell.

Box turtle inside its shell

Turtles Soft-shelled turtles lack peripheral ossifications and epidermal scutes. Instead the plastron and carapace are covered with skin.

Turtles Turtles have no teeth and instead have a keratinized beak.
This does not mean they can’t have an impressive bite as snapping turtles demonstrate.

Alligator Snapping Turtle
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Body size Turtles are unusual among the reptiles in having a large number of species that achieve very large body sizes. Large size means thermal stability because larger animals heat and cool more slowly than smaller ones, but large size may make temperature regulation difficult in habitats where shade is scarce.

Body size The marine turtles are the largest members of the group and leatherbacks (the largest species) can weigh 1,500 lbs and are more than two meters in length (largest ever was just over 3m). Their large body size plays a major role in allowing them to range into very cold ocean waters yet maintain a body temperature that may be as much as 18º C higher than the surrounding water. The largest land dwelling members are the Giant tortoises of the Galapagos.

Leatherback Turtle

Galapagos Giant Tortoises
18.8

Ecology and Behavior of Turtles
Turtles are very long-lived. Even small species such as the painted turtle do not mature until aged 7 or 8 and even box turtles may live to be 50 years old. Large tortoises and turtles can live at least as long as humans and perhaps longer, although accounts of several hundred year old turtles are likely exaggerated.

Ecology and Behavior of Turtles
Not surprisingly, being naturally long-lived, turtle populations are vulnerable to increased adult mortality (as e.g., are sharks). Thus, increased adult mortality in sea turtles as a result of fishing has severely reduced their populations. However, the use of turtle excluder devices on shrimp nets has reduced mortality.

Loggerhead turtle escaping through Turtle excluder device

Turtle Reproduction All turtles are oviparous and the eggs are laid in a nest in sand or soil that the female excavates using her rear limbs. As is true of a number of other reptiles (including crocodiles, tuataras and some lizards), incubation temperature plays a major role in determining the sex of individual turtles. Higher incubation temperatures produce the larger sex, which in turtles is female.

Loggerhead Turtle laying eggs
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Turtle Reproduction Young turtles when they hatch are on their own because adults provide no parental care. Marine turtles lay their 100 or so eggs on sandy beaches. When the young hatch they must escape a host of waiting predators to get to the sea and mortality is high.

Green turtle hatchlings

Turtle Reproduction Simultaneous emergence of large numbers of young turtles from multiple nests swamps the predators and allows some to escape.

Turtle Reproduction Where young marine turtles go once they reach the sea is a mystery. Most nesting beaches are upcurrent from feeding grounds so the young likely drift to suitable nursery areas. In areas where currents meet, accumulations of weed and other flotsam provide refuge from predators and a supply of invertebrate food, and these are likely nursery areas for young turtles.

Movement and Navigation
Although where young sea turtles go remains a mystery we know that adults when ready to nest return to the beaches where they hatched. Given the lack of landmarks in the ocean and the often huge distances between nesting and feeding grounds the navigational success of these animals is remarkable.

The movements and navigation of green turtles has been extensively studied for more than 50 years. Green turtles use four major nesting sites including Tortuguero on the Caribbean coast of Costa Rica and Ascension Island in the mid-Atlantic east of Brazil. Mating takes place off the nesting beaches where males congregate to wait for the females.

Adult Green Turtle

Studies of tagged green turtles at Tortuguero have shown that in a nesting season females typically lay three clutches with about 12 days between clutches. However, they do not lay every year. One third lay every second year, the remainder every third year. Information from tag recoveries shows that after breeding the turtle disperse throughout the Caribbean.

The ability of female turtles nesting at Tortuguero to return to the same kilometer of nesting beach is impressive, but pales in comparison to the challenge of locating Ascension Island, which is 2,200 km east of Brazil and only 20km in diameter.

In navigating to Ascension it appears that chemosensory cues provide important information. The South Atlantic Equatorial current passes Ascension and flows west towards Brazil. Young turtles that drift on this current as hatchlings may learn its odor signature. Satellite-tracking studies of nesting females have shown that they take a quite direct route to Ascension from off the coast of Brazil and travel much of the way along the current apparently working their way up the odor plume.

