trabecular cornu 



superior gnathal cartilage 



^Meckel's cartilage 



.inferior gnathal cartilage 

 ^quadrate cartilage 



B 



pterygoid process 



processus oscendens 

 ntermondibular cartilage 

 trabecule communis 



intermondibular cartilage 

 trabecular cornu 



inferior gnathal cartilage 



Meckel's cartilages. 



trabeculo communis 

 prootic fenestra 



otic cap sule quadrate cartilage 

 hypohyal 



occipital arch 



ceratohyol. 



superior gnathal cartilage 

 igoment 



metotic foramen 



superior gnathal cartilage 



A 



carotid conalX o^ic operculum 

 fenestra vestibull 

 otic process of quadrate cartilage 



processus oscendens. 



C 



fenestra vestibuli 

 otic operculum 



jugular foramen (metotic foramen) 



Figure 4-22. Cartilaginous jaws and chondrocranium of tadpole. A, lateral view of entire chondro- 

 cronium; B, medial view of rigfit half of jaws; C, ventral view of chondrocranium. 



type of chondrocranium also is peculiar in that the anterior 

 parts appear before the posterior region. 



Metamorphosis is a rather lengthy process— from an alga- 

 feeding existence to a carnivorous one, from small pre- 

 mandibular jaws armed with horny teeth to large mandi- 

 bular jaws armed with bony teeth. The superior gnathal 

 cartilage, the upper jaw of the larva, gradually disappears 

 and is replaced by the premaxilla. The inferior gnathal 

 cartilage, which supports the horny teeth of the larval lower 

 jaw, becomes attached to the anterior end of Meckel's car- 

 tilage, and the dentary arises lateral to it. Ossification 

 converts the inferior gnathal to the mental bone. 



As these changes occur, the gap of the mouth moves back 

 and the articulation between Meckel's cartilage and the 

 quadrate moves back with it. At first the palatoquadrate 

 has its mandibular articulation anterior to the vertical of 

 the eye. With the posterior shift of this articulation, the 

 palatoquadrate undergoes drastic shortening and rotates 

 backward to form the definitive suspensorium. In this rota- 

 tion, the otic process of the otic capsule becomes shortened 

 and flexed and gradually disappears. At no time in its 

 development does the frog have a basitrabecular (basiptery- 

 goid) process or a basal attachment of the palatoquadrate 

 in the region of such a process. 



The development of bones begins during metamorphosis. 

 The bones of the jaws appear early, the teeth later. Several 

 bones present problems as to their homology. The palatine 

 and pterygoid appear to be properly named but the pre- 

 frontal, frontoparietal, and squamosal are in doubt. These 

 problems cannot be solved until transitional fossil forms. 



Unking the anurans with the basic skull pattern, are found. 

 However, speculations on the probable homology of these 

 parts are in order. 



The prefrontal is usually identified as the nasal, perhaps 

 because of its arising above the orbitonasal lamina. This 

 position is more typical of the prefrontal. A true nasal lies 

 over the nasal capsule and forms a part of the margin of the 

 external naris— this is not true of the bone of the frog. The 

 lack of contact between this bone and the nasal processes of 

 the premaxilla and the presence of a strong orbital process 

 extending down medial to the lacrimal duct support its 

 identification as the prefrontal. 



The frontoparietal presents a more easily solved problem. 

 According to Gaupp's (1906) account, there are two pairs 

 of centers for this bone. Recent observations suggest that 

 these bones arise from both single and double pairs of 

 centers. The general pattern among tetrapods, where large 

 orbits are involved, has been elongation of the frontals and 

 reduction in the size of the parietals. It is likely that re- 

 duction of ossification in the anuran line has involved sup- 

 pression of the parietal and its replacement by or hision 

 with the frontal. 



The squamosal of the anuran is a double structure in its 

 embryological development. One part is associated with 

 the muscular process of the palatoquadrate; the other 

 arises over the otic capsule above the paroccipital or parotic 

 process. The latter ossification center has been equated with 

 the postfrontal (or the supratemporal), but its lack of con- 

 tact with the frontal and its association with the otic capsule 

 is against the assumption that is belongs to the circumor- 



84 • HEAD SKELETON OF OTHER TETRAPODS AND CHOANATES 



