CHAPTER-5
Amniotes
 The amniotes are a group of tetrapod vertebrates
• have a terrestrially adapted amniotic egg.
• They are named based on the presence of amnion.
• Amniotes include most of the land-dwelling vertebrates alive
today, namely, mammals,
• Turtles,
• lizards,
• crocodylians and birds.

1. The origin of the Amniotic cell and its structure
• The possession of a shelled egg (amniotic egg) unites
 the mammals, birds and reptiles into a monophyletic group
the amniotes.
• The evolutionary origins of the amniotic egg are unclear
because early amniote fossils are scarce and eggs especially
so.

The Amniotic egg
• The shelled egg freed the amniotes from the need to
reproduce in water that hindered the amphibians
ability to spread into harsh or dry environments.
• Evolution of the amniotic egg expanded the
success of vertebrates on land
• The amniotic egg is hard shelled and is called an
amniotic egg because the embryo develops within a
sac called the amnion.

The Amniotic egg
• The amniotic eggs have extra embryonic membranes that
provide complete support for embryonic development.
• Extra embryonic membranes that classify the interior and
give it several different functions.
• These includes:
• One membrane is the amnion, which is a fluid filled sac
that acts as
 a support for the embryo and also
 prevents desiccation.

Cont…
• The yolk sac contains food for the embryo, thus
eliminating the need for a larval stage.
• Unlike amphibians amniotes lack a larval stage and after
hatching develop directly into the adult form (because of
presence of yolk sac).
• Another membranous sac is the allantois which is serves
 both as a respiratory surface and
 a storage reservoir for metabolic waste products such as
nitrogenous compounds.

Cont…
• A third membrane, the chorion provides a special hard
covering that is permeable to respiratory gases (O2 and
CO2) while being impermeable to water vapor.
• Finally, surrounding and protecting everything is a porous,
parchment like or leathery shell.

Figure 26.04

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

Classification of Amniote
• 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

Classification of Amniote
• 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.

Classification of Amniote
• 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 only living representative
turtles and their ancestors) have a solid skull with no
temporal openings behind the eye sockets.
• Synapsids (which include the mammals and their
ancestors) have one pair of openings in the skull associated
with the attachment of jaw muscles.

Cont…
• Diapsids (lizards, snakes, crocodilians, birds, and ancestors)
have two pairs of openings in the skull roof.
• 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).

20.1
Figure 28.01

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).
• 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

18.2
Figure 26.02

3. Biological contributions:
• The shelled, amniotic egg. This helps amniote to lay large
eggs and in drier habitats.
• A tough, dry, heavily keratinized skin that provides
protection.
• Powerful jaw- temporal openings in the diapsid skull provide
the space for bulging temporal muscles.
• Internal fertilization
• Effective adaptation for water conservation. Metanephric
kidney that excretes nitrogenous wastes as uric acid and urea.

4. Characteristics of class reptilian
• Body covered with keratinized epidermal scales, integument
with few glands.
• Two paired limbs, usually with five toes.
• Skeleton well ossified; skull with one occipital condyle
• Respiration by lungs; no gills but some use cloaca (e.g. turtle),
pharynx or skin for respiration.
• They have pulmonary and systemic circuits, ventricle
incompletely divided, heart has 3 chambers (two atria & one
ventricle semi separated); but in the group crocodilians have 4
chambers (two atria and two ventricles).

Cont…
• Exothermic; many thermoregulatory behaviorally
• Metanephric kidney (paired); uric acid main N2 west.
• Nervous system with optic lobes on dorsal side of brain; 12
pairs of cranial nerves.
• Fertilization is internal; separate sexes
• Egg’s covered with calcareous or leathery shell extra
embryonic membranes (amnion, chorion & allantois)
present during embryonic life. They have no aquatic larval
stages.

5. Advancement of reptile over amphibians
• They have tough, dry scaly skin offering protection against
desiccation and physical injury.
• Their amniotic egg permits rapid development of large young in
relatively dry environment.
• Their jaws are efficiently designed for applying crushing or
gripping force to prey.
• Copulatory organ, permitting internal fertilization the first to
have penis.
• They have an efficient and flexible circulatory system and
higher blood pressure; complete separation of ventricles.

cont…
• This greatly reduces the mixing of oxygenated and deoxygenated
blood.
• Nervous systems are considerably more complex than
amphibians system.
• Efficient water conservation strategies.
• Reptiles depend almost entirely on lungs to oxygenate their
blood and reptilian lungs are more developed than those of
amphibians.
• Efficiently designed limbs for travel on land except limbless
members; have better body support.

