bital series. The two centers observed in the toad appear to 

 belong to a single bone, the squamosal. Separation can be 

 attributed to the shortening and rotation of the palatoquad- 

 rate during metamorphosis. 



What has been called the angular of the lower jaw is 

 described here as the prearticular in spite of the lack of a 

 foramen for the medial mandibular (chorda tympani) divi- 

 sion of the hyomandibular nerve. 



General observations on living amphibians The frog head 

 skeleton differs from that of the salamander in several fea- 

 tures. The frog has a prootic foramen for the trigeminal and 

 facial nerves anterior to the otic capsule and the prootic 

 ossification. The palatine branch of the facial nerve passes 

 over the area of contact between pterygoid and otic capsule 

 rather than behind and under it. The orbitosphenoid of 

 the frog lies anterior to the optic nerve and invades the or- 

 bitonasal lamina and nasal septum. That of the salamander 

 lies behind the nasal capsule and encloses the optic foramen. 

 The frontal and parietal of the frog are fused, the quadrato- 

 jugal is distinct and continues the labial arch back to the 

 quadrate— with which it is fused in both types. The frog has 

 a tympanum, ringed with cartilage or calcified cartilage, 

 and the stapes extends out to this membrane suspended in 

 the cavity of the middle ear. The para.sphenoid of the frog 

 has a narrow rostral part, a processus cultriformis, whereas 

 that of the salamander is broad. The inner aspect of the 

 otic capsule of the frog is ossified, isolating the various open- 

 ings in bone. 



The two groups a.gree in that the orbital series of bones is 

 undeveloped: both lack the postfrontal, the postorbital, the 

 jugal, and usually the lacrimal (some salamanders have the 

 latter). Both lack the tabular and postparietal; of the tem- 

 poral series, the supratemporal is questionably retained in 

 the anurans. Both have a large interpterygoid fontanelle, the 

 opening between the pterygoid and the parasphenoid 

 rostrum, and a reduced or absent palatine. There is no 

 ectopterygoid or epipterygoid. The teeth of the jaws are 

 numerous and pleurodont and agree in having a tooth 

 denticle fused to a hollow bony base which in turn is fused 

 along its outer aspect to the jaw margin. In both, the otic 

 capsule is usually formed by the prootic and exoccipital; usu- 

 ally a basioccipital is lacking. A supraoccipital is always 

 lackmg. The stapes is usually associated with an operculum 

 m the frog and some salamanders. In both the upper jaw is 

 fixed in position, the palatoquadrate connecting through 

 cartilage with the neurocranium. 



Fossil Amphibians 



Early Amphibians In terms of the head skeleton the frogs 

 and salamanders show much agreement, even though this 

 has been judged by some as indicative of parallel changes 

 rather than direct common ancestry. The frogs perhaps extend 

 back in time to Protohatrachus from the eariy Triassic of Mada- 



gascar. The salamanders trace back to the lower Cretaceous 

 and are then lost. Salamander-like organisms are known 

 from the Upper Pennsylvanian; these are the branchiosaure. 

 The branchiosaurs are small in size. In their life history, 

 which is known in some detail, they went through a period 

 with internal and external gills followed by a period with 

 only external gills. As adults they were lung breathers. 

 These small animals have been viewed as the larvae of 

 larger forms, but, if not adults themselves, they approximate 

 the adult form of the ancestral amphibian better than the 

 larger forms. At least some of the genera of this group agree 

 with the living forms in having four digits on the forelimb; 

 these are Amphibamus and Branchiosaums (Figure 4-23). 



The dermal skulls oi Amphibamus and Branchwsaurus agree 

 m most details. The roof consists of paired postparietals, 

 paired parietals between the anterior ends of which is a 

 parietal foramen, then paired frontals and nasals. Lateral 

 to the postparietals is a small tabular in Branchiosaums but 

 not in Amphibamus. Lateral to the parietal is a large sup- 

 ratemporal in both, but neither has an intertemporal. 



The circumorbital ring of bones consists of prefrontal, 

 postfrontal, postorbital, jugal, and lacrimal in both. The 

 lacrimal and jugal do not meet below the eye in Amphibamus. 

 The lacrimal in at least one species of Branchwsaurus con- 

 tains a canal for the lacrimal duct extending from the orbit 

 margin, where it has two openings, to below the ventral mar- 

 gin of the external narial opening. There is no septoma.xilla 

 at the posterior margin of the external narial opening. The 

 squamosal is a large plate forming much of the anterior 

 and ventral margins of the tympanic notch. 



The upper jaw consists of premaxilla, maxilla, and 

 quadratojugal and the palatoquadrate complex of ptery- 

 goid, epipterygoid, and quadrate. There is a broad vomer 

 plate to either side of the midline. The parasphenoid has a 

 broad basiotic expansion and a long, narrow, cultriform 

 process. 



The endocranium was largely cartilaginous but exocci- 

 pital ossifications have been observed as well as a center in 

 the otic capsule (opisthotic?). The delicate stapes had a foot 

 plate expansion and a shaft perforated by a foramen. There 

 was a sclerotic ring of more than twenty overiapping, bony 

 plates. The lower jaw is not well known but had dentary, 

 angular, supraangular, prearticular, coronoid, and artic- 

 ular ossifications. The cartilaginous branchial skeleton is 

 not known. 



Even more primitive than the branchiosaur, in terms of 

 having a larger number of bones in the cranial roof, and in 

 having grooves for sensory canals, is Eugyrinus wildi from 

 the Lower Pennsylvanian. In general size, shape, and flat- 

 tening of the head skeleton, this is a branchiosaur. In addi- 

 tion to the bones described above there was an intertem- 

 poral and a septomaxilla. The parasphenoid is grooved 

 and perforated by the internal carotid and there are slight 

 basipterygoid processes. In the palate there is a large pair 

 of vomers and, to the sides behind the internal nares, a 



OTHER TETRAPODS • 85 



