4 



The Head Skeleton of other 



Tetrapods and the 

 Choanate Fishes 



OTHER TETRAPODS 



The tetrapods are the "four-footed" animals and include 

 the mammals, reptiles, birds, and amphibians. The head 

 skeletons of living examples of these several kinds of tetrapods 

 must be reviewed in order to establish a basic pattern for 

 all and also to indicate some of the variations that charac- 

 terize each of these end products of the evolutionary process. 

 These several evolutionary lines can then be traced back 

 through their fossil representatives so as to arrive at some 

 notion of the head skeleton of their common ancestor. 



Reptiles 



The reptiles are generally presumed to have evolved from 

 the amphibians and to have given rise to the mammals and 

 birds. The first known reptiles are from the Upper Pennsyl- 

 vanian (see Table 4-1), but it is not until the Permian that 

 enough details are revealed to compare them with living 

 representatives. By the Upper Pennsylvanian the reptiles 

 had already radiated into many diverse types, and during 

 the Mesozoic there were further radiations of types giving 

 rise to birds and mammals. 



There are three main lines of reptiles: turtles (Chelonia), 

 Lepidosauria, and Archosauria. The snakes, lizards, and 

 Sphenodon are lepidosaurs, whereas the dinosaurs and 

 crocodilians are archosaurs. The birds are viewed as a deriv- 

 ative of the archosaurs. Of the living reptiles, the snakes 

 and lizards are the most widespread, and the lizard retains 

 a close resemblance to what might be imagined as the an- 

 cestral reptile, at least in terms of the larger number of 

 bones in the head skeleton. On this basis it can be consid- 

 ered as less modified than the alligator or turtle. 



The lizard will be considered first. A check-list of the 

 bones to be observed is given in Table 4-2. 



Lizard Species of Tupinamhis and Iguana are readily avail- 

 able as representatives of this type (Figure 4-1 ). The skull 

 is diapsid (Figure 4-26), that is, it has two temporal fene- 

 strae, but the inferior fenestra lacks a ventral, labial bar. 



Both fenestrae are large and the skull roof is reduced and 

 separated from the endocranium. This separation is effected 

 by the confluence of the orbit cavity with the temporal and 

 posttemporal fossae and the inward extension of the roofing 

 bones around the edges of the muscle masses as this sepa- 

 ration occurred. The lizard thus has parts of two cranial 

 walls, one lying outside the temporal musculature and one 

 inside. The mammal fully developed the inside wall but 

 lost the outer. 



In sharp contrast to the mammal, there is ajoint between 

 the fused otic capsule-occipital segment of the skull and the 

 overlying roof (Figure 4-2). This joint permits some move- 

 ment between these parts. The quadrate can be rocked 

 back and forth and the snout raised and lowered. Shifting 

 of parts within the skull is identified as kineticism. 



The roof lacks a postparietal bone, a case of loss. In 

 Iguana there is a parietal foramen, a case of retention of a 

 primitive feature. Prefrontal, postfrontal, and postorbital 

 bones are present. The squamosal is much reduced and 

 there is a supratemporal. The cranial cavity is open ante- 

 riorly. The anterior margin of the oticoccipital segment of 

 the endocranium is notched by the foramen of the fifth 

 nerve, and ventrolaterally it bears a flange under which the 

 facial nerve exits. Below this flange, the splint-like stapes 

 extends out parallel to the strong paroccipital process of 

 the opisthotic bone. The paroccipital processes of either 

 side along with the supraoccipital articulate with the roof 



There is a single rounded occipital condyle rather than 

 the two separate articular processes (condyles) of the mam- 

 mal. The exoccipital and basioccipital contribute to this 

 condyle. The basioccipital has strong, bilateral ventral 

 processes. A flange of the opisthotic bone connects this 

 ventral process with the cranium and forms the posterior 

 margin of the tympanic fossa. This rim divides the metotic 

 fissure into a posterior jugular foramen for the tenth and 

 eleventh nerves and a recessus tympanicus for the ninth 

 nerve and perilymphatic fenestra. The exoccipital is per- 

 forated just behind the jugular foramen by the hypoglossal 

 roots. 



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