118 VISION 



The eyes were sightless in the black, and the other senses transmitted 

 nothing extraordinary to the small, primitive brain. 



Peter Benchley, Jaws, 1974 



Instead of a "swimming nose," a picture is beginning to develop of a 

 shark as a sensory wonder, attuned to sights, sounds, smells, movement, 

 electrical impulses, and even the movement of the earth. None of 

 these sensory abilities implies any more intelligence than was previously 

 attributed to the sharks; we can still assume that they react pretty 

 much on the basis of instinct. . . . 



Richard Ellis, The Book of Sharks, 1975 



INTRODUCTION 



Until recently elasmobranchs were considered primitive fish with small, 

 simple brains mediating a behavioral repertoire limited compared to those of 

 bony fish or land vertebrates. The elasmobranch telencephalon was said to 

 function primarily in olfaction, and its efferents were believed to project 

 principally to epithalamic and hypothalamic centers integrating olfactory 

 and gustatory behavior. The roof of the midbrain was believed to be the 

 highest visual center— capable of only crude visual analysis— where ascending 

 somatic and visual sensations were integrated into a few stereotyped 

 behavioral responses. The well-developed cerebellum was believed to relate 

 to powerful, well-coordinated trunk movements, yet sharks were said to be 

 clumsy. 



These conclusions are rapidly being replaced by a newer picture of elasmo- 

 branch central nervous system (CNS) organization. However, it is important 

 to understand how sharks came to be viewed as primitive, robotlike smelling 

 and feeding machines. Until the 1950s, most comparative studies were 

 framed within typological considerations. Nonmammalian vertebrates were 

 assumed to represent earlier, and thus simpler, stages in the evolution of 

 mammals. Attention focused on recognizing morphological features common 

 to all vertebrates (yielding a common pattern or vertebrate Bauplan), and on 

 assigning different vertebrates to different "phylogenetic levels" or "stages." 

 Elasmobranchs have cartilaginous skeletons, which were believed to predate 

 bone in vertebrate evolution. Thus, they were assigned to a primitive ("low," 

 in typological thinking) position in vertebrate evolution. 



The myth was easily perpetuated. Most biologists, as students or re- 

 searchers, have examined the spiny dogfish (Squalus acanthias) or the lesser 

 spotted dogfish (Scyliorhinus caniculus). Squalus is representative of less 

 than 24% of the living sharks, and is a member of the most primitive group 

 of living sharks (squalomorphs), while Scyliorhinus is one of the most primi- 

 tive members of the advanced sharks (galeomorphs). Examination of either 

 species gives little idea of the range of morphological complexity exhibited 

 by elasmobranchs. Even worse, one might assume that all elasmobranchs are 

 similar to these two taxa. The magnitude of this misconception can be 



