Synapsids/Basal Mammaliaforms

Question 1

Basal Mammaliaforms

Answer 1

• Complicated teeth replaced twice, tightly tied to lactation
• 2º palate
• No postorbital bar
• Atlas/axis complex
• Reduced cervical ribs
• Dexterous hands
• Opposable big toe (hallux)

Question 2

Cranial bones over evolution of synapsids

Answer 2

Basal eupelycosaur:
- No mobility between atlas and axis vertebrae
- Single occipital condyle
- Coronoid, articular, angular bones in lower jaw
Therian:
- Smaller incisors
- Temporal fossa and orbit moved dorsally and cranially
- Dentary bone = all lower jaw
- Coronoid process, angular process
- Condyloid process attaches jaws
- Paired occipital condyles
- Opening to mid-ear cavity
- Rotation between atlas and axis vertebrae (from paired occipital condyles)

Question 3

Synapsids/Basal Mammal Changes

Answer 3

• Changes to jaw joint
• Changes to teeth -> multicupsed, occlude
• Changes to muscles -> separated temporalis and 2 layers of masseter
• Tribosphenic teeth
• Ancestrally nocturnal and small

Question 4

Evolution of mammal jaw and mid-ear ossicles

Answer 4

Jaw joint between quadrate and articular ->
Dentary gets bigger, tympanum develops, back teeth get smaller ->
Huge synapsid fenestra, squamosal moves down and touches dentary, and 2 jaw joints ->
Dentary forms around Meckel’s cartilage, malleus, incus, and stapes detached from back of jaw and move into ear ->
Dentary is the only bone in the lower jaw, 1 jaw joint between squamosal and dentary bones, tympanum and middle ear bones are now associated with the skull

Question 5

Evolution of jaw closing muscles

Answer 5

Adductor mandibular in temporal fossa on coronoid bone of lower jaw, pulls jaw up and back ->
2 jaw closing muscles: temporalis (up and back pull) and masseter (low jaw, pull up and forward) ->
3 jaw closing muscles: temporalis (lower jaw up and back pull), deep masseter (low jaw up, back, and sideways), and superficial masseter (low jaw up and forward)