SHARKS OF THE FAMILY LAMNIDAE 

 HISTCRY 



The history of modern shaxks can be traced back to the 

 Jxirassic, but fossil remains prior to this period axe too 

 scanty to determine the derivation of modern groups. The 

 bulk of evidence consists of teeth, head spines, and fin spines, 

 since the cartilaginous elasmobranch skeleton normally is 

 poorly and incompletely preserved (Moy- Thomas, 1939) • Available 

 fossil material suggests that the modern sharks probably have 

 arisen from fonns such as the Hybodontii ( Hybodus) , which appear 

 to possess characters intermediate between those of the modern 

 Notldanoidea and Heterodo ntoidea . Further evidence suggests that 

 the hybodonts can be traced through the apparently intermediate 

 CtenacBjathi, to the Upper Devonian Cladoselache , the earliest 

 known shark. 



The chief diatinctions in the development of modern sharks 

 are the change from araphlstyly to hyostyly, greater development 

 of vertebreil centra, and the progressive disappearance of the 

 notochord through segmental constriction. Among the last to 

 evolve were the lamnids, which first appeeired in the Cretaceous. 



CLASSIFICATION 



At present, there appear to be three well-defined genera in 

 the family lamnidae, separable according to the following key 

 (modified from Blgelow and Schroeder, 19^8): 



la. Upper teeth broadly triangulajj with serrate edges. 



Car char odon A. Smith, I838 



lb. Upper teeth slender, with smooth-edged cusps. 



2a. First two teeth in each jaw similar in shape to the 

 succeeding teeth; most or all of teeth with lateral 

 denticles in most species, and perhaps in all; origin 

 of first dorsal about over or anterior to inner corner 

 of pectoraJL when latter is laid back; anterior peirt 

 of caudal fin with a secondary caudal keel on either 

 side below the primary keel formed by the lateral 

 expansion of the caudal peduncle. 



T.amna Cuvier, I8I7 



2b. First two teeth in each jaw noticeably more slender 

 and more flexuous than the others; latereG. denticles 

 absent or present only as a trace; origin of first 

 dorsal definitely posterior to inner corner of pectoral 



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