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In a message dated 8/14/01 3:22:50 PM EST, Tetanurae@aol.com writes:

<< Where do you get your data for this, and why is it only irreversible in 
dinosaurs? >>

[1] Nobody has found an unassailable instance in dinosaur evolution of the 
reappearance of digital phalanges, or of the reapparence of a metapodial once 
lost. (Also true of mammalian evolution, I believe.) This is actually rather 
odd, because supernumerary vertebrae--particularly cervicals and caudals--do 
appear from time to time in dinosaur lineages; why not supernumerary 

[2] Extra phalanges and (perhaps--don't recall for certain) extra digits do 
appear in the paddles of marine reptiles. We know they're extras because the 
phalanx counts exceed the primitive 2-3-4-5-4 (or is it 2-3-4-5-3?) count in 
reptiles. That's why I had to specify dinosaurs above.