THE PHYLUM CHORDATA 



sphere, while the spoonbill is found elsewhere only in the Yangtze River 

 of China. With the exception of the African species, these living fossils 

 are rather degenerate. They have largely lost the ganoid scales of their 

 ancient progenitors. The internal skeleton is largely cartilaginous, in con- 

 trast to the bony skeletons of their ancestors. Only the African forms retain 

 the lungs. 



The Holostei appears in the record in the Triassic, but probably took 

 origin from chondrostean progenitors in the Permian. They rapidly rose 

 to dominance, reaching a maximum in the late Jurassic. In these fishes, 

 the tail shortened and became symmetrical, thus losing its shark-like as- 

 pect. The scales tended to lose their ganoine covering, thus leaving thin, 

 simple, flexible bony scales, similar to those of the famihar fishes of today. 

 But heavy scales were retained by some holosteans, including Lepisosteus. 

 Furthermore, the holosteans invaded the seas, and their greatest diversifi- 

 cation took place there. Yet it is a curious fact that the two genera which 

 have persisted to the present are both fresh-water fishes. These are Amia, 

 the bowfin, and Lepisosteus, the garpike, both of which occur in the 

 United States. The "lungs" have fused to form a single sac, which func- 

 tions as a hydrostatic organ or swim bladder. Yet its respiratory function 

 is still important, for they come to the surface frequently to gulp air, and 

 a garpike can be drowned by holding it under water. 



Finally, the superorder Teleostei, in which the skeleton is almost en- 

 tirely bony, appears to have originated from holostean ancestors early in 

 the Mesozoic (Figure 54). They remained unimportant until the Creta- 

 ceous, when they began a rapid expansion which is still in progress. By 

 the end of the Cretaceous, they were the dominant fishes both of the seas 

 and of the fresh water of the world. Today they comprise upward of 

 95 per cent of the world's fishes. In the oceans, only the Chondrichthyes 

 compete with them. In fresh water, only the few stragglers of the chon- 

 drosteans and holosteans and a very few unusual sharks are their com- 

 petitors. The skeleton is always entirely bony in the teleosts. The scales 

 are always thin, flexible chips of bone; the tail is invariably symmetrical; 

 and the lungs, if present, are fused to form a swiin bladder. As all of the 

 familiar fishes of today are teleosts, it is obvious that they have undergone 

 an immense adaptive radiation. The taxonomy of the teleosts is still quite 

 controversial, but one widely accepted classification lists no less than 

 twenty-eight orders. The teleosts, then, are one of the most successful and 

 varied groups of vertebrates. 



CROSSOPTERYGIANS, DIPNOANS. AND THE 

 ORIGIN OF THE SUPERCLASS TETRAPODA 



Soon after their origin, the subclass Choanichthyes became the dominant 

 fishes, although they quickly yielded this position to the Chondrostei. The 

 subclass included two orders, the Crossopterygii and the Dipnoi. The 

 Dipnoi, or lungfishes, are represented by three living genera, one each in 

 Australia, Africa, and South America. In the early evolutionary studies, 

 they were given great theoretical importance as the probable little- 



163 



