A news story about an ordinary octopus that developed only six legs prompted PZ Myers at Pharyngula to refer readers back to one of his detailed articles, “Cephalopod development and evolution.”
One… question is how oysters could be related to squid; one’s a flat, sessile blob with a hard shell, and the other is a jet-propelled active predator with eyes and tentacles. Any family resemblance is almost completely lost in their long and divergent evolutionary history….
One way to puzzle out anatomical relationships and make phylogenetic inferences is to study the embryology of the animals. Early development is often fairly well conserved, and the various parts and organization are simpler; I would argue that what’s important in the evolution of complex organisms anyway is the process of multicellular assembly, and it’s the rules of construction that we have to determine to identify pathways of change. Now a recent paper by Shigeno et al. traces the development of Nautilus and works out how the body plan is established, and the evolutionary pattern becomes apparent.
He’s referring to “Shigeno S, Sasaki T, Moritaki T, Kasugai T, Vecchione M, Agata K. (2007) Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from Nautilus embryonic development. J Morphol. [Epub ahead of print].”