VI 



EVOLUTION AND ADAPTIVE RADIATION OF 

 ELASMOBRANCHS 



1 . Characteristics of elasmobranchs 



The organization of a shark used to be considered to show the earlier 

 stages of fish evolution, but we have seen evidence that this is a mis- 

 take (p. 131). The sharks and skates and rays are highly developed 

 creatures; in particular, the absence of bone is a secondary feature; 

 they have been able to give up their defensive armour because of the 

 development of other means of protection, swift swimming, good 

 sense-organs and brain, and powerful jaws. We can now examine the 

 history of these changes and study the varied creatures that can be 

 classified as elasmobranchs. As usual in examining such histories we 

 must try to discover evidence about the forces that have operated to 

 produce the changes of type, and look for signs of any consistent 

 trends, persisting for long periods of years. 



2. Classification 



Superclass Gnathostomata 

 Class Elasmobranchii ( = Chondrichthyes) 

 Subclass 1. Selachii 

 *Order 1. Cladoselachii. Devonian-Permian 



*CIadoselache; *Goodrichia 

 *Order 2. Pleuracanthodii. Devonian-Trias 



*Pleur acanthus 

 Order 3. Protoselachii. Devonian-Recent 



*Hybodiis; Hetcrodontus 

 Order 4. Euselachii. Jurassic-Recent 

 Suborder 1. Pleurotremata. Jurassic-Recent 

 Division 1. Notidanoidea. Jurassic-Recent 

 Hexanchus ; Clilamydoselache 

 Division 2. Galeoidea. Jurassic-Recent 



Scyliorhinus; Mustelus; Cetorhinus; Carcharodon 

 Division 3. Squaloidea. Jurassic-Recent 

 Squalus; Sqaatina; Pristiophorus ; Alopias 

 Suborder 2. Hypotremata. Jurassic-Recent 

 Raja; Rhinobatis; Pristis; Torpedo; Trygon 



