Whence the Shadows? 



209 







Fossil remains of this Selachian ancestor, Vleur acanthus, have been found in the 

 Carboniferous and Permian Period rocks of North America, Australia, and Europe. 

 It ranged in size from 18 inches to 6 feet or more. Note its claspers, which show that 

 it was more shark-like than teleostean. Its teeth are shark-like, but the fossils do not 

 indicate the presence of dermal denticles. After Hussakof 



modern shark. The typical Hybodont had the basic appearance of a 

 modern shark and a remarkable arrangement of teeth— sharp ones in 

 the front for seizing prey, and flat ones in the back of the jaw for crush- 

 ing the shells of mollusks. Thus, they could alternate between two kinds 

 of diet— fast-swimming fish and sedentary bottom-dwellers. This abil- 

 ity to vary feeding habits in the event of a shortage of one kind of 

 food undoubtedly aided the Hybodonts' survival. For, by the Triassic 

 Period (155 to 185 million years ago), the Hybodonts apparently were 

 the only cartilaginous fish in the sea. 



At this pivotal era in the dynasty of the shark, the hardy little 

 Hybodonts were relatively rare. They struggled in the primitive seas 

 against the hard-skeletoned ancestors of the bony fishes of today. Their 

 principal enemies— or at least competitors— were probably carnivorous, 

 fish-shaped reptiles, some almost 30 feet long, which roamed the open 

 seas using sharp teeth on the same prey the Hybodonts sought. These 

 marine reptiles were abundant, but the shark line did not die out. 



The Hybodonts eventually gave way to new shark forms, but at 

 least one of their descendants still thrives today, little changed from 

 Triassic times. This is the Port Jackson shark {Meter odontus portiis- 

 jacksoni), which still retains the dual-denture system of the Hybodonts. 

 With its blunt, bull-like head and sway-backed body, this usually small 

 Australian shark somehow looks as if it belonged to the past. 



The Hybodonts of the Triassic Period were the harbingers of the 

 modern sharks, which evolved in the next geologic period, the Jurassic. 

 The spectacular flying pterosaurs and the great dinosaurs were spawned 

 in the Jurassic, but they were forms ultimately abandoned by nature 

 because they failed to adjust to changing conditions. The shark, a tried 

 and tested form, had apparently already reached a stage of nearly perfect 