Other studies of marine turtles have shown other cues are also important in navigation. For example, when initially trying to get to sea young loggerhead hatchlings respond first to light and crawl towards the brightest visible light, which in a natural situation would lead them to the sea.

Once in the water the baby loggerheads swim into the waves and this moves them offshore and ultimately to the Gulf Stream. This current carries them up the east coast of the U.S. and across the Atlantic. Off the coast of Portugal, the Gulf Steam splits into northward and southward branches. The turtles need to take the southward branch which will bring them back across the Atlantic and a lot of evidence suggests they use the Earth’s magnetic field to orient themselves correctly.

Turtle Conservation Turtles and tortoises because of their delayed maturity and slow growth rates are very vulnerable to increased adult mortality or reduced juvenile recruitment. Marine turtles are threatened by coastal development that destroys nesting beaches and generates light pollution that fatally disorients young turtles. In addition, adult mortality caused by entanglement in fishing nets and long lines has put additional stress on populations.

Turtle Conservation Smaller freshwater turtles are also under severe threat in China and southeast Asia in general. Turtles have traditionally been used for food and medicine in China and millions are consumed each year. Chinese populations have been severely depleted and as a result China has been importing large numbers from neighboring countries.

Turtle Conservation Tortoises are also threatened, but instead of being taken for food they are illegally taken for the pet trade. In addition, in the southwestern U.S. deserts degradation of desert habitat and bacterial disease (likely introduced from pet tortoises released back into the wild) have caused desert tortoise populations to fall by 30-70%.

Turtle Conservation All of these threats coupled with widespread habitat degradation and enormous numbers of road deaths mean that turtles and tortoises face as severe a global crisis as amphibians do.

Tuataras: Order Sphenodonta
The order is represented by two living species found only on offshore islands in New Zealand. They are the last survivors of a group that was much more diverse 200 million years ago.

Tuataras Tuataras retain many features of their distant ancestors including a diapsid skull with two openings and associated complete arches and a well developed parietal “third eye” on the top of its skull.

Tuataras The parietal eye has a lens, cornea, and retina, but a degenerated nervous connection to the brain. It is not used for vision, but may help regulate day-night cycles or absorb UV rays to manufacture vitamin D.

Tuataras Adult Tuatara are about 2 feet long, nocturnal and live in seabird burrows. Tuatara have two rows of teeth on the upper jaw (one on the maxilla, the other on the palatine bones). When they bite the single row of teeth on the lower jaw fits between those on the upper jaw.

Tuataras The feeding ecology of Tuatara is dictated by their association with seabird colonies. They eat seabirds, which are most vulnerable to attack at night. In addition, the birds guano, food scraps and dead bodies attract lots of invertebrates that the Tuatara also eat and in fact invertebrates make up most of their diet.

Modern reptiles: diapsids Squamata
Subclass Diapsida: Order Squamata. The Squamata includes about 95% of all living reptiles including three suborders: Sauria: lizards, Serpentes: snakes Amphisbaenia: worm lizards.

Modern reptiles: diapsids
The diapsid skull of squamates has been modified from the ancestral condition by the loss of bone behind and below the temporal opening. Most squamates have a kinetic skull, which has movable joints that allow the snout and upper jaw to be moved against the skull and raised.

Kinetic skull Mobility of the skull allows squamates to seize and manipulate prey and also increases the force of the bite. Snakes show the most extreme development of the kinetic skull and are capable of swallowing prey several time their own diameter.

Order Squamata: Suborder Sauria the lizards
Lizards are a very diverse group that includes terrestrial, burrowing, aquatic, arboreal and even gliding members. There are about 4800 species ranging in size from about 3cm to 3m long. Most lizards are insectivorous and small (80% weigh 20 grams or less).

Lizards Lizards have invaded many of the world’s hottest areas by evolving a suite of adaptations that make survival in deserts possible. These include a thick skin that contains lipids, which reduce water loss, and the excretion of uric acid which minimizes water loss.

