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Origin of Land-Living Tetrapods (What is a tetrapod? (Arms & legs…
Origin of Land-Living Tetrapods
Organisms
Venture onto Land
First organisms
probably arthropods in Cambrian
Significant step
Digits and limbs develop
Tetrapods venture onto land
Vertebrates - organisms with bones
Life Transition
Ocean dominated by fishes
Land inhabited by tetrapods
Fish evolve to live on land
What is a tetrapod?
Vertebrate living on land
Arms & legs with digits
Robust limbs
hold up against gravity
Girdles
can rotate to support body weight
Discrete neck to support head
with bones & strong muscles/ligaments
Nostrils
for breathing air
Rigid ribcage
provides support & protection & muscle anchorage
Where tetrapods come from?
Broadly tetrapods evolve from fish
Gnathostoma
fish with jaws
Ostheichthyes
fish with bones
Sarcopterygians
fleshy fin with main axis
Tetrapods
turned fleshy lobed fins into arms & limbs
digits on each limb
muscles btwn fin bones
Closest Living Tetrapod Relatives
Coelocanths
Lungfish
Fleshy muscular lobed fin
Walks along bottom river for short periods
Lungs to breathe air
secondary breathing system
Fossil Relatives
Devonian
1. Osteolepis
Sarcopterygians
fleshy lobed fins
internal nostril on palate
must've had lungs
Fins reduced
2 paired bones in fin
2. Eusthenopteron
Bigger size
Sarcopterygian feats
fins
internal nostril
Teeth with infolded enamel
Simplified skull
roof bones
reduced number and larger plate like bones
Shared characters with tetrapods despite being fish
3. Panderichthys
Flattened body
doesn't look like fish anymore
Close eyes atop snout
Large & long humerus
(upper arm bone)
Tiktaalik - 'fishopod'
Fish on verge of becoming tetrapod
Large weight-bearing limb
attached to rigid shoulder
Wrists rotate & flex
Beginnings of
discrete neck
Ribs = broad & overlapping
Tiktaalik
up close
Short neck
Large, distinct radius & ulna
(forearm)
Eyes spaced closely on skull
Wrist rotates, flexes
& supports weight / walks
Large humerus
(upper arm)