Doubting Didymus

Quite. This relates entirely to my original point about just what would be neccessary to achieve something that we would call 'fundamentally different' in a bodyplan. The new bodyplan would have to evolve practically from scratch, from a single cell. The vast majority of modern animals will never ever evolve enough for us to call their bodyplan fundamentally new. Tetrapods will always have four limbs, arthropods will always be segmented etc. Even if an animal achieves something quite novel, like a massive pair of wings, they are still simply derived characteristics, 'just' a modification to a fundamentally similar bodyplan.

lpetrich

As to jawed vertebrates' side fins / limbs, their origin is indeed a riddle, because they seem to come out of nowhere. But one theory I've seen states that they were originally ectopic top/bottom fins.

And furthermore, there is the question of how one defines "body plans". The cases of Sacculina, pentastomids, etc. are cases of drastic simplification of body architecture as an adaptation to being parasites, but there is another difficulty that has emerged as a result of work in evolutionary developmental biology ("evo-devo").

Deep homology.

Arthropods and vertebrates have a variety of body-plan differences, but several of the underlying development mechanisms are remarkably similar, something which suggests that their shared ancestor was a rather sophisticated sort of late-Precambrian worm. It had:

* Front-to-rear specification with at least 7 Hox genes, which are always expressed in the same order

* Belly/back distinction, though the vertebrate interpretation is the reverse of the arthropod one (Geoffroy St. Hilaire)

* Segmentation, with new segments added at the rear end

* One-way gut extending through most or all of the body

* A heart

* A central nervous system

* Some simple sort of eyes

* Some sort of appendages

Other phyla have not been studied as much, but their shared ancestor with arthropods and vertebrates often have several of these features.

[ October 01, 2002: Message edited by: lpetrich ]</p>