Lizards Reptiles are ectothermic and adjust their body temperature by moving from one microclimate to another to bask or cool down. Cold climates do not suit lizards as there are too few opportunities to warm up. Because they spend relatively little energy keeping warm, ectotherms in general do well in low productivity ecosystems such as tropical deserts and grasslands.

Lizards Lizards are very adaptable and occupy a wide range of habitats. In addition to deserts and grasslands they occur in swamps, along coasts, above timberline on some mountains and many species are arboreal.

Lizards Lizards have good vision and an external ear, which snakes lack. They also have eyelids, also a trait that snakes lack. Most lizards have four limbs, although some species (the Amphisbaenians) are completely legless.

Lizards Well known species of lizards include: chameleons, geckos, iguanas, and monitor lizards, which include the largest species, the Komodo dragon.

Chameleons Chameleons are the most arboreal lizards.
Their zygodactylous feet (the toes are fused together) allow them to grip branches firmly and they have a prehensile tail. The eyes are raised on small cones that can rotate independently. This arrangement allows chameleons to gauge distance accurately, which is very important is prey capture. They catch prey by projecting their long tongue

Chameleon catching an insect with its sticky extensible tongue.

Geckos Geckos are among the smallest lizards (3cm to 30cm), but they are very successful with more than 1,000 species and they occur on every continent but Antarctica. They have modified scales on their feet (setae) that allow them to cling to vertical surfaces

Gecko (note the flattened pads on the toes
Gecko (note the flattened pads on the toes. Ridges on these pads enable the gecko to cling to smooth surfaces).

Iguanas Most large lizards are herbivorous and many iguanas are arboreal. In areas without mammalian predators (e.g. islands in the West Indies) larger species have evolved that spend much of their time on the ground. Iguanas occur throughout South and Central America and some species (e.g. the Chuckwalla) occur in the western U.S. The marine iguanas of the Galapagos Islands are behaviorally very specialized and they dive and swim to obtain seaweed.

Green Iguana /Shared/StaticFiles/animals/images/primary/ green-iguana.jpg Galapagos Marine Iguana 2008/Belcher/marine-iguana.jpg

Monitor Lizards Unlike other large lizards monitor lizards are active predators and feed on a wide variety of prey. Monitors have evolved a positive pressure gular pump to assist the axial muscles in lung ventilation. This enhanced respiration enables them to sustain high activity levels.

Water Monitor Lizard malaysia/06/malaysia0513.JPG Komodo Dragon

Monitor Lizards Monitor Lizards are widely distributed throughout the Old World with large species found throughout the range. In Australia and New Guinea a diverse array of smaller monitors occur and this appears to be due to a lack of small placental mammal carnivores.

Monitor Lizards Monitors display complex hunting behavior and will adjust their strategies depending on the behavior of their prey. For example, Komodo dragons hunting deer wait in the morning to ambush deer as they move along paths between resting and feeding areas. If they are unsuccessful, they then switch to active stalking for deer in the thicket habitats where they are most likely to occur.

Monitor Lizards Komodo Dragons can dispatch smaller prey easily, but do not have to kill larger prey in their initial attack. Komodo mouths contain a diverse stew of bacteria and bites inevitably become infected. A bitten animal rapidly develops sepsis and dies. The monitor that bit it merely needs to trail the victim for a few days until it succumbs to its wounds.

Amphisbaenians Leglessness has evolved multiple times among lizards and one large group the Amphisbaenians is exclusively legless (apart from 4 species in one genus that retain forelimbs). These are tunneling lizards and have a variety of specialized adaptations for digging and moving in burrows.

Amphisbaenians Amphisbaenians burrow using by ramming their heads against the soil and pushing dislodged material to the sides. The head is heavily keratinized and there is variation in head shape that relates to the particular mode of tunneling used. For example, those with shovel-shaped snouts ram their heads into the end of the tunnel and then compress the material into the roof.

