Italian Journal of Zoology 66, 99-119 (1999)
Hypotheses on the origin and early evolution of chordates in the light of recent zoological and palaeontological evidence

Alberto M. Simonetta, Antonella Pucci, & Jerzy Dzik
Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa, Poland. e-mail: dzik@twarda.pan.pl

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.