SKELETAL SYSTEM: SKULLS

I.	Chondrocranium (neurocranium):

	A.	Endochondral bone or cartilage.
	B.	Surrounds and protects brain & sense organs in elasmobranchs.
	C.	Elements appear to lie in series w/ vertebrae.
	D.	In most verts, cartilage serves as scaffold, replaced by bone.
	E.	Centers of development - see Kardong Table 7.1 & Fig. 7.3

II.	Splanchnocranium:  (Fig. 7.5)
	A.	Structure:
		1.	Derived from neural crest cells which migrate into walls of pharynx.
		2.	Consists of non-dermal bone, cartilage, or both.

	B.	Origin of jaws:
		1.	From one of anterior pair of gill arches.
			a.	In sharks, jaws and branchial arches develop similarly in series.
			b.	Spiracle appears to be a reduced gill slit that was crowded forward by 
                                enlarged hyoid arch.
			c.	Similar nerve and blood vessel distribution patterns.
			d.	Musculature appears to be modified branchial arch musculature.

		2.	Alternate hypotheses:  (Fig. 7.7)
			a.	Serial theory: first or second branchial arch --> mandibular arch.
				Next branchial arch --> hyoid arch
			b.	Composite theory: mandibular arch derived from losses or fusions of parts of 
                                 several anterior branchial arches.

	C.	Types of jaw attachments:
		1.	Euautostyly: mandibular arch suspended from skull by itself (no help from hyoid 
                        arch).  eg. placoderms, acanthodians.

		2.	Amphistyly:  jaws attached to braincase via two articulations: 1) ligament 
                        connecting palatoquadrate to skull, 2) posteriorly by hyomandibula.  
                        eg. early sharks, some osteichthyans; modern sharks slightly modifies.

		3.	Hyostyly: mandibular arch attached to braincase via hyomandibula.
			a.	Modern bony fishes.
			b.	New dermal bone, symplectic bone, may aid in jaw suspension.
				eg. modified hyostyly of teleosts.

		4.	Metautostyly: jaws attached to braincase directly through quadrate (a bone 
                        formed in posterior part of palatoquadrate).  eg. most amphibians, reptiles, birds. 
                        (Fig. 7.8)
			a.	Hyomandibula not involved in jaw suspension; instead gives rise to columella 
                                (stapes), which is involved in hearing.
			b.	Other elements of second and third arches contribute to hyoid apparatus that 
                                 supports tongue & floor of mouth.

		5.	Craniostyly: entire upper jaw is incorporated into braincase, lower jaw articulates 
                        with squamosal bone (dermal origin).
			a.	Occurs in mammals.
			b.	Lower jaw consists of pair of dentary bones (dermal origin).
			c.	Shift from articular/quadrate to dentary/squamosal jaw joint.
			d.	Palatoquadrate and Meckel's cartilages give rise to quadrate and articular 
                                elements, which ultimately become the incus and malleus of the middle ear.
			e.	Therefore, in mammals, the splanchnocranium does not contribute to jaws or 
                                 their suspension.  Does contribute to hyoid apparatus and middle ear bones.

III.	Dermatocranium:

	A.	Structure:
		1.	Dermal bones derived from dermal armor of integument, due to shift from dermal 
                        (early fishes) to subdermal (some fishes and all tetrapods) development of bones.
		2.	Forms sides and roof of skull to complete braincase.
		3.	Fuses with or surrounds chondrocranium in most vertebrates.
		4.	Some bones incorporated into jaws as tooth-supporting elements.

	B.	Evolution of dermatocranium:
		1.	Trend toward reduction in number of dermal elements thru loss or fusion.

		2.	Squamate reptiles: erosion of dermal elements correlates w/ skull kinesis.

		3.	Some dermal bones have fused w/ endochondral bones to give rise to bones w/ 
                        dual origin.  eg. temporal

		4.	Temporal fenestrae (openings):  (Fig. 7.34)
			a.	Anapsid: no temporal openings.  eg. cotylosaurs, turtles.

			b.	Diapsid: two temporal fenestrae separated by postorbital and squamosal.
				eg. crocodilians, Sphenodon.
				Modified in snakes, lizards, birds.

			c.	Synapsid: single temporal fenestra.  eg. pelycosaurs, therapsids. May be 
                                modified by loss or reduction of postorbital bone in modern mammals, 
                                allowing merging of fenestra w/ orbit.

			d.	Eurapsid: single temporal fenestra, bounded by different bones.  eg. extinct 
                                 ichthyosaurs & plesiosaurs.

		5.	Function of temporal fenestrae:  Fig. (7.35)  {open up space in skull for 
                        attachment of jaw muscles, eg. masseter.  Probably led to increase in size of 
                        dentary.}

		6.	Reduction in size of postdentary elements --> increased oscillatory response.  
                        (Figs. 7.11, 7.55, 7.56)

		7.	Formation of secondary palate in crocodilians and mammals.  (Fig. 7.57, 7.58).