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>Aren't tetrapods sarcopterygians?


>Wouldn't "Choanata" be the same as Sarcopterygii?

Uuuhhhhh, no.  Sarcopterygii contains some fish without a true choana 
such as coelacanths and lungfish.  It should be noted, though, that in 
1981 Don Rosen and collegues (one of whom was Brian Gardiner) argued 
that lungfish primitively had a true choana and were the closest group 
to tetrapods rather than osteolepoformes and relatives.  Anyway, I think 
that some still hold to the dipnoan+tetrapod arrangement rather than the 
better-supported panderichthyid+tetrapod arrangement (panderichthyids 
show a true choana, but this was disputed by Rosen's group in 1991).  

Another note should be made.  Chan Mee-Mann (sp?) has argued that 
tetrapods form a sister-group to the groups that typically form the 
choanates as well the lungfish.  She argues that the only creatures with 
a true choana are tetrapods and not any other group.  She also proposes 
a dipnoan+porolepoformes relationship, placing dipnoans deep within the 
animals typically called choanates. 

My personal opinion is that panderichthyids are the sister-group to 
tetrapods and they (and their relatives) did show a choana.  As for the 
dipnoan-tetrapod link?  I like the correlation and I wish there was more 
evidence for it, but unfortunately there is not.  (However, Forey, one 
of the original Rosen et al. authors, argued that panderichthyids 
bridege the gap between dipnoans and tetrapods.  I don't buy it, but it 
is interesting.)

Matt Troutman

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