Fish to Tetrapod Transition

Tiktaalik - 'fishopod'

Fish on verge of becoming tetrapod

Large weight-bearing limb attached to rigid shoulder

Wrists rotate & flex supports weight / walks

Beginnings of short discrete neck

Ribs = broad & overlapping

Large, distinct radius & ulna (forearm)

Eyes spaced closely on skull

Large humerus (upper arm)

Tiktaalik

= crucial link between fish and tetrapods

Tetrapodomorphs

Lived in the water

Did not have fins with true digits

Not true tetrapods

True tetrapods

Had fins with true digits

Capable of moving on land

e.g. Tiktaalik, Panderichthys

Could not support self on land

Fossils found in tidal estuary rocks

Oldest Fossil Tetrapods Greenland

Icthyostega

Acanthostega

Most primitive tetrapod?

8 digits on each hand & foot

7 digits on each hand and foot

Longer neck

More rigid ribcage

More robust limbs bigger muscles / robust girdle

Lived in water, capable of venturing onto land

Inhabited land-water interface

Modern Tetrapods

baseline 5 digits

Why Venture onto Land?

STORIES
∴ hard to test

Habitat Loss
(Classic View)

Arid climate with lakes drying up in Devonian

Driven by envmtal change

Predation

Escape predators i.e. large arthopods

Protection - lay eggs on land

Thermoregulation

bask in warmth of sun

Move btwn drying pools

What we can say

Features allowed it - ventured onto land because they could

Tetrapods Diversify into Carboniferous

Become more suited to land

More derived, more terrestrial

Pederpes

More terrestrial animal

Looks less fishy

Only 5 digits

Metatarsal & metacarpal bones clearly distinguished

Muscle attachment
to support & control digits

Fish-Tetrapod Transition

Gradual transition / morphological change

Changes in skull / neck / rib / limb anatomy

Metacarpal - palm of hand

Metatarsal - foot

Devonian

became dominant

developed many ecologies

Growing diversity in size & ecologies

Tetrapods = primitive & experimentation with digits

Some lobe-finned fishes evolved features well-suited to life on land

Carboniferous

Diverse sizes and ecologies developed

Rapid evolution

Tetrapods became more fine-tuned to land

i.e. fleshy lobed fins, lungs