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