NO. 3539 HAMMERHEAD SHARKS — GILBERT 17 



to determine the exact sequence of events. Judging from morpho- 

 logical evidence, however, S. tudes evolved first, later followed by 

 differentiation of the other two species. The presence of S. tudes 

 in both the Mediterranean and western Atlantic is unusual in that 

 few species display such a distribution pattern. This common occur- 

 rence may be due either to recent migration in one direction or the 

 other, or to a long period of isolation during which the two populations 

 have, for some reason, failed to differentiate to the species level. 



The distribution of Sphyrna zygaena is a classic example of an 

 antitropical distribution (Ekman, 1953, p. 250). Presumably the 

 now-disjunct north-south distribution of this species (map 2) was 

 continuous during one or more of the Pleistocene glacial periods, 

 when equatorial waters were cooler than at present. As the glaciers 

 retreated and seas became warmer, S. zygaena gradually disappeared 

 from the tropical regions, perhaps remaining for a time in deeper 

 waters where temperatures were cooler (equatorial submergence). 



One may conclude from present distributions that the subgenus 

 Platysqualus evolved in the western Tethys Sea, a prehistoric body of 

 water that formerly extended from the Eastern Pacific-West Indian 

 region to what is now southeast Asia (Ekman, 1953, pp. 63-67). 

 Should it have evolved in the Indo-Pacific region, one or more species 

 of the subgenus would likely still be found there, since environmental 

 conditions in this area have remained stable over a long period of time. 

 The subgenus Eusphyra apparently evolved in the Indo-Pacific, as 

 the only included species, Sphyrna hlochii, is restricted to that region. 

 It is impossible to determine the center of origin of the subgenus 

 Sphyrna, since three of the species in this group have a worldwide 

 distribution. 



Key to Species of Sphyrna 

 (Based primarily on specimens 1,000 mm. TL or smaller) 



la. Greatest maximum width of head almost 50 percent of total body length; 

 outer narial groove present; nares not present near eyes, situated more 

 than half distance from eyes to anterior- median notch; upper precaudal 

 pit as in figure 21o; anterior-median pores on ventral surface of head as in 

 figure 22a; chondrocranium as in plate 5. . . . Sphyrna blochii (Cuvier) 

 Indo-Pacific region, from Persian Gulf to northern Australia and southern 

 Philippines. 

 lb. Greatest maximum width of head not more than 32 percent of total body 

 length; outer narial groove absent; nares present near eyes; upper pre- 

 caudal pit not as in figure 21a; anterior- median pores on ventral surface of 



head not as in figure 22a; chondrocranium not as in plate 5 2 



2a. Anal fin and base longer, former measuring from 9.8 to 13.5 percent, and 

 latter from 6.5 to 9.9 percent of total body length; fifth gill slit about as 

 long as first gill slit, both somewhat shorter than three middle slits; 

 fifth gill slit situated above insertion of pectoral fin; orbit smaller, its 

 horizontal diameter less than shortest distance from anterior edge of 

 219-937—67 2 