Gray Amphisbaenian 3gDlu3kFXlk/s400/puerto+rican+gray+amphisbaenian_kingsnake1com.JPG

Amphisbaenians Amphisbaenians skin is distinctive and rings called annuli encircle the body. The integument has only a few connections to the body so that the trunk is free to move within a tube of skin. To move, the animal contracts integumentary muscles between selected annuli. This bunches the skin so it presses against the tunnel and the trunk then slides forward within the tube of skin.

Order Squamata: Suborder Serpentes: the snakes
There are approximately 2900 species of snakes and they range is size from 10cm long burrowing forms that eat termites to almost 10m long anacondas and pythons.

Snakes Snakes are limbless and usually lack both the pectoral and pelvic girdles. They have numerous vertebrae, which are shorter and wider than those in other vertebrates and allow them to make undulatory movements.

Snakes There are three major lineages of snakes:
Scoleophidia: more than 300 species of small burrowing (fossorial) snakes. Alethinophidia: About 160 species that include the boas, pythons and a variety of boa-like snakes. Colubroidea: more than 2400 species including the Colubridae, Elapidae and Viperidae.

Aletinophidia Alethinophidia: Boidae: Includes the 26 species of pythons (Pythoninae) and 33 species of boas (Boinae). The pythons are Old World constrictors that are large to enormous (approaching 10m) in size. The boas are the New World equivalent of the pythons and have a similar range of sizes.

Emerald Tree boa

Anaconda

Snakes The large constrictors primarily use rectilinear motion to move. Alternate sections of the ventral integument are raised off the ground and pulled forward by muscles that connect the ribs and ventral scales. Waves of muscles contraction travel down the snake which moves in a straight line.

Colubroidea Colubroidea includes most of the living species of snakes and the Colubridae alone contains 2/3 of all snakes. Many colubroid snakes are venomous and the Elapids and Viperids possess hollow fangs at the front of the mouth and have highly toxic venom. Many colubrids possess venom glands but they do not have the hollow teeth specialized to inject venom.

Colubroid movement Several different forms of motion are used by colubroids, but horizontal undulations and concertina-like movements are the most common.

Colubridae The group is a bit of a phylogenetic dumping ground and includes more than 1800 species that occur worldwide (except Antarctica). Most are medium sized, all lack a pelvid girdle, have no vestigial hindlimbs and in all the left lung is absent or very reduced in size. North American colubrids include garter snakes, kingsnakes, hognose snakes, racers, and corn snakes.

Corn Snake

Prairie Kingsnake Lc_calligaster.html Common Garter snake Striped whipsnake

Viperidae In members of the Viperidae the long fangs rest horizontally when the mouth is closed. Viperids range in size up to about 2m and include both the true vipers, which occur in Eurasia and Africa and the pit vipers, which occur in New World and Asia.

Viperidae True vipers include the Gaboon Viper and Puff Adder.
Pit vipers include rattlesnakes.

Gaboon Viper

Gaboon Viper Skull

Puff Adder

Rattlesnake

Elapidae Elapids have functionally hollow fangs (the tooth is folded over to form a groove that is almost closed down which the venom runs) that are shorter than those of the viperids, but they are permanently erect. Elapids include the mambas, cobras, kraits and sea snakes.

King Cobra 2009/01/a-full-sized-indian-king-cobra.jpg

Black Mamba

Sea snakes Sea snakes (members of the Elapidae) are morphologically specialized for life in the water. The tail is laterally flattened so it can act as an oar. Nostrils are located dorsally on the snout and are equipped with valves to keep water out. More primitive sea snakes lay eggs on the land, but the more derived species give birth to live young.

Yellow-bellied sea snake
yellow%20bellied%20sea%20snake.jpg

Snakes Snakes are an extremely successful group of predators. Although most have poor vision (with the exception of arboreal species) and limited hearing ability they use other sense organs to track prey. Snakes have pit-like Jacobson’s organs in the roof of the mouth, which are olfactory organs. The forked tongue when extended samples the air and picks up molecules that are delivered to the Jacobson’s organ when the tongue is withdrawn.

