Origin of the holothurians (Echinodermata) derived by constructional morphology

According to a recent hypothesis, the holothurians originated by a process of extreme paedomorphism as “giant larvae”, with “de novo” developed radial systems. However, the present approach, which follows the principles of constructional morