206 Shark and Company 



armored. Others had jaws, and among these jawed fish appeared a new 

 breed. These had developed "lungs" and paired fins strong enough to 

 perform a function that changed forever the course of life. For, in times 

 of drought, when tidal rivers or estuaries dried up, some of these 

 fish were able to crawl, and so move to new and undried ponds, or 

 else bury themselves in the mud— and breathe air. From them came 

 the primitive amphibians with their tenuous grasp on the land. 



The Age of Fishes apparently dawned in the sea, but it is at the 

 beginning of the Devonian Period that the first records of shark-like 

 creatures appear. These ancestors of the shark were already highly 

 developed, and their progenitors had doubtless been spawned in the 

 previous Silurian Period, for some Silurian rocks contain faint evidence 

 of shark-like fishes. Since fossils provide the only tangible clues to 

 prehistoric life, the shark's origin is a matter of some speculation. But 

 fossils themselves are but perplexing pieces of a gigantic puzzle that 

 seems destined to remain forever unsolved in its entirety. Sir Arthur 

 Smith Woodward, an outstanding authority on fossils, wrote in 1898 

 of the difficulties in gleaning knowledge from fossils. There have been 

 great paleontological discoveries since he made his observations, but 

 what he said is still true: 



We may, in fact, without exaggeration declare that every item of knowl- 

 edge we possess concerning extinct plants and animals depends upon a chapter 

 of accidents. First, the organism must find its way into water where sediment 

 is being deposited and there escape all the dangers of being eaten: or it must 

 be accidentally entombed in blown sand or a volcanic accumulation on land. 

 . . . Lastly, man must accidentally excavate at the precise spot where entomb- 

 ment took place, and someone must be at hand capable of appreciating the 

 fossil, and preserving it for smdy when discovered. 



The oldest fossil records of sharks were found in what are known 

 to paleontologists as the American Middle Devonian beds— limestone 

 deposits in Ohio, rich in marine fossils. The beds gave up a few specimens 

 of the type of tooth known as Cladodus. These primitive teeth, amazingly 

 similar to the teeth found in some species of modern sharks, are out- 

 standing for their dagger-Uke points. Another American fossil-hunting 

 ground, the Cleveland Shales, has given posterity one of the most valu- 

 able records of a prehistoric shark. These late Devonian fossils show, in 

 delicate traceries, not only the bodily outline of a shark, Cladoselache, 

 but also the imprints of its muscles— and even its kidneys. From these 

 dim outlines, more than 265 million years old, paleontologists have been 

 able to reconstruct a shark from 1^/2 to 4 feet long. The Cladoselache 

 and another primitive shark with similar characteristics, the Ctenacan- 

 thus, are believed to be close to the source of the shark's earUest an- 

 cestors. 



