Tetrapoda

Question 1

Tiktaalik synapomorphies/characteristics?

Answer 1

• Neck
• Short hyomandibula
• Loss of opercular bone

Question 2

Evolution of nostrils (of sarcopterygians)

Answer 2

• Coelacanth: excurrent nostril and incurrent nostril both open on snout
• Neoceratodis: Incurrent nostril is inside upper lip and excurrent nostril opens inside upper lip/mouth?
• Tetrapodomorph: Incurrent nostril on surface of head, choana (homologue of excurrent nostril) opens inside oral cavity

Question 3

Tiktaalik Skeleton: fish-like features

Answer 3

• Unossified vertebrae, no zygapophyses
• Long trunk
• Ulna shorter than radius and lacks olecranon process
• Short fin rays retained
• No sacral articulation
• Pubis but no ischium, left and right halves of pelvis not fused
• Hip socket located posteriorly

Question 4

Tiktaalik Skeleton: tetrapod-like features

Answer 4

• No supracleithral or opercular bones
• Process for forelimb flexor muscles
• Homologues of wrist bones
• Elbow can flex
• Ulna can rotate hand
• Robust, over-lapping ribs
• Ilium ascends dorsally
• Hip socket faces laterally
• Hindlimb smaller in size to forelimb

Question 5

Devonian tetrapods: skeleton of Acanthostega

Answer 5

• No fin rays, now digits
• Soft tissue operculum
• Weak zygapophyses
• Small ribs
• Tripartite pelvis
• Weak sacral connection
• but caudal fin with fin rays

Question 6

Devonian tetrapods: skeleton of Ichthyostega

Answer 6

• Olecranon process of ulna
• Robust, overlapping ribs
• Robust tripartite pelvis
• Robust paddlelike hindlimb with 7 digits
• Strong zygaphophyses
• Strong sacral connection
• but small caudal fin with fin rays

Question 7

Evolution and development of Sarcopterygian fins and limbs

Answer 7

• Metapterygial axis bends through evolution of tetrapoda
• Gene expression of fin development: hoxd13 expression on postaxial side of fin (“top”) and alx4 is expressed on preaxial side (“bottom”), basically half and half of fin
• Gene expression during forelimb development: early de v resembles fin dev, but domain of hoxd13 increases disproportionately = becomes digits, alx4 expression is restricted to arm and forearm (above thumb, bottom part of wrist)

Question 8

Tetrapoda crown

Answer 8

All living and most recent common ancestor