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.
2.
3.
4.
limbs/girdles- better support & muscle
attachments
skull/jaws- new feeding modes
vertebrae- better support
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