Snakes Crotaline vipers (pit vipers such as rattlesnakes) have heat-sensitive pit organs on their heads between the nostrils and eyes. These are very sensitive to radiant heat and can detect temperature differences as slight as 0.003ºC. The vipers use the organ to track prey and to aim their strike when biting.

Predation Snakes use one of three methods to catch and kill prey.
Most catch prey by grabbing it and swallowing it alive. Most such species are quick and concentrate on small, easy-to-handle prey. The other two group kill their prey either by constriction or with venom.

Constrictors A variety of snakes including pythons and boas kill by constriction. They coil around their prey and every time the prey breathes out they tighten their coils a little more until the prey can no longer breathe and suffocates. Most constrictors are large, slow-moving ambush predators and the largest snakes, the anaconda, boas and pythons are all constrictors.

Venomous snakes About 20% of all snakes are venomous (although in Australia 80% of snakes are venomous). About 50,000-60,000 people die annually worldwide from snake bite, most of them in the Indian subcontinent. Snakes with venom lethal to humans include the vipers (including the American pit vipers) which have large movable tubular fangs at the front of the mouth; elapids (cobras, mambas, coral snakes, kraits, sea snakes) which have shorter, but permanently erect fangs in the front of the mouth;

Venomous snakes Snake venoms are highly modified salivas and complex in constitution including a variety of proteins and enzymes. Elapid venom is neurotoxic and works by shutting down the respiratory system whereas viper venom is more painful and attacks the vascular system bringing about coagulation of blood and clotting of arteries as well as often severe tissue damage.

Result of a rattlesnake bite
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Crocodiles and Alligators: Order Crocodilia
Modern crocodiles and birds are the only survivors of the Archosaurian lineage that included the dinosaurs. Crocodiles have changed little in almost 200 million years a testament to the success of their design.

Crocodiles All crocodiles have their teeth set in sockets a trait found otherwise only in mammals and fossil birds and also like mammals have a complete palate which enables them to breathe even if the mouth is filled with water or food. They alos possess a four chambered heart as do the only other extant members of the Archosauria, the birds

Crocodiles Crocodiles are ambush predators that kill by grabbing and drowning their prey. The largest Nile and Estuarine crocodiles (called “salties” in Australia) can exceed 1000 kgs in weight and can attack and kill almost anything.

Crocodiles The muscles used to open a crocodile’s mouth are quite weak, but those used to close the jaws are massive and powerful. Broad nosed crocodiles can for example crush an adult turtle. A crocodile’s snout contains large numbers of touch and pressure receptors. These enable the animal to lunge at a prey animal in darkness or immediately snap the jaws closed on a fish or other animal that brushes against the animal’s open mouth.

Crocodiles Crocodiles do not chew their prey. Smaller prey animals are swallowed whole, but larger animals are eaten piecemeal. Crocodiles often allow the animal to decompose for several days to make it easier to tear chunks off.

Classification There are 23 species of crocodile divided into three lineages: Alligatoridae, Crocodilidae Gavialidae.

Alligatoridae The Alligatoridae includes the alligators and caimans and, with the exception of the Chinese alligator, is solely a New World group. Alligators and caimans are exclusively found in freshwater and, in general, they have broader snouts than crocodiles.

Alligators The American Alligator is found throughout the Gulf states and caimans occur in Central America, South America and the Caribbean. Alligator populations in the U.S. had declined enormously as a result of hunting for meat and especially skins, but Federal protection has caused their numbers to rebound so that they are again common.

American Alligator

Crocdiles In contrast to alligators, crocodiles occur in both freshwater and salt water and readily move from one to the other.

Crocodiles The saltwater crocodile is probably the largest living crocodile and may be capable of reaching 7m in length although hunting pressure in recent history means there may not be old enough individuals around for maximum size to have yet been attained.

Australian saltwater Crocodile with a hooked Barramundi
barra_croc.jpg

Gharial There is only a single species in the Gavialidae: the gharial.
Gharials were once widespread in large rivers in India and Burma but are now threatened species. It has a very narrow snout and is a specialist fish predator.

Gharial picture Gharial

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