Jaws

Question 1

jaw formation

Answer 1

jaws from jawless ancestors

first appearance in placoderms; continued in extant gnathostomes

Question 2

jaw origin

Answer 2

one of the anterior pairs of gill (branchial) arches

Question 3

10 pairs of gill arches in ancestral agnathans

Answer 3

terminal
premandibular
mandibular
hyoid
branchial arches I-VI

Question 4

7 arches in extant gnathostome fishes

Answer 4

mandibular
hyoid
branchial arches I-V

Question 5

evidence for pharyngeal arch origin

Answer 5

arches develop in series; jaws appear as “enlarged anterior arches”

blood vessel and nerves in jaws with similar pattern as arches

jaw muscles appear to be modified pharyngeal arch muscle

spiracle in Chondrichthyes is “crowded” gill slit

Question 6

2 jaw origin theories (Fig 7.7)

Answer 6

serial theory

composite theory

Question 7

serial theory

Answer 7

one arch = jaws

recall 10 gill arches in ancestral condition (terminal, premandibular, mandibular, hyoid, 6 branchial arches)

serial theory: from mandibular arch exclusively; loss of terminal and premandibular

Question 8

composite theory

Answer 8

loss of terminal; fusion of part of premandibular and mandibular

Question 9

palatoquadrate

Answer 9

upper jaw

Question 10

Meckel’s

Answer 10

lower jaw

Question 11

hyomandibula

Answer 11

upper half of hyoid arch

Question 12

mammalian middle ear bones/ossicles

Answer 12

ossic (L) - “little bone”

quadrate (from palatoquadrate) = incus
articular (from Meckel’s) = malleus
stapes

forms from upper hyoid arch component: hyomandibula

So… all ear ossicles in mammals derived from splanchnocranium

Question 13

reptile/amphibian ear ossicles

Answer 13

only have one - stapes

Question 14

columella

Answer 14

stapes

Question 15

mammalian splanchnocranium forms…

Answer 15

middle ear bones
initial scaffold for dentary
“hyoid bones”

Question 16

hyoid bones in mammals

Answer 16

corpus (main body)
cornua (horns)

part of each hyoid horn fuses with braincase; contributes to styloid process; styloglossus and stylohyoid muscles