2009 BIO153: Lecture 18
The Tetrapods
Mar 25 th, 2009
The tetrapods are the vertebrate lineage that
successfully colonized the terrestrial environment.
The transition from an aquatic to a terrestrial
lifestyle involves:
1. Changes in skeletal system
2. Changes in gas exchange system
3. Changes in circulatory system
4. Changes in reproduction
1. Changes in skeletal system:
▯ ribs to protect internal organs
▯ interlocking vertebrae (to prevent rotation
of the body during movement)
▯ pectoral & pelvic girdles (for articulation of
the limbs; separation of the axial and
appendicular skeleton)
▯ mobile neck (for improved feeding, use of
sense organs)
▯ limbs
▯ changes in the skull (discussed further in
the amniotes)
2. Changes in gas exchange system: there are
fundamental differences between breathing O 2
dissolved in water vs. breathing O 2dissolved in air:
▯ air: O = 210 ml/l vs. water: O = 3-9 ml/l.
2 2
The amount of oxygen in water is
temperature dependent – amount of O 2
decreases as temp increases – while the
amount of O 2 in air is constant with
temperature, but varies with altitude
(atmospheric pressure)
▯ water is denser, more viscous & diffusion
slower
▯ harder to extract O2from H O2than from air
Breathing water:
▯ gill filaments are oriented to the direction of
water flow
▯ unidirectional flow of water over respiratory
surface
▯ highly efficient: because water is dense and
viscous, flow-through system is better than
a tidal system
1 The transition to air breathing: a respiratory
exchange surface must be moist and highly
vascularized.
a. vascularized pharyngeal sac
▯ found in lungfish
▯ amphibian lung
b. skin respiration
▯ salamanders (skin, oral mucosa)
Air breathing in tetrapods: “tidal” breathing:
because air is easier to move than water, tidal (i.e.
in and out) breathing is possible.
2 types of tidal breathing:
a. positive pressure:
“swallowing air” (amphibians)
▯ inspired air is gulped, creating increased
pressure in the internal cavity (like filling a
balloon)
b. negative pressure:
▯ ribs & diaphragm create vacuum that
draws air in passively
▯ reptiles, birds & mammals
3. Changes in the circulatory system:
▯ in fish, the 2-chambered heart receives only
deoxygenated blood from the body
▯ blood is pumped to the gills to pick up
oxygen before continuing to the rest of the
body
▯ single circuit
In tetrapods:
▯ double circuit: pulmonary and systemic
▯ different blood pressures in the 2 systems;
separate controls
▯ pulmonary circuit at low pressure (to
protect delicate lung capillaries); systemic
circuit at high pressure (to quickly deliver
oxygenated blood to working tissues)
▯ 3 or 4 chambered heart (3 chambered in
amphibians; 4 chambered in reptiles, birds
and mammals)
2 4. Changes in reproduction:
▯ external fertilization (common in fish) to
internal fertilization: less sperm is
needed; ensures paternity; protects
gametes and zygotes
▯ protected egg: shell; membranes;
internal development
▯ parental care is common in many
tetrapod groups (also common in many
fish!)
Major lineages in the tetrapods:
1. Amphibians
▯ most have smooth, moist skin; often have
chromatophores, poison glands
▯ some (e.g. toads) have dry skin
▯ respiration through lungs, skin, gills
▯ 3-chambered heart; double circuit
▯ metamorphosis (egg and larval stages in
water (or moist environments; adult stage
may be completely terrestrial)
Amphibians, as their name implies (amphi = both;
bios = life) are incompletely transitioned to the
terrestrial environment; their reproduction is still
tied to water.
▯ prone to desiccation (most frogs can survive
after losing 50% of their body water – this
is an adaptation that can permit some frogs
to survive complete freezing of body
tissues)
▯ aquatic reproduction (metamorphosis)
Some adaptations that allow amphibians to
reproduce outside of standing water:
▯ eggs may be laid in moist terrestrial
environments (e.g. leaf litter)
▯ eggs and/or larvae may be retained by
parent: in skin, in stomach…
2. Amniotes:
▯ Carboniferous period (360-285 mya)
dominated by amphibians, but a new form
appearing…
▯ cotylosaurs: early reptiles
▯ Amniota: all vertebrates with extra-
embryonic membranes surrounding the egg
▯ reptiles, birds & mammals
3 Amniote characteristics:
▯ lungs: complex; used for CO 2umping
▯ no more positive pressure respiration; head
can be smal
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