Paleozoic Tetrapod Origins/Radiation:

Introduction/Overview

The Movement to Land

• origins (mid-late Devonian)

• sarcopterygian ancestry

• land invasion of plants & invertebrates

Rhipidistian Crossopterygian

Ancestry • radiations: late Paleozoic/early Mesozoic

• ancestors to modern amphibians (Temnospondyls)

• data: skulls, labyrinthodont teeth, limbs/girdles

advantages/selective forces influencing the move to land

1. increasingly xeric environment

2. predation pressure

3. absence of competition on land

4. untapped food resource on land (insects)

5. low O2 levels in warm stagnant pools; lungs preadaptive

6. dispersal opportunities

physical properties of air & water

1. density & viscosity

2. gravity

3. temperature extremes

4. oxygen & dissolved gases

key problems associated with terrestrial existence

1. Respiration

2. Desiccation

3. structural support & locomotion 4. sensation

5. feeding

gravity & support/locomotion;

solutions • modifications of vertebral column

1. zygapophyses- interlock/link adjacent vertebrae to prevent sagging/twisting

2. notochord replaced by centrum

3. pectoral & pelvic girdles strengthened

major structural changes

1. limbs/girdles- better support & muscle attachments

2. skull/jaws- new feeding modes

3. vertebrae- better support

4. lungs

Labrynthodont Tooth

modifications to the pelvic girdle

1. pectoral girdles free from head; allowing head movement

2. pelvic girdle fused to vertebrae allowing thrust

3. enlarged muscle attachment

Pelvic Girdles

zygapophyses

respiration

• gills do not function efficiently in air; gill filaments adhere reducing surface area & evaporation

• lungs are internal respiratory structures that protect from desiccation

• cutaneous respiration

sensory

• lateral line system relies on density of water

• changes required for sound detection; sound transfer from air--->fluid

reproduction

• fishes generally have external fertilization & development

• external fertilization & development is rare among terrestrial organisms; gametes can't withstand desiccation

• solutions: amniotic/cleidoic egg; air/water barrier

Ichthyostega

Acanthostega

Skull Anatomy

Amniotic Egg