these are usually thought to be the most primitive and there- 

 fore should retain more of the original skeletal plan rather 

 than less. 



The living amphibians and reptiles agree in such improb- 

 able details as having a septomaxilla in the wall of the 

 nasal passage; however, this may not be the same structure 

 in the two groups nor the homolog of the similarly named 

 bone of the early amphibian. The reptile and amphibian 

 also agree in retaining all or most of the bones of the lower 

 jaw and the primitive articulation. The mammal line differs 

 in retaining the postparietals right down to the present and 

 in its reorganization of the jaw apparatus which features an 

 articulation of the dentary with the squamosal and conver- 

 sion of the posterior jaw elements and quadrate into two 

 middle-ear bones and the tympanic ring. Another part of 

 the palatoquadrate complex, the epipterygoid, has become 

 an integral part of the brain capsule in the mammal. 



The convergence of these three lines as one goes back in 

 time suggests a common ancestor not unlike some of the early 

 amphibians. The problem now is to trace the tetrapod pat- 

 tern into the fishes to see if it arose there or is a common 

 inheritance of all vertebrates. 



CHOANATE FISHES 



The choanate fishes are of special interest because they 

 are the assumed ancestors of the tetrapods. A study by Erik 

 Jarvik contended that the choanate group, as usually de- 

 fined — including crossopterygians and dipnoans — is not a 

 natural one. The argument was based on the idea that the 

 dipnoans lacked a true internal naris. The opening into the 

 mouth, which has been called the internal naris, actually 

 corresponds to the posterior external nasal opening of the 

 crossopterygian, in the opinion of Jarvik. To avoid the con- 

 sequences of this lack of agreement in choanae, these same 

 fishes have been re-identified as the Sarcopterygii or the 

 "fleshy-finned fishes" (see Chapter 6). 



As suggested by the lack of agreement in nasal capsule 

 development, there are several well-marked groups of choa- 

 nate fishes, and each of them should be examined in attack- 

 ing the problem of the origin of the tetrapod from a 

 fish ancestor. 



Crossopterygians 



Osteolepiform The osteolepiform is usually described as 

 the representative, or type, of the crossopterygian fish. Its 

 selection is based on its presumed similarity to the amphib- 

 ian. There are two well-known genera, Osteolepis and Eusthen- 

 opteron (Figure 4-28). These genera represent two different 

 families, the Osteolepidae and Rhizodontidae. 



The head skeleton in this group is made up of the bones 

 listed in Table 4-4. The osteolepiform skull shows many 

 variations; bones appear to be fragmented in some individ- 

 uals and occasionally extra bones, anamestics or Wormian 



bones, appear between those regularly observed. The snout, 

 the lower jaw, and to a lesser extent the cheek do not show 

 suture lines because of the continuous cosmine and enamel 

 layers over them. Sutures can be distinguished in these area 

 when the outer layers of bone are removed or when viewed 

 from the inside. 



Although the osteolepiform skull and the amphibian skull 

 have always been Compared, opinions differ as to the nature 

 of the comparisons. The difficulty stems in part from the addi- 

 tion of the extrascapular series to the roof (Figure 4-29) and 

 the lack of paired nasal and frontal plates. These are repre- 



TABIE 4-4 THE BONES OF THE HEAD SKELETON OF THE CROS- 

 SOPTERYGIAN FISHES ARRANGED ACCORDING TO 

 TYPE AND PLACE OF ORIGIN 



Cranium (brain and sense 

 orgon capsule) 



Splanchnocronium or Visceral 

 Skeleton (dermal bones with*} 



Endocranium (bones of chon- 

 dral origin) 



ossified in two pieces 

 anterior: sphenethmoid 

 posterior: oticoccipital 



Eye 



sclerotic plates 



Dermocranium (bones of 

 membrane origin) 



Roof, snout, and floor 



extrascapulars (median 

 and lateral) 



postparietal 



parietal 



tabular 



supratemporal 



intertemporal 



Mandibular arch — Palato- 

 quadrate and Meckel's 

 cartilage 



premaxilla* 



ma-xilla* 



quadratojugal* 



suprapterygoid(s)* 



pterygoid* 



ectopterygoid* 



palatine* 



palatoquadrate 



articular 



mental 



angular* 



supraangular* 



posterior splenial* 



anterior splenial* 



dentary* 



prearticular* 



coronoids* 



92 • HEAD SKELETON OF OTHER TETRAPODS AND CHOANATES 



