The condition here is very similar to the lobe-fish, except that the gills have been lost (although note that the earliest tetrapods actually retained internal gills, not shown here). The eight-fingered limbs here reflect the condition in the earliest tetrapods. The posterior cardinal veins not shown here are still present in some modern amphibians (they have been reduced to the azygous and hemiazygous veins in most living tetrapods). The loss of the second aortic arch by this level is conjectural, but this arch is not retained in any adult tetrapod today. The ventricle is shown as undivided, as in modern amphibians, although modern lungfish have a partially divided ventricle (it is not clear if this is represents the primitive condition for lungfishes plus tetrapods, lost in modern amphibians, or a parallel development - but see comments below about the likely protoamniote condition). Note that, while the fate of the spiracle is to become the middle ear cavity plus Eustachian tube in most extant tetrapods, an enclosed middle ear was evolved a number of times convergently within tetrapods.
© 2007-2010 Christine Janis, Professor of Ecology and Evolutionary Biology
Colleen Brogan '10, Student Technology Assistant
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