Evolution of the Jaw

Question 1

What did jaws evolve from?

Answer 1

Evolved from the anterior branchial arch (mandibular arch)
•gnathostome condition - first branchial/mandibular arch is enlarged
•subsequent arches like the hyoid arch can be used to support the jaw

Question 2

Process of the jaw’s evolution

Answer 2

Debatable still on how…
1996 Ventilation hypothesis -probably initially was a ventilation device for gills
•allows mouth to open and close - water can be forced into mouth effectively and over the gills
•1st branchial gill arch became enlarged to open and close entrance to pharynx
•vertebrates getting larger and more active, needed efficient gas exchange
-selection acting for a better mechanism

Question 3

Problems with ventilation hypothesis

Answer 3

•mandibular arch doesn’t actually form a functional gill arch in any living vertebrate or any fossil forms that have been found so far
•never been used in respiration - why would they enlarge for it?
•molecular evidence shows that gill arch has different origin to others
•agnathan arches on outside, and gills inside bt opposite for gnathostomes
-very different so agnathans not good comparison to make

Question 4

Why is the ventilation hypothesis good?

Answer 4

Explanatory power- makes sense
•increasing ventilation over gills would’ve improved suction in the mouth
-could suck larger prey in and keep in mouth - secondary function

Question 5

Paired fins and what they could be used for…

Answer 5

•gnathostomes have paired pelvic fins and pectoral fins
•midline fins only has 1 - not paired, dorsal and anal fin (obviously caudal fin -tail)
•fish can control their body & movement more efficiently
-tail gives thrust
-fins stabilise the body
-other functions too like spines in fin for defence or colour warnings etc.

Question 6

Function of fins (terms)

Answer 6

Pitch - movement up and down, controlled by pelvic and pectoral fins
-stops moving backwards or forwards
Yaw - movement side to side, controlled by dorsal and anal
Both helped by pushing against the dense water

Question 7

Development of paired fins

Answer 7

Genes responsible for dorsal fin of lamprey also responsible for midline, anal and paired fins of bony fish
The same genes (as fin formation) are responsible for limb development in the tetrapods

Question 8

Gnathostome features

Answer 8

•presence of jaw - supported by 2nd gill arch
•spiracle - remanence of first gill slit
•teeth present in jaw - very similar to the denticles of skin
•2 nostrils at from of head, leads to olfactory tracts and bulbs
-could jawless have nostrils without jaws?
•musculature in gill region - aid ventilation
•different musculature between dorsal and ventral - middle = lateral line
•paired fins and pelvic
•duplication of Hox genes

Question 9

Placoderms

Answer 9

Lived 430-460mya
•bony plates on their skin covering head and trunk
•hinge between bony plates of head and trunk so can move jaw
•had teeth? debate about this
•has no modern equivalent
•variable - some like sharks others like rays
•showed viviparity (birth to live young - fossil carrying one)
•fossils found male claspers - itromittent organ, but separate from pelvic fin (different to modern) = copulation