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 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.
https://youtu.be/mVkBEqO4Nuc
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.
 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 welldeveloped 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.