Amphbian predecessors

Question 1

first appeared? age of amphibians? radiations?

Answer 1

• First appeared in devonian (419-359 mya)
Carboniferous period 359 mya - the age of amphibians (large in number)
• Climatic and biotic changes
○ Major radiation of insects (food)
○ Different selective pressures on land vs water
○ Adapted to land 3 separate times in late Devonian: Eifelian/-/Frasnian/-Famennian extinction events and final bottleneck land invasion

Question 2

terrestrial selection pressures for amphibians

Answer 2

1. Maintaining water balance
   • Air is dry relative to water
   • Humidity directly affects activities- sharp drop on land
   
   2. Gas exchange with air - diffusion - no longer simple
      • Air has 20x the O2 conc. Than water
      • Air sac = not as effective as lungs, had to develop
      • Evolution of internal lung
      • Major evolutionary event driving evolution of internal skeleton
   3. Gravity
      • Water is 800 times denser than air
      • Buoyancy of water counteracts most of the forces of gravity
      • On land, there is no buoyancy to counteract the forces of gravity
      • Support in terrestrial vertebrates provided by lateral appendages and axial skeleton - limbs provide cushioning affect
      • Different selective pressures

Question 3

amphibian ancestors?

Answer 3

• Tetrapodomorph fishes: Eusthenopteron,
Panderichthys & Tiktaalik
• Early tetrapods: Acanthostega & Ichthyostega
•

Question 4

• Tetrapodomorph fishes

* Eusthenopteron,

Answer 4

Question 5

• Tetrapodomorph fishes

panderichthys

Answer 5

Question 6

describe Tiktaalik roseae

Answer 6

• Late devonian 375 mya
  
  • Tiktaalik - a large freshwater fish seen in the shallows - in the local inuktitut language
  • Has a mix of fish and tetrapod/ amphibian traits = transitional form
  • Fishapod- intermediate between fish (eusthenpteron) and early tetrapods (acanthostega and ichthyostega)
  • 1-3 m long - based on over 10 specimens
  • Eyes located dorsally
  • Crocodile like flattened head with sharp teeth
  • Well developed lung

Question 7

tiktaalik modifications skeletal. primitive? advanced?

Answer 7

○ Skeletal modifications:
○ Pectoral girdle
§ Like a lobe finned fish but also a distinct shoulder, elbow, wrist
§ Fish like fins (no toes)
§ Neck could move independent of body (unlike other fish)
○ Pelvic girdle
§ Primitive features: no ischium; no attachment for sacral rib
§ Advanced features: enlarged; paired with broad iliac processes; flat and elongated pubic bones
○ Rib cage
§ Well-developed and tetrapod like
§ Suggest benthic (shallow h2o); could move on land for short periods

Question 8

what link did tiktaalik provide? Features (fish vs fishpod vs tetra)?

Answer 8

• Tiktaalik provided the “missing link”(mosaic
of features) between sarcopterygian ancestors
and their tetrapod descendents
• Summary:
• Fish-like: gills, scales, fin rays, caudal fin
• Fishapod-like: limb bones and joints
• Tetrapod-like: ribs, neck, lungs

Question 9

Devonian tetrapods from east Greenland?

Answer 9

• Acanthostega - more fish like features, but still had tetrapod features
Ichthyostega: modifications for both aquatic and terrestrial life
• Acanthostega & Ichthyostega were primarily aquatic
Both had:
○ Internal fish like gills - > fishlike aquatic respiration
○ Probably also had lungs
○ Polydactyly having more than 5 toes
Reduced scale cover - more glandular like body

Question 10

Answer 10

Question 11

why move onto land?

Answer 11

redator protection
○ Selection favored forms in shallow water habitats
• Water shortage
○ Devonian was a period of warming
○ Shrinking aquatic habitats
• O2 shortage
○ Low O2 In warm shallow H2O
○ Atmospheric O2 possibly lowest in Earth’s history
○ Structures of “lungs” and “legs” pre-adaptations for life on land