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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.
Figure 26.04
The Amniotic egg
Unlike amphibians amniotes lack a larval stage
and after hatching develop directly into the adult
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.
Amniote origins and classification
 There
is considerable disagreement
between cladistic and traditional
classification of the amniotes.
 Traditional
classification recognizes three
Reptilia: reptiles
Aves: birds
Mammalia: mammals
Amniote origins and classification
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
Figure 26.02
Amniote origins and classification
 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
Amniote origins and classification
 One
of the major characteristics used to
classify the amniotes is the structure of the
 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
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.
 The
anapsids are characterized by having
no temporal opening behind the eye
 They
are represented today by the turtles
a group that has changed little since it
evolved about 200 mya.
Figure 28.01
Figure 26.02
 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).
 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;
280-260 mya)
 The
pelycosaurs were the dominant group
of the Permian period, but disappeared in
the Great Permian extinction (approx 245
 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
Figure 26.01
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.
Therapsid to mammal transition
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
Turbinate bones in nasal cavity: increase retention
of body heat.
Therapsid to mammal transition
 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 midTriassic (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.
 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
Figure 26.02
Figure 26.01
Differences between reptiles and
 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
 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
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
Differences between reptiles and
 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
This greatly reduces the mixing of oxygenated and
deoxygenated blood.
Vertebrate circulatory systems
Lung and skin capillaries
Lung capillaries
Lung capillaries
Gill capillaries
ventricle (V)
Atrium (A)
Vein circulation
Systemic capillaries
Systemic capillaries
V aorta
Systemic capillaries
Systemic capillaries
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.
Differences between reptiles and
amphibians: respiration
 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.
Differences between reptiles and
amphibians: respiration
 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
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.
Figure 26.06
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
 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
 Soft-shelled
turtles lack peripheral
ossifications and epidermal scutes.
 Instead
the plastron and carapace are
covered with skin.
 Turtles
have no teeth and instead have a
keratinized beak.
 This
does not mean they can’t have an
impressive bite as snapping turtles
Alligator Snapping Turtle
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
Figure 26.08
Galapagos Giant Tortoises
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
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
Turtle Reproduction
 Young
turtles when they hatch are on their
own because adults provide no parental
 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
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.
Movement and Navigation
The movements and navigation of green turtles
has been extensively studied for more than 50
Green turtles use four major nesting sites
including Tortuguero on the Caribbean coast of
Costa Rica and Ascension Island in the midAtlantic east of Brazil.
Mating takes place off the nesting beaches
where males congregate to wait for the females.
Adult Green Turtle
Movement and Navigation
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
 Information from tag recoveries shows that after
breeding the turtle disperse throughout the
Movement and Navigation
 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.
Movement and Navigation
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.
Movement and Navigation
 Other
studies of marine turtles have
shown other cues are also important in
 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.
Movement and Navigation
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
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
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.
Figure 26.02
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
Figure 26.26
 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.
 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.
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
When they bite the single row of teeth on the
lower jaw fits between those on the upper jaw.
 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
 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.
Figure 26.11
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.
Figure 26.18
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
have invaded many of the world’s
hottest areas by evolving a suite of
adaptations that make survival in deserts
 These
include a thick skin that contains
lipids, which reduce water loss, and the
excretion of uric acid which minimizes
water loss.
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
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
 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.
 Well
known species of lizards include:
chameleons, geckos, iguanas, and
monitor lizards, which include the largest
species, the Komodo dragon.
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
Figure 26.14
Chameleon catching an insect with its sticky extensible tongue.
 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
 They
have modified scales on their feet
(setae) that allow them to cling to vertical
Figure 26.12
Gecko (note the flattened pads on the toes. Ridges on these pads enable the gecko
to cling to smooth surfaces).
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
Galapagos Marine Iguana
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
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
 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 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
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
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.
 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
Colubroidea: more than 2400 species
including the Colubridae, Elapidae and
 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
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 includes most of the living species of
snakes and the Colubridae alone contains 2/3 of all
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.
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
Corn Snake
Prairie Kingsnake
Common Garter snake
Striped whipsnake
 In
members of the Viperidae the long
fangs rest horizontally when the mouth is
 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.
 True
vipers include the Gaboon Viper and
Puff Adder.
 Pit
vipers include rattlesnakes.
Gaboon Viper
Gaboon Viper Skull
Puff Adder
 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
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
Yellow-bellied sea snake
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
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.
Figure 26.24
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.
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
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 50,00060,000 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;
Figure 26.25
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
Crocodiles and Alligators: Order
 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.
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
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.
The muscles used to open a crocodile’s mouth are quite
weak, but those used to close the jaws are massive and
Broad nosed crocodiles can for example crush an adult
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
do not chew their prey.
Smaller prey animals are swallowed
whole, but larger animals are eaten
 Crocodiles
often allow the animal to
decompose for several days to make it
easier to tear chunks off.
 There
are 23 species of crocodile divided
into three lineages:
 The Alligatoridae
includes the alligators
and caimans and, with the exception of the
Chinese alligator, is solely a New World
 Alligators
and caimans are exclusively
found in freshwater and, in general, they
have broader snouts than crocodiles.
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
 In
contrast to alligators, crocodiles occur in
both freshwater and salt water and readily
move from one to the other.
 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
Australian saltwater Crocodile with a hooked Barramundi
 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