Whence the Shadows? 



207 



In this restoration of life in a Devonian Period shallow sea, a prehistoric shark 

 (Cladoselache) swims toward a large coiled cephalopod crawling about amid several 

 types of primitive sponges growing on the sea floor. The form of the modem shark 

 can easily be seen in this ancestor of eons ago. (This Devonian sea model was prepared 

 by George Marchand, under the direction of Irving G. Reimann. ) 



Courtesy, Rochester Museum of Arts and Sciences 



From more indistinct clues than the Cladoselache skeleton scientists 

 have reconstructed prehistoric sharks of astounding size. In Devonian 

 deposits and in strata of the next geologic period (the Carboniferous of 

 210 to 265 million years ago), fossil spines called ichthyodorulites have 

 been unearthed. These are dart-like quills found in the dorsal fins of various 

 fishes. These quills are similar to the strong, sharp quills found in some 

 present-day sharks, such as the Spiny dogfish {Squalus acanthias), one 

 of the most prolific species of shark in today's seas. The modern Spiny 

 dogfish is a small shark; it is no longer than 4 feet, and its spines, or 

 fin-quills, are usually only 1 or 2 inches long. Some fossil spines, how- 

 ever, are 3 feet long, and may even have carried venom. 



Shark skeletons, for the most part, have vanished from the geologic 

 record because their skeletons contain no true bone. Shark skeletons 

 were— and are— cartilaginous, and usually the relatively soft cartilage 

 is obliterated due to its being soluble. The fascinating ichthyodorulites, 

 made of tougher dentine, often survive, however, as the only remains 

 of some gigantic shark-like creatures. 



The Cladoselache disappeared during the Permian Period, from 185 to 

 210 million years ago, leaving the continuance of the evolutionary drama 

 to the Hybodonts, from which can be drawn almost direct lines to the 



