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Tetrapods
From Water to Land
TETRAPODS
tetra = four
podia = foot
Tetrapod = animal with four feet (or limbs)
and those descended from them
LOBE FINNED FISHESAncestor of all tetrapods – extant and extinct
Fleshy “fins” – lobe fins – not highly effective for swimming
Lobe fins more likely used to assist locomotion along sea
bottom
Decedents similar to these early lobed finned fishes are still extant today –coelacanths
Structure of fin –
bone surrounded
by flesh
Other populations branched into lungfish
Provides insight into development of lungs
lamprey
sharklobed finned fishes
ray finned fishes with swim
bladders evolved from early
lungs
tetrapods
FIRST TETRAPODSEarly tetrapods began developing increased articulation and
range of motion in limbs
Weight
bearing
elbow; wrist
bends
humerus
radiusulna
Tiktaalik
375 mya
Tiktaalik Mudskipper
Limbs and skull much like amphibians, still retain fish like tail
and gills.
Front limbs could bear some weight.
Strong, supportive internal skeleton to counteract gravity.
Hind limbs not likely useful for land locomotion.
Ends of limbs differentiated into digits.
Icthyostega
365 mya
Likely spent most
of time in water
with only brief
sojourns on land.
The Devonian
DEVONIAN MASS EXTINCTIONClose to the time the earliest tetrapod ancestors made tentative
forays onto land, a mass extinction was underway.
The Devonian mass extinction event was one of the longest in
life’s history and marked the end of the “Age of Fishes.”
It lasted approximately 20-25 million
years – most mass extinctions
happen over the course of about 1
million years.
During this mass extinction, 70-80%
of all animal species went extinct.
Shallow marine life appears to have
been amongst the most common
species to go extinct.
Thousands of fish species went extinct, including
the 35 foot, 8000 lb apex predator, Dunkleosteus.
DEATH MEANS OPPORTUNITY
The massive die out of species inhabiting shallow seas provided opportunity for the survivors.
New survival necessities were available to them, such as shelter and space.
On the other hand, food resources became more scarce.
This combination is believe to have created remarkable selection pressures.
Natural selection favors
individuals most able to
adapt to change – and this
was a massive change.
INTERTIDAL ZONESIntertidal zones are near oceans and experience dramatic differences in water levels
throughout the days as the tide shifts.
During low tide, sea bed that is usually under water, becomes exposed…. and so do the
smaller animals that inhabit that shallow sea bed.
Tetrapods are thought to have started to flourish and diversify during in these areas during
or right after the Devonian mass extinction event.
Consider the mass extinction
combined with the
characteristics of intertidal
zones.
What is a selection pressure
that would have resulted in the
success of early tetrapods?
ACCESS TO FOOD
One very plausible theory
is that early tetrapods were
able to exploit the food
resources stranded in such
intertidal regions while
true fish, without the ability
to breathe out of water,
could not.
This could create a very strong selection pressure favoring those
individuals with the ability to survive short periods of time out of
water.
Because the food sources were
stranded and relatively
immobile, early tetrapods did
not have to be particularly
agile.
They simply had to be able to
breathe out of the water and
move enough to get from one
piece of food to the next.
Why weren’t predators a
problem for them?
Analogous Mudskipper
The Tetrapod Blueprint -- Tiktaalik
Role of Predators