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Posted by SNYDER on March 13, 1999 at 13:36:56:
In Reply to: Re: Snake Origins posted by Troy H. on March 08, 1999 at 21:42:43:
: the way i understand it (and its been about 4 years since my grad school days, so pardon if i mess up a few details) is that snakes and amphisbaenians are clades within the lizard clade that includes geckos, skinks, anguids, teiids, and of course monitors. The other lizard clade includes all of the iguania (agamas, chameleons, and all the divers species of "iguanids" (in the sense used prior to Hillis and Frost).
: however, unless i've missed something since i left academia, no one has yet shown that snakes are not monophyletic -- that is, there is no extant lizard species or group nested within the snake clade, even at its most basal position. So while snakes may have evolved more than once, they very likely did so from very closely related lizard groups. What is more likely, however, is that snakes in fact did evolve once, and that the anatomical differences you point out (left or right lung vestigial) result from the fact that the original snake had both lungs (this original is prior to the splits between blindsnakes and other snakes) . . . however, this original snake taxa later evolved to lose the lung, and different subsequent groups lost different lungs (hope that all makes sense ;-) )
: Troy
...Well thanks, but I seem to be havin' a little trouble with understanding what is ment by Monophyletic and paraphyletic. I have misquoted Pough et al (1998 "Herpetology") They say "Lacertilia is paraphyletic with respect to Serpentes". The definition I learned is: Paraphyletic-Genus or species level groups which due to different habits, morphology or physiology have emerged from their ancestral families and, rather than replacing them now exist alongside their progenitors in different niches. Now I can see how you could say this about Serpentes (as I wrote originally, but to say that Lacertilia is paraphyletic to Serpentes doesn't make sense to me. Monophyletic just means "derived from a single ancestral clade." Right?
...I was mistaken in thinking that the Amphisbaenians are considered as members of the Serpentes. Pough et al does not show them so. They show a seperate derivation for the four families of Amphisbaenians, the Dibamidae, and the Serpentes(with a question mark as to their ancestral line). A possible derivation from Varanoidea is suggested for Serpentes is suggested. Also suggested is a derivation from Scinomorpha for Amphisbaenians.
...But I'm really confused by the definition of paraphyletic above. I had infered from my reading that it ment a taxon that actually contained descendents from more than one ancestral line. ???
Dave Snyder