6. Classification of modern Reptiles
• Classification of reptiles based on temporal fossa (lateral
window) number and presence. Temporal fossa is an opening in
the temporal bone of cranium that found behind the eyes orbit.
• The modern reptiles being a paraphyletic group include anapsids
and diapsids. They have four living order:
 Testudines (Chelonia)
 Sphenodonta
 Squamata
 Crocodilia

6.1. Order Testudines (Chelonia): turtle
• Turtles descended from anapsids lineages; no temporal fossa.
• They can live in water or on land.
• They are enclosed in shells which are broad and flattened
consisting of dorsal part carapace and ventral part plastron.
• Soft-shelled turtles lack peripheral ossifications and epidermal
horny plates.
• Instead the plastron and carapace are covered with skin.
• Turtles have no teeth and instead have a keratinized beak or
horny plates for gripping food.

18.6
Figure 26.06

Box turtle inside its shell

Body size of turtle
• Turtles are unusual among the reptiles in having a large
number of species that achieve very large body sizes.
• The marine turtles are the largest members of the group; For
example, leatherbacks (the largest) can weigh 725 kg and
attaining a length of 2 m.
• The largest land dwelling members are the Giant tortoises of
the Galapagos.

Leatherback Turtle

18.8
Figure 26.08
Galapagos Giant Tortoises

Order Testudines:
• Turtles are very long-lived: for some are believed to live
more than 150 years.
• The turtle live in land, water or in sea and it applied to
semi- aquatic species and tortoise to land species.
• Some turtles only eat animals, such as insects, worms, and
small fish. Some turtles only eat plants. Most turtles eat
both animals and plants.
• Green turtles are herbivores.
• They feed on marine grass and algae.

Order Testudines
• Cloaca respiration in the turtle occurs by taking water in to
the cloaca and the accessory bladders and then forcing it out
through the cloacal opening.
• Many aquatic turtles respired by cloaca when inactive.
When active they must use lung breath.
• They have poor perception of sound but they have good
sense of smell and acute vision and color perception as
good as that of humans.

Order Testudines
• All turtles are oviparous and the eggs are laid in a nest in
sand or soil.
• Fertilization is internal.
• Nest or incubation temperature (To) determines sex of
hatchlings (primitive feature), low To produces more males;
high To produces more females, in turtles.
• All reptiles with To dependent sex determination during
embryogenesis lack sex chromosomes.

Loggerhead Turtle laying eggs

• Young turtles when they hatch are on their
own because adults provide no parental care.
Picture that show Green turtle hatchlings

Adult Green Turtle

Threats of Turtle
• Turtles and tortoises because of their delayed maturity and
slow growth rates are very vulnerable to increased adult
mortality or reduced juvenile enrollment.
• Predators on turtle eggs and baby turtles.
• In addition, adult mortality caused by entanglement in fishing
nets and long lines has put additional stress on populations.
• Smaller freshwater turtles are also under severe threat in China
and southeast Asia in general.

Threats of Turtle
• Turtles have traditionally been used for food and medicine in
China and millions are consumed each year.
• Tortoises are also threatened, but instead of being taken for food
they are illegally taken for the pet/favorite trade.
• In addition, degradation of desert habitat and
• bacterial disease have caused desert tortoise populations to fall
by 30-70%.
• 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.

6.2. Order Sphenodonta: Tuataras
• The order is represented by two living species found only on
offshore islands in New Zealand.
• They are lizard like forms 66cm long or less and nocturnal that
live in the burrows.
• Tuataras retain a primitive diapsid skull with two openings 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.

18.23
Figure 26.26

6.3. Order Squamata: lizards, worm lizards & snakes.
• Subclass Diapsida: These are reptiles that have been a skull
with two pairs of temporal openings.
• Squamata are the most recent and diverse products of
diapsida evolution.
• Most lizards and snakes of a mobile skull having movable
joints, such skull is called kinetic skull,
• such exceptional skull mobility is considered a major
factor in diversification of lizards and snakes.

18.9
Figure 26.11

18.16
Figure 26.18

Gaboon Viper Skull

A. Suborder Sauria: the lizards
• Most lizards are insectivorous and small.
• 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 are very adaptable and occupy a wide range of
habitats.

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 are
completely legless.
• Well known species of lizards include: chameleons, geckos,
iguanas, monitor lizards, which include the largest species,
the Komodo dragon, Glass lizards and Gila monsters.

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
determine 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
• 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 except Antarctica and mostly
nocturnal forms.
• They have modified scales on their feet (setae) that allow
them to cling to vertical surfaces.
• Geckos are exception because males are strongly vocal (to
announce territory and discourage approach of other males).

Figure 26.12
Gecko (note the flattened pads on the toes. Ridges on these pads enable the gecko
to cling to smooth surfaces).

Monitor Lizards
• Unlike other large lizards monitor lizards are active
predators and feed on a wide variety of prey.
• Monitor Lizards are widely distributed throughout the Old
World with largest species found throughout the range.

Komodo Dragon
Water Monitor Lizard

Glass lizards
• Glass lizards are completely limbless.
• Glass lizards distinguish from snakes by the deep,
flexible groove running along each side of the
body.

Gila monsters
• Gila monsters (Heloderma suspectum) store fat in their
tails, which they use during drought to provide energy and
metabolic water.
• Gila monsters are the only lizards capable of delivering a
venomous bite.

B. Suborder Amphisbaenia: worm lizards
• The name of the suborder means double walk.
• In reference to their peculiar ability to move back ward
nearly as effectively forward.
• They have no external limbs;
• they have no visible eye and ear (hidden in the skin).
• They have a variety of specialized adaptations for digging
and moving in burrows.

Gray Amphisbaenian

C. Suborder Serpentes: the snakes
• Snakes have elongated body structure;
• Snakes are limbless and usually lack both the pectoral and
pelvic girdles.
• They have no external ears or tympanic membrane, but
• have internal ear;
• they are quite sensitive to vibrations carried in the ground.
• Most snakes have relatively poor vision except arboreal
snakes of the tropical forest have excellent vision that helps
them track prey.

Snakes
• Tongue of snakes forked and protrusible.
• There are approximately 2900 species of snakes and they
range is size from 10cm long burrowing forms to almost
10m long anacondas and pythons.

Emerald Tree boa

Anaconda

Pattern of movement in Snakes
• They have numerous vertebrae, which are shorter and
wider than those in other vertebrates and allow them to
make undulatory movements.
• To solve problems of movement with out limbs snakes use
different pattern of movement.
• Lateral undulation- this is the most typical pattern of
movement follows S-shaped path, with a snake propelling
itself by exerting lateral force against surface irregularities.

Pattern of movement in Snakes
• Concertina- this movement enables a snake to move in a
narrow passage, as when climbing a tree by passing
irregular channels in the bark.
• Rectilinear- many heavy bodied snakes employ this
movement. It is slow but effective way of moving towards
prey.
• Side-winding- this movement enables desert vipers to move
with surprising speed across loose, sandy surface with
minimum surface contact.

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.
• These are lined with richly innervated chemosensory
epithelium.
• They flick its forked tongue to smell its surroundings.

Snakes
• Scent particles trapped on the tongues surface are transferred to
Jacobson’s organ, olfactory organs in the roof of the mouth.
• This is the heat-sensitive pit organ between the nostril and eye.
• Snakes of subfamily Crotalinae with in family Viperidae;
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.

18.22
Figure 26.24

Rattlesnake

Predation
• Snakes use one of three methods to catch and kill prey.
 Most snakes capture their prey by grasping it with their
mouth and swallow it still 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.

Corn Snake

Constrictors
• Snakes that first kill their prey by constriction often
specialize on large mammalian prey.
• 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
(Zendo) and the largest snakes, the anaconda, boas and
pythons are all constrictors.

Emerald Tree boa

Anaconda

Venomous snakes
• Other snakes kill their prey before swallowing of by
injecting it with venom/poison.
• 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.
• Venomous snakes are usually divided in to 5 families,
based in part on types of fangs:

Venomous snakes
• Colubridia the third lineage of snakes, which is including
the Family Colubridae, Elapidae and Viperidae.
• Many colubridia 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.
• North American colubrids include garter snakes, king
snakes, hognose snakes, racers, and corn snakes.

Common Garter snake
Striped whipsnake
Prairie Kingsnake

Venomous snakes
1. Family Viperidae
• Vipers (including the American pit vipers) which have
large movable tubular fangs at the front of the mouth;
• True vipers include the Gaboon Viper and Puff Adder.
• Pit vipers include rattlesnakes.

Gaboon Viper

Gaboon Viper Skull

Puff Adder

Venomous snakes
2. Family Elapidae
• Elapids include cobras, mambas, coral snakes, kraits, and
sea snakes;
• Aggressive king cobra, which may greater than 5.5 m in
length, is the largest and perhaps the most dangerous of all
poisonous snakes.
• Elapids have functionally hollow fangs that are shorter than
those of the viperids, but permanently erect in the front of
the mouth;

King Cobra

Black Mamba

Venomous snakes
3. Family Hydrophiidae, highly venomous sea snakes found
in this group.
4. Family Atractaspididae, the fossorial mole vipers usually
have fangs similar to vipers.
5. Family Colubridae, includes most of the living species of
snakes and contains 2/3 of all snakes; very large families,
which contain most familiar and non poisonous snakes, does
include a few snakes that have been responsible for human
fatalities.

Venomous snakes
• Venomous snakes have fangs on the upper jaw, can be erected
• All vipers have a pair of teeth on the maxillary bones
modified as fangs. When viper strikes, a special muscle and
bone lever systems erects the fangs when its mouth open.
• The venom gland modified salivary glands, is connected by a
duct to the hollow fang.
• The fangs are driven in to the prey by the force of the beat and
venom is injected in to the wound through a channel in the
fangs. The bit of a pit viper can be dangerous to humans.

18.20
Figure 26.25

Venomous snakes
 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.
• Most snakes are oviparous, very few snakes are viviparous.
• Snakes are able to store sperm and can lay several clutches
of fertile eggs at long intervals after a single mating.

Result of a rattlesnake bite

6.4. Order Crocodilia: Crocodiles and Alligators
• All crocodilians have an elongated, robust, well reinforced skull
and
• massive jaw musculature arranged to provide a wide gape and
rapid, powerful closure.
• Teeth are set in sockets, type of dentition called thecodant.
• Crocodilians have a four chambered heart with completely
divided atria and ventricles as do the only other extant members
of the Archosauria, the birds
• Crocodile is one of the largest living reptiles.

Crocodiles
• Crocodiles are ambush predators that kill by grabbing and
drowning their prey. Crocodiles are known to attack animals as
large as cattle, deer, and people.
• The largest, the estuarine crocodile, Crocodylus porosus, found in
S. Asia and Nile crocodile; C. niloticus can exceed 1000 kgs in
weight and can attack and kill almost anything.
• Alligators are usually less aggressive than crocodiles and far less
dangerous to humans.
• Broad nosed crocodiles can for example crush an adult turtle.
• A crocodile’s snout contains large numbers of touch and pressure
receptors.

Classification of Modern crocodilians
• Modern crocodilians are the only surviving reptiles of the
archosaurian lineage.
• There are 23 species of crocodile divided into three
lineages:
Alligatoridae, (Arjano)
Crocodilidae (Azo)
Gavialidae.

1.Alligatoridae
• The Alligatoridae includes the alligators and caimans and,
mostly they are a new world group .
• Alligators and caimans are exclusively found in freshwater
and,
• in general, they have broader snouts than crocodiles.
• In USA, Alligator mississippiensis is the only species of
alligators.
• Alligators are able to make definite vocalization.
• Male alligators give loud bellows in mating season.

2. Crocodiles
• Alligators and crocodiles are oviparous; usually 20 to 50
eggs are laid in a mass.
• As with many turtles and some lizards, incubation
temperatures of the eggs determine sex ratio of the
offspring.
• However, unlike turtles, low nest T0 produces only
females, while high nest T0 produces only males.
• The saltwater crocodile is probably the largest living
crocodile and may be capable of reaching 7m in length.

American Alligator

Australian saltwater Crocodile with a hooked Barramundi

Crocodiles and alligators
• Crocodylus acutus, restricted to extreme S. Florida is the only
species of crocodile.
• In contrast to alligators, crocodiles occur in both freshwater and
salt water and readily move from one to the other. Crocodiles
and alligators can be distinguished on the basis of head
morphology.
• Crocodiles have a relatively narrow snout, and when their
mouths are closed, the fourth lower jaw tooth is visible.
• Alligators generally have a broader snout, and their fourth lower
jaw tooth is hidden by fitting in to a notch in the upper jaw.

3. Gavialidae
• Gavials represented by only a single species in India and
Burma; the gharial.
• Gharials were once widespread in large rivers in India and
Burma but are now threatened species.
• Gavials have a very narrow snout and is a specialist fish
predator.

Gharial picture
Gharial