SARCOPTERYGIANS

["Matthew Troutman" <m_troutman@hotmail.com>]

<<Actually livestock is a fairly small group (tens of species at most
I'd guess) whereas "fish" generally encompasses some number in the
tens of thousands of species.  Nevertheless comparative molecular
biology indicates that the most recent common ancestor of
Crossopterygian and Actinopterygian fish had four cone classes (i.e.
potentially tetrachromatic vision).  That split predates the evolution of 
Tetrapoda (which derive from the Crossopterygia), so it encompasses
almost all extant fish.  Mammals lost two of those cone classes.
Primates re-evolved a third.  We've thus almost caught up to our 
ancestors...  Many fish are known to have retained all four cone classes, 
and very few are known to have fewer than three.>>

Sorry to nitpick but instead of "crossopterygian" the term should have been 
osteolepiform (Osteolepiformes).  I'm a bit rusty with my sarcopterygians 
but I believe that Crossopterygia is redundant with Sarcopterygia, 
actinistians, dipnoans and osteolepiforms (including tetrapods).

Out of curiosity, since I believe that something relating dipnoans to 
tetrapods based on cone classes was written up in Nature a few years ago, 
how do actinistians and dipnoans (which share many soft-part features with 
tetrapods including eye lens proteins) fit into this bracket in terms of eye 
structure?  Anyone who has read Rosen et al. (1980) will know the arguments 
for a dipnoan-tetrapod relationship: are eye characters shared between the 
groups primitive for sarcopteygians (if found in actinistians) or are 
diagnostic of whatever level dipnoans and tetrapods share a common ancestor 
at exclusive of actinistians?  (Jarvik and ?Holmes didn't answer this did 
they?  I'll have to recheck.)

Matt Troutman
m_troutman@hotmail.com


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