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David Marjanovic wrote:
In this case I don't have a better idea. However, gradual changes are also
suspected to occur.
Yes. I suspect that gradual change was responsible for the shift in caudal
morphology from basal to derived titanosaurs. But within derived
titanosaurs, it is more likely that the change in coely (e.g., procoelous to
opisthocoelous) was a single-step process, at least for adjoining vertebrae.
Given what we now know about homeotic genes, this idea isn't as
revolutionary as it used to be.