Return to BSC 310 Page.

BSC 310. COMPARATIVE VERTEBRATE ANATOMY Mar. 7, 2000

LECTURE 13.            THE SKULL II.

DEVELOPMENT OF MAMMAL SKULL FROM PRIMITIVE REPTILE
SKULL .
(See Walker and Homberger 79-83)
1. Secondary Palate. (Walker 79-81).
Primary palate. in primitive tetrapods and many reptiles. Palate is the roof of the nasal and oral cavities which are the same, forming a common path for food and air. See ventral views of labyrinthodont (Walker fig. 4-8 p.64 ) and tegu lizard (Walker fig. 4-13 p. 73).
In mammals, a secondary palate develop to separate air and food passages, elongated in life by soft palate over pterygoid processses (see ventral cat skull) (Walker fig. 4-25, p.85)
Hard palate formed by medial extensions of premaxilla, maxilla and palatine forming hard palate (compare labyrinthodont and therapsid on p. 81). Therapsids have a short secondary palate (Walker Fig. 4-20, p. 81)
Mammals are warm-blooded. High Metabolic Rate (5-10 times that of reptiles). Must breath continually, while eating to support need for constant oxygen supply.

2. & 3. Temporal fenestra and Braincase Ossification.
Temporal Fenestrae: Openings in dermal skull roof lateral to braincase, allowing attachment of muscles, and perhaps laterall bulging of contracting muscles below skin.Anapsid condition: no temporal fenestrae. Found in labyrinthodonts, primitive extinct reptiles such as cotylosaurs, and retained in turtles.
See:Walker Fig. 4-19 p. 80 three cross-sectional views of dvlpmt. of temporal fenestrae from anapsid stem reptile (=cotylosaur), to therapsid (mammal-like reptile) to mammal;  figure on handout on opposite side of Dermal cranium page.
See Walker (fig. 4-21. p. 81), showing enlargement of temporal fenestra in Thrinaxodon, a cynodont (advanced therapsid), relative to condition in Dimetrodon, a pelycosaur.
(One opening in synapsids, two openings in diapsids).
Stage A. Cotylosaur (primitive reptile). No temporal fenestra. Jaw muscles from jaw to underside of skull.Braincase walls unossified anteriorly, between orbits, as in turtle and lizards.
Stage B. Pelycosaur to Therapsid.(Recall pelycosaurs, therapsids, and mammals are all synapsids). Temporal fenestra opens. (Postorbital decreases in size). Frontal and parietal bones lower a portion to form lateral braincase wall. Jaw muscles attach to edges of temporal fenestra, and perhaps bulge out through hole. Development of zygomatic arch (jugal=zygomatic and squamosal), serving for muscle attachment.  Brain itself enlarges , pushing out sides of braincase.  Downgrowths of parietal and frontal bones and upgrowth of former epipterygoid (a splanchocranium element, derived from palatoquadrate cartilage) becomes alisphenoid form ossified braincase, medial to muscles and to orbit.
[ postorbital-squamosal connection lost in therapsid cynodont Thrinaxodon.]
Stage C. Mammal. Downgrowth of parietal and frontal meets upgrowth of alisphenoid to form braincase wall, while roofing bones of parietal lost.  Zygomatic arch remains as remnant of lower border of temporal fenestra. Great lateral growth of braincase walls, bulging out into former fenestra.

4. Mandible lower jaw) reduced from 8 bones in reptiles and pelycosaurs to the single tooth-bearing dentary in mammals. Therapsids: dentary of increased importance. (Walker fig. 4.22, p. 82; Kardong fig. 7.55, p. 263.)

5. Jaw-joint. Quadrate-articular in reptiles and pelycosaurs, both dentary-squamosal and quadrate-articular in therapsids; dentary-squamosal in mammals. [POSTDENTARIES angular, articular, coronoid, prearticular, splenial, surangular; dermal forms tympanic part of temporal(=bulla)].

6. Ear. Middle ear bones: Stapes (columella) in reptiles and pelycosaurs (hyomandibular cartilage, splanchnocranium.) Quadrate and articular involved in sound transmission from the lower jaw in therapsids Itself specialized to receive sound in therapids).
In mammals, quadrate and articular become incus and malleus.
Malleus-incus-stapes chain constitute middle ear. Amplifies sound from tympanic membrane.
Sound transmission: Tympanic membrane (eardrum) to middle ear ossicles to (oval window of) cochlea of inner ear.
(Discussed  in Walker pp.234-235; Kardong p. 262;
Reptile and mammal ear bones illustrated Walker Fig. 8-16, p. 234).

7. Dentition. Reptiles: homodont. All teeth the same form. Pelycosaurs: homodont but with 2 canines on each upper jaw (see Dimetrodon). Therapsids and mammals: heterodont. Teeth have different forms and functions. e.g. mammals, incisors, canines, premolars, molars (recall cat skull lab).

8. Occipital condyles. One in reptiles and pelycosaurs; two in therapsids and mammals.

Other Skull Changes.  (mostly not covered in lecture).
nasal capsules (chondrocranium) unossified in reptiles and pelycosaurs, ossified as nasal turbinates in mammals, and some therapsids.
lacrimals shrink.
Lost:prefrontal, postfrontal, and postorbital; quadratojugal, intertemporal, supratemporal and tabular.

REPTILE-MAMMAL TRANSITION
(See Kardong 257-269)
FEATURE   Reptiles           Pelycosaurs         Therapsids         Mammals                
PALATE     Primary             Primary      Add Secondary            Add Secondary Pal.
TEMPORAL None              One              Loss of squamosal.      Large
FENESTRA                                            post-orbita. junction

BRAINCASE Incompl     Downgrowth of flange from parietals   B'case walls Sealed by
           ossified                 & frontals, upgrowth of epipterygoid    downgrowth of  parietal    Cartilaginous walls         Walls still partly cartilage.                    and frontal to meet upg
anteriorly (between          Decrease in dermal skull roof .             upgrowth (of  epipteryg.)
orbits).                                                                                        Most of  dermal roof lost
                                                                                                 lost, except zygomatic arch.

MANDIBLE Eight bones Eight bones       Dentary large:           Dentary only (others
                                                                  others reduced          present in prim. mammals)
                         TREND: Dentary of increasing importance

JAW-          Quadrate-     Quadrate            - Dentary-squamosal.     Dentary-squamosal.
JOINT         Articular      Articular          & Quadrate-Articular       only

EAR:            Stapes         Stapes              Stapes, but Q-A          Malleus,  Incus & Stapes
                    (=Columella)                       transmit sound           (=Artic. -Quad.--Stapes)

Splanchnocranium
Hyomandibular = Stapes = Stapes
Palatoquadrate=  Quadrate = Incus
Meckel's (mandib.)=Articular=Malleus)

OCCIPITAL One          One                      Two                               Two
CONDYLE
 
DENTITION Homodont Homodont         Heterodont                  Heterodont (C, I, P, M)
                                        (+ 2 canines)
 

_____________________________________________________________________________