terms of the large size of the orbit and brain case. Restric- 

 tion of weight is also suggested by the relatively thin or very 

 spongy bones. The bones have dense surface lamellae joined 

 by strands or trabeculae of bone. Fusion of the component 

 bones adds to the strength of the brain case. Reduction of 

 the jaws to a horn-sheathed beak may be related to both 

 weight restriction and to the style of feeding. 



Because of the fusion of parts in the cranium, the head 

 skeleton of the adult chicken is not appropriate for study. 

 Chicks taken during the first month of development can be 

 cleared and stained. Such preparations clearly indicate all 

 of the bones as well as something of their origin (Figures 

 4-13,4-14). 



Although of diapsid origin, the bird possesses only a labial 

 arch, a part of which is the quadratojugal. The posttem- 

 poral fossa has been obliterated by expansion of the brain 

 case out to the skin. The parietal, frontal, and squamosal 

 make important contributions to the endocranial wall. 



There has been some loss of bones in the skull of the bird; 

 a lacrimal (the prefrontal is usually identified as the lacri- 

 mal), postfrontal, postorbital, supratemporal, ectopterygoid, 

 and epipterygoid are lacking. The pterygoid is peculiar in 

 that the anterior part fuses to the palatine and articulates 

 with the posterior part. In the primitive bird, the pterygoid 

 was a single element suturing with the vomer anteriorly. 

 The quadrate is streptostylic, that is, capable of rocking; the 

 kinetic skull is described as prokinetic, with flexion occur- 

 ring at a hinge between the nasals and the frontals. Air sacs 

 from the middle ear enter the bones of the cranium; a 

 eustachian tube passes through the cranial base to reach the 

 pharynx. The mandible has been modified by loss of the 

 coronoid. 



The head skeletons of the alligator and the bird have some 

 distinctive common features. These include the pneumatiza- 

 tion of bones and several details of the chondrocranium, such 

 as the extensive flooring of the nasal cavity by the anterior 

 cupola (Figure 4-12 B), the fusion of the posterior cupola of 

 the nasal capsule with the interorbital septum, the loss of 

 the organ of Jacobson early in development, the position of 

 the orbitosphenoid in the pila antotica, the loss of the hyoid 

 arch component of the hyoid apparatus, and the lack of an 

 ascending process of the synotic tectum. 



The most marked differences involve the relationships of 

 the palate to the endocranium, a feature in which the alli- 

 gator is undoubtedly atypical, and the outlines of the skulls, 

 which reflect the functional differences between these forms. 



Origin of birds and mammals 



The origin of the bud from the reptiles, at least in terms 

 of the skull, is revealed by the remarkable fossils identified 

 as Archaeopteryx (and Archaeornis). Although this fossil type 

 is not known in detail, it links the bird and the archosaur 

 (Figure 4-15). There is some question as to whether 

 Archaeopteryx has achieved the status of a bird or is still a 



reptile. In terms of the skull, the latter view seems more 

 reasonable. In spite of the reptilian nature oi Archaeopteryx, 

 the exact source of the birds from the archosaurs is not 

 known. 



The origin of the mammal is better documented than that 

 of the bird. It is now generally assumed that the mammals 

 are polyphyletic; that is, they stemmed from several lines of 

 mammal-like reptiles identified as therapsids and not from 

 an ancestral mammal. They can also be viewed as mono- 

 phyletic on the grounds that all stem from therapsids. 



The head skeletons of mammal-like reptiles range from 

 highly unlike to quite similar to the mammal. The cyno- 

 donts are usually referred to in contrasting these two groups 

 because of the similarity of their skulls. 



The brain case of the cynodont is very small, rising to a 

 sagittal crest formed by the parietals. A parietal foramen is 

 present. The squamosal, jugal, and postorbital enclose the 

 single, large temporal fossa (synapsid type) and outline the 

 suborbital fenestra. A prefrontal and septomaxilla are 

 present. On the broad occipital surface there are large 

 tabular bones as well as a posttemporal canal opening into 

 the confluent orbital and temporal spaces. The opisthotic 

 forms a distinct paroccipital process. A secondary palate 

 is present which includes a palatine contribution. Ecto- 

 pterygoids as well as pterygoids are present, and the latter 

 suture with the basis cranii. An epipterygoid (alisphenoid) 

 is present and is sutured into the cranial wall, contacting 

 the parietal above and the prootic behind. The quadrate is 

 no longer connected with the palatal complex (it is in some 

 of the cynodonts) but is sutured to the squamosal and 

 much reduced in size. The stapes articulates distally with 

 the quadrate. The lower jaw is basically dentary but a 

 small coronoid, splenial, prearticular, and angular are 

 present. The angular, with its reflected lamina, is usually 

 thought to have given rise to the tympanic ring, but, at least 

 in one species of cynodont, this ring is a part of the 

 prearticular. 



The intermediate nature of the cynodont is quite appar- 

 ent. The approach to the mammal is carried even further 

 in an ictidosaur, Diarlhrognathus. in which the posterior 

 process of the dentary articulates with the squamosal lateral 

 to the much reduced quadrate (Figure 4-16). Establishment 

 of the dentary-squamosal articulation frees the articular and 

 quadrate; the quadrate is already in contact with the stapes, 

 for conversion into middle-ear bones. With this complex 

 conversion, identified as Reichert's theory, the full mam- 

 mal type is reached. 



The morphological sequence between primitive reptile 

 and mammal is in the process of undergoing an almost final 

 delineation. Huxley has commented that mammals, like 

 birds, are only extremes of the reptile plan (a grade), not a 

 nionophyletic new functional type derived from reptiles (a 

 clade). Many taxonomists are beginning to ask, what is a 

 mammal? An arbitrary line must be drawn across a con- 

 tinuous and overlapping series of species separating mammal 



76 • HEAD SKELETON OF OTHER TETRAPODS AND CHOANATES 



