THE HETERODONTID SHARKS: THEIR NATURAL HISTORY, 



AND THE EXTERNAL DEVELOPMENT OF HETERODOJiTUS 



(CESTRACIOK) JAPOHICUS BASED ON NOTES 



AND DRAWINGS BY BASHFORD DEAN 



By Bertram G. Smith 



Professor of Anatomy 



New York University College of Medicine 



New York City 



INTRODUCTION 



Sharks of the family Heterodontidae (Cestraciontidae) have an especially well' 

 defined pedigree. The genus Heterodontus (Cestracion), which includes the only species 

 living at the present time, dates at least from the Upper Jurassic; the family Cestracion ti' 

 dae, as defined by Zittel (1932), from the Lower Jurassic. The closely related family 

 Hybodontidae, represented only by fossils, dates from the Devonian or Lower Carbonifer' 

 ous to the Cretaceous. Therefore the geologic histories of the two families overlap; but 

 the Hybodonts were approaching extinction when the Heterodonts came into being. 

 Since there appears to be genetic continuity between the two families, one might readily 

 conclude that the recorded lineage of sharks of the genus Heterodontus is more ancient 

 than that of any other living vertebrate. In this circumstance we find the key to Dean's 

 interest in the embryology of Heterodontus. 



At the time when Dean began collecting the eggs and embryos of the Japanese 

 Bullhead Shark, Heterodontus japonicus, all that was known concerning the embryonic 

 development of any species of Heterodontus was contained in HaswelPs brief account 

 (1898) of the blastula and gastrula of H. phiUipi. This deficiency was the more notable in 

 view of the fact that the family Heterodontidae has no other genus, besides Heterodontus, 

 represented by living species. But Heterodontus was not, from Dean's point of view, 

 merely another kind of shark to be studied in order to fill a gap in our knowledge of 

 comparative embryology. It is well known that Dean, like many other biologists of his 

 generation, was interested in the study of animals chiefly from the viewpoint of organic 

 evolution. Thus it is not surprising to find in his notebook the following carefully 

 worded statement : 



The embryology of the Cestracionts [Heterodonts] is expected to prove of value not 

 merely in comparison with other sharks, but in estimating the general significance of develop- 

 ment in "recapitulating" ancestral characters. For granting that these sharks represent 

 a peculiarly primitive branch of the descent-tree of Selachians, we would reasonably expect 

 to find in their embryonic stages certain simpler, more archaic characters than in the cor- 

 responding stages of the commoner groups of sharks. Furthermore, and this is the importance 

 of such a study, if we do find that Cestracion [Heterodontus] presents definitely more primi- 

 tive embryonic characters than sharks of a more modern type, we can certainly maintain 



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