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Keeping a cool head
  1. Ivan R Schwab1,
  2. D Michelle McComb2
  1. 1University of California, Davis, Sacramento, CA, USA; irschwab@ucdavis.edu
  2. 2Florida Atlantic University, Florida, USA

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    Sharks always draw a crowd. We have a macabre fascination with these creatures because of their commanding presence and predatory lifestyle. The hammerhead shark evokes further interest because of the bizarre morphology of its head. What purpose would such a head serve and how does the creature manage its sensory input?

    The class Chondrichthyes arose in the Silurian approximately 415 million years ago, and includes all fish having a cartilaginous skeleton. Evolutionarily, these fish followed a different path by separating from teleosts or bony fish. Teleosts eventually led to tetrapods and terrestrial creatures (BJO September 2006), but the chondrichthyes evolved in a different direction that now includes the modern elasmobranchs—sharks, skates and rays. The last common ancestor of both cartilaginous and bony fish probably arose approximately 430 million years ago. That ancestor had a primitive piscine-like eye and first showed a superior oblique muscle attaching anteriorly to the globe, a key step in evolution. Extant cartilaginous and bony fish have similar eyes that possess more similarities than differences from each other, …

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