Earth Sciences 2266F/G Lecture Notes - Lecture 25: Mandible, Squamosal Bone, Occipital Condyle

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Lec 25
- Most of the features evolved from cynodonts later appear in mammals
- Review: zygomatic arch, coronoid process (strong muscles to control lower jaw)
- Reduction in lumbar ribs, double occipital condyle
Double occipital condyle is clear to see in humans
In dinosaur head, there is a single knob; so advanced synapsids are quite diff from advanced
diapsids
Modification of jaw joint
In cynodont, started off with 3 bone pieces: dentary, angular, articular that probably came from
the 3 gill arches in fishes
Angular and articular bones become reduced until they were tiny pieces in cynodonts
Not ear bones yet
These 2 bones got sucked into the ear canal and became true ear bones later in evolution
Single dentary bone in lower jaw didn’t happen until true mammals
Angular and articular become further reduced in cynodonts b/c development of 2nd jaw joint
Orig jaw joint is the articular-quadrate jaw joint, in the back of the skull, seen in reptiles
Later on in evolution, b/c of the enlargement of the coronoid process, a 2nd jaw joint starts to
develop (seen as the other blue dot in therapsids), between dentary and the squamosal bones; this
2nd jaw joint is called the dentary-squamosal jaw joint
Quadrate is embedded in the squamosal bone
When we lost the orig jaw joint, the “2nd” jaw joint becomes the only jaw joint in mammals
Stapes: originally reptiles only have this stapes inherited from hyomandibular of fish
Whenever this vibrates, nerves sense the vibration
Eventually, the quadrate and articular got reduced here, but there’s still a primary jaw joint so it’s
still mobile
In cynodonts, they’re a small bone but not ear bones yet
True mammal middle ear (DMME): true ear bones
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