[Taxacom] Two kinds of nictitating membranes

Kenneth Kinman kennethkinman at webtv.net
Mon Mar 23 10:09:53 CDT 2009

Dear All,
      It appears that vertebrates evolved nictitating two separate
times.  The first time was apparently in the Devonian (or Silurian?)
during the tetrapod transition to land.  The second time was in sharks
(presumably at a later time).               
   Two questions:               
(1) Are there any fish (either actinopterygian or sarcopterygian) that
also have nictitating membranes?    
(2) Are the tetrapod and shark types homologous structures (sort of a
distant parallelism), or are they merely analogous and convergent?
                      Ken Kinman

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