[Taxacom] Two kinds of nictitating membranes
kennethkinman at webtv.net
Mon Mar 23 10:09:53 CDT 2009
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).
(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?
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