158 



INTRODUCTION TO EVOLUTION 



An aberrant group of placoderms was noteworthy for producing the 

 largest animal of the time, Dinichthys ("terrible fish"). This somewhat 

 sharklike animal reached a length of 20 or 30 feet. It appears to have in- 

 habited brackish and salt water. The head and forepart of the body were 



covered with armor; the armor 

 plates covering the jaws took the 

 place of teeth in forming a shear- 

 ing device. 



We have noted that Chondrich- 

 thyes (sharks and dogfishes) have 

 skeletons of cartilage. Conse- 

 quently prehistoric representatives 

 are known mostly from such hard 

 parts as teeth, spines, and scales. 

 In the case of Cladoselache (Fig. 

 8.16), however, we are more for- 

 tunate; the outline of the body and 

 some details of skin and muscles 

 were preserved in the fossilization. 

 Turning to the Osteichthyes, we 

 note that they are commonly di- 

 vided into two subclasses. Subclass 



FIG. 8.13. Devonian ammonites (goniatites) 

 showing wavy suture lines. (Reprinted by 

 permission from Jexibook of Geology, Pari 

 II, Hisforical Geology, by Louis V. Pirsson 

 and Charles Schuchert, published by John 

 Wiley & Sons, Inc., 1915, p. 709.) 



Actinopterygii (ray-finned fishes) 

 includes most of the forms we are familiar with as food and sport fishes. 

 Subclass Sarcopterygii (Romer, 1959) (fleshy-finned fishes) includes the 

 lungfishes (Dipnoi) and the Crossopterygii, a group represented by the an- 

 cestors of the first amphibians and by the coelacanth fishes. (The Sar- 

 copterygii are often called Choanichthyes, meaning the fish with nostrils, but 

 since coelacanths lack internal nostrils the name is not particularly appro- 

 priate.) Because of their ancestral position, the Crossopterygii (lobe-finned 

 fishes) are of particular interest to us. Each pectoral and pelvic fin had a 

 thickened, fleshy base (Fig. 8.17). Within these fleshy bases in such a spe- 

 cies as Eusthenopteron were skeletal elements capable of developing into 

 the stiffening supports for limbs of terrestrial vertebrates (Fig. 8.18). 



Crossopterygians, like lungfishes and like some modern actinopterygians 

 such as the gar pike, had air bladders connected to the pharynx. Such a 

 connection makes possible filling of the bladder with air from the exte- 

 rior. Thus, gar pikes and some modern lungfishes, when the surrounding 

 water becomes stagnant and unfitted for respiration by means of gills, 

 rise to the surface and gulp in air. Their air bladders function as lungs. 



