1

Uroch

orda

ta

Cephal

oc

h

o

rdata

My

xi

ni

Cephal

aspi

domorp

h

i

Cho

ndric

hth

y

es

Actin

o

p

terygii

Actinis

tia

Dipn

oi

Amphibia

Reptili

a

Mammalia

Tetrapods

Many intermediate fossils have recently been discovered

Millions of years ago

265

280

355

340

295

420

415

400

385

370

Devonian

Carboniferous

Silurian

Paleozoic

Permian

Ray-finned fishes

Coelacanths

Lungfishes

325

310

To
present

Acanthostega

Amphibians

Figure 34.20

2

Acanthostega was a fish-like tetrapod

-

aquatic (gills)

- four limbs

Devonian period (405-360 mya) had frequent droughts

3

Class Amphibia: amphibians

MEIs:

1. Adults partially terrestrial

a. better lungs

- still not very efficient (few, large alveoli)

- cutaneous breathing also

b. stronger skeleton

c. tetrapod (4 limbs)

- from paired fins

But larvae are still aquatic (gills, fins)

4

b. single ventricle

2. 3-chambered heart

a. separate pulmonary and systemic circulation

3. Poison glands in skin (alkaloids)

5

2. Includes sharks, skates, rays

- 750 species

4. Lateral line senses pressure

3. Cartilaginous skeleton

- reversal

Arose ~360 mya
5,000 species, mostly frogs and toads

Adaptive radiation during warm, wet

Carboniferous period (360-300 mya)

“Age of Amphibians”

6

Earth was warmer, so sea level was much higher