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<<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

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