Italian
Journal of Zoology
66, 99-119 (1999) Abstract.
Both new
palaeontological and embryological evidence on chordates require a
reassessment of traditional ideas on their ear ly phylogeny and on the
evolution of their Bauplan and tissues. A revision of the available data from
both palaeontological and zoo logical points of view is useful both to clarify
the better justified possible hypotheses and to point to the more profitable
lines of investigation for further advancing our understanding of these
problems. Recent advances in the knowledge of the morphology of Cambrian
protochordates and of conodonts provide significant evidence for the
understanding of the earliest phases of develop ment of typical vertebrate
morphology. It appears that, in. the old est lineages related with the
evolution of vertebrates, mineralized tissues may have been restricted to the
bucco-pharyngeal region and be of ectodermal and ectomesenchymal origin. The
develop-ment of dermal and visceral skeletal structures in living vertebrates
further supports the overall significance of ectodermal-ectomesenchymal
interactions as the first step towards the origin of the vertebrate skeleton. |