Early Tetrapods and Modern
Amphibians
Chapter 25
Early Tetrapods

Tetrapods are
gnathostomes that have
limbs and feet.
 Tetrapods are a
monophyletic group.
 One of the most
significant events in
vertebrate history was
when the fins of some
lobe-fins evolved into
the limbs and feet of
tetrapods.
Early Tetrapods

The movement from water to land
required more than the change from fins
to feet.
Early Tetrapods

The terrestrial environment is different
than the aquatic environment.
A higher oxygen content means oxygen can
diffuse more rapidly.
 Air is less dense – less buoyant.


Stronger skeletal structure, sturdier limbs.
Temperature fluctuates more.
 Terrestrial environment offers a whole new
array of habitats.

Early Tetrapods

Tetrapods have many adaptations
including:
Lungs for efficiently breathing air.
 Four limbs and feet with digits.
 Ears for detecting airborne sounds.

The Origin of Tetrapods

The Devonian (beginning 400 mya) had
mild temperatures and alternating
droughts & floods.
Freshwater sources unstable.
 Reduced oxygen content as pools &
streams dried up.

The Origin of Tetrapods

Freshwater fishes alive at this time had a
kind of lung that developed as an
outgrowth of the pharynx.
Efficiency of this air-filled cavity was
enhanced by improving the vascularity with
a capillary network.
 Oxygenated blood was then sent to the
heart & pumped to the rest of the body.

The Origin of Tetrapods

This was the origin of the double
circulation that we find in all tetrapods.
A systemic circulation serving the body.
 A pulmonary circulation supplying the
lungs.

The Origin of Tetrapods

Vertebrate limbs also arose during the
Devonian period.
Fins of lobe-finned fishes have bony
elements that are homologous to tetrapod
limbs.
 Some Devonian lobe-finned fishes have
identifiable humerus, radius, & ulna, and
could sort of walk on the bottom of pools.

The Origin of Tetrapods

In one lineage of lobe-fins, the fins became
progressively more limb-like while the rest of
the body retained adaptations for aquatic life.
The Origin of Tetrapods

Extraordinary fossil discoveries over the
past 20 years have allowed
paleontologists to reconstruct the origin
of tetrapods.
The Origin of Tetrapods

One theory about how early tetrapods
became adapted for life on land
suggests that they had to be able to
move from pool to pool to find water.
Those with stronger fin/limbs would survive
& reproduce.
 Alfred Romer

The Origin of Tetrapods

Acanthostega had well
developed limbs with
digits, but was fully aquatic
in other ways.



The limbs were not
adequate for proper
walking on land.
Only later did tetrapods
move onto land.
We do not currently know
what caused this move.
The Origin of Tetrapods

Lobe-finned fishes
are probably the
closest relatives of
tetrapods.


Sister group
Share several
characteristics with
early tetrapods like
Acanthostega &
Ichthyostega.

Characteristics of
the skull, teeth, &
pectoral girdle.
The Origin of Tetrapods

Ichthyostega had characteristics of a terrestrial
vertebrate:







Jointed limbs
Stronger vertebrae & associated muscles to support
the body in air.
Muscles to elevate the head.
Stronger shoulder & hip girdles.
Protective rib cage.
Modified ear structures for detecting airborne sounds.
Modifications for detecting airborne odors.
The Origin of Tetrapods

Ichthyostega also
retained some aquatic
characteristics:



The tail has fin rays.
Opercular (gill cover)
bones are present.
Ichthyostega represents
an early offshoot of
tetrapod phylogeny, not
an immediate ancestor
of amphibians.
The Origin of Tetrapods


Several extinct lineages
plus the Lissamphibia
with modern amphibians
formed the
temnospondyls.
Lepospondyls and
anthracosaurs are,
based on skull structure,
closer to amniotes than
temnospondyls.
The Origin of Tetrapods

The warm and
wet
Carboniferous
period followed
the Devonian.

Tetrapods
radiated
quickly.
Class Amphibia
Class Amphibia is represented by about
5,000 species.
 Most amphibians have moist skin that
complements the lungs in gas exchange.



Amphibians require moist environments.
Amphibians are ectothermic with a low
metabolic rate.
Metamorphosis

Amphibian means “two lives”, a reference to
the metamorphosis of an aquatic larva into a
terrestrial adult.
Terrestrial Amphibians

Some amphibians
have adapted to a
more terrestrial life.


Some caecilians
and salamanders
have direct
development.
Some frogs have
unusual
adaptations for
caring for their
young.
Order Gymnophiona

Order
Gymnophiona
includes caecilians,
which are legless
and resemble
worms.

Burrowing &
aquatic forms.
Order Gymnophiona – Reproduction
Fertilization is internal in this group.
 Larvae are aquatic.



Some complete larval development inside
the egg.
Some species are viviparous with the
young obtaining nourishment by eating
the wall of the oviduct.
Order Urodela

Order Urodela
includes
salamanders,
which have tails.
Order Urodela – Metamorphosis

The ancestral
condition in
salamanders is to
have aquatic larvae
and terrestrial adults
that live in moist
places.

Some species are
fully aquatic, others
are fully terrestrial.
Order Urodela – Reproduction

Fertilization is internal.

The male deposits a spermatophore
(package of sperm) on the substrate, the
female then picks it up.
Order Urodela – Reproduction

Aquatic larvae have external gills and a
finlike tail.
Blue-spotted Salamander larva (Photo from the
Massachussetts Vernal Pool Association )
Order Urodela – Reproduction

Terrestrial species
have direct
development –
no larval form.

Eggs are laid in
moist areas and
are sometimes
guarded by the
parents.
Order Urodela – Life Cycle

Some American
newts have a complex
life cycle:


Aquatic larvae
metamorphose into a
terrestrial red eft
stage that later
metamorphose again
into aquatic adults.
Some skip the
terrestrial stage and
remain aquatic.
Order Urodela – Respiration
Salamanders have a diverse array of
respiratory mechanisms.
 Respiration through the skin is important.
 At some point in the life cycle they may
have:

External gills
 Lungs
 Both
 Neither

Order Urodela – Paedomorphosis

Paedomorphosis is found in some salamanders.


Larval characteristics are retained in mature adults.
Some species do not metamorphose, retaining their
external gills and finlike tail and remaining aquatic while
mature.

Mudpuppies
Order Urodela

Some species only metamorphose under
certain conditions.


Gilled forms called axolotls.
Metamorphose when the pond evaporates.
Order Urodela

Another example of paedomorphosis occurs in
the feet of salamanders in the genus
Bolitoglossa.

Growth of the feet stops early leading to padlike feet that are useful for climbing.
Order Anura

Order Anura includes frogs and toads, which
lack tails.
Order Anura – Habitats

Adult anurans may
be found near water
throughout their
lives, or in more
terrestrial habitats.

Some return to
water only to breed.
Order Anura – Habitats

Some species are
fully terrestrial with
tadpoles living in
small water
accumulations in
plants, or in the
mouth or vocal sac
of the parent.
Order Anura – Defense

Adult frogs show a variety of defense
mechanisms.
Leaping away is a commonly seen defense.
 Most anurans can inflate their lungs so they
look to big to swallow.
 Some have poison glands.

Dendrobatids (poison dart frogs)
 Toads


Some will bite at a potential predator.
Order Anura – Coloration

Skin color in amphibians
is produced by pigment
cells called
chromatophores.



Xanthophores contain
yellow, orange or red
pigment.
Iridophores contain a
silvery, light reflecting
pigment.
Melanophores contain
black or brown melanin.
Order Anura – Coloration

Green hue is an interaction of xanthophores
containing yellow pigment and underlying
iridophores.
 Many can adjust color to match with
background and camouflage themselves.
Order Anura – Respiration

Amphibians can use three respiratory
surfaces for breathing:
Skin – cutaneous breathing
 Mouth – buccal breathing
 Lungs


Anurans are more reliant on lungs than
salamanders.

Carbon dioxide still lost through skin.
Order Anura – Respiration

Air must be forced
into lungs – positive
pressure breathing.
Order Anura – Vocalization

Males have better
developed vocal
chords in their
larynx (voice box)
than females.
 Males use their
voices to attract
females.

Calls are species
specific.
Order Anura – Circulation

Tetrapods have separated
pulmonary and systemic
circulation.



This is made more efficient
by partitioning the heart.
This partitioning is not
complete in amphibians.
Frog hearts have two atria,
but a single ventricle.

Blood still remains mostly
separated.
Order Anura – Feeding

Frogs, like most adult amphibians, are
carnivorous.
Most will feed on just about anything that
moves and is small enough to swallow
whole.
 Most have a sticky, protrusible tongue.


Tadpoles are usually herbivorous.
Order Anura – Senses
Lateral line system present in larval
forms only.
 Ears are used for detecting airborne
sounds.
 Vision is important in most amphibians.


Frogs have color vision provided by rods
and cones on the retina.
Order Anura – Senses

Frogs also have tactile & chemical
receptors in their skin, taste buds on the
tongue and palate, and a well-developed
olfactory epithelium in the nasal cavity.
Order Anura – Reproduction

Spring weather signals breeding season and is
marked by calling frogs.
Order Anura – Reproduction

Frogs pair up with the
male riding on the
females back –
amplexus.


Fertilization is external.
Eggs usually deposited
in water, anchored to
vegetation.
Order Anura – Reproduction

Tadpoles, the aquatic
larval stage, hatch
after 2-21 days
depending on
temperature.


Usually herbivorous
with keratinized jaws
for scraping algae.
Three pairs of
external gills which
become internal gills.
Order Anura – Reproduction

Hind limbs appear
first, followed by
front limbs.
 The tail is gradually
resorbed.
 The tadpole stage
may last a few
weeks or a couple of
years, depending on
species.