Re: dinosaur eyeballs

[Adam Yates <zooamy@zoom.zoo.latrobe.edu.au>]

>         
>        Snakes don't blink, but they are much more 
>      primitive a form than a dinosaur is, and the demands on their eyeballs 
>      aren't as important to a snake's survival as they have developed other 
>      senses more important than sight (to a snake).   Birds (some) have 
>      inner eyelids for whatever reason.  Is this an extremely advanced 
>      trait?       

        Snakes don't blink because their eyes are covered by a 
        transparent scale (spectacle) that is a derived eyelid that is 
        fused shut. Hence the apparently lidless eyes of snakes is the 
         derived condition and the presence of eyelids is the ancestral 
        ("primitive") condition. Is the nictating membrane of sharks 
        homologous with the inner eyelids of some  birds and mammals? If 
        so it would not  be a derived condition within the vertebrata    


        Adam Yates