440 VERTEBRATE LIfE AND ORGANIZATION 



Groups of bony fishes that have remained in fresh water throughout 

 their history tendetl to retain kmglike organs, but those that went to 

 sea no longer needed lungs, for ocean waters are rich in oxygen. Their 

 useless lungs evolved into useful hydrostatic organs. What are presumed 

 to be intermediate stages in this shift can still be seen in certain species. 

 Later, when conditions were more favorable, many salt-water bony fishes 

 reentered fresh water, but retained their swim bladders. The fresh-water 

 perch has had such a history. Its ancestors first evolved lungs in a fresh- 

 water environment, then went into the ocean where the lungs changed 

 into swim bladders; later the fish reentered fresh water and retained 

 the swim bladders. 



201 . Evolution of Bony Fishes 



Bony fishes have enough features in common to indicate their evolu- 

 tion from a common ancestral stock, but they early diverged into two 

 separate lines— the subclasses Actinopterygii and Sarcopterygii. The 

 actinopterygians are the ray-finned fishes like the perch (Fig. 22.9 A). 

 Their paired appendages are fan-shaped and are supported by numerous 

 dermal rays derived from bony scales. Their paired olfactory sacs connect 

 only with the outside. The sarcopterygians are the fleshy-finned fishes 

 such as our present day lungfishes (Fig. 22.9 B). Their paired appendages 

 are typically elongate and lobe-shaped, supported internally by an axis 

 of flesh and bone, fn many species, each of the olfactory sacs connects to 

 the body surface through an external nostril and to the front part of the 

 roof of the mouth cavity through an internal nostril. 



Ray-Finned Fishes. Actinopterygian evolution presents a good ex- 

 ample of a succession in which early dominant groups became replaced 

 by more successful types. Three superorders are recognized, and each in 

 turn had its day (Fig. 22.2). Currently the superorder Chondrostei have 

 dwindled to a few species of which the Nile bichir (Pulypterus) and the 

 sturgeon {Scaphirhynclnis) are examples (Fig. 22.12). The superorder 

 Holostei have also dwindled and are represented today by such relict 

 species as the gar (Lepisosteiis) and bowfin (Amia). The superorder 

 Teleostei, in contrast, have been continuously expanding since their 

 origin near the middle of the Mesozoic era. It is to this group that the 

 perch and most fishes belong. 



Various evolutionary tendencies can be traced through this suc- 

 cession. The functional lungs of early actinopterygians (still retained in 

 Polypterus) became transformed into swim bladders with little respira- 

 tory function. Correlated with increased buoyancy and better streamlin- 

 ing, we find that the primitive heterocercal tail of most chondrosteans 

 (Polypterus is an exception) became superficially symmetrical in teleosts, 

 but the caudal skeleton still shows indications of the upward tilt of the 

 vertebral column. Such a tail is said to be homocercal (Fig. 22.9 A). 

 Holosteans have an intermediate abbreviated heterocercal tail. Early 

 actinopterygians were clothed with thick, bony scales characterized by 

 having many layers of enamel-like ganoin covering the surface. During 

 subsequent evolution the superficial layers were lost, and the bone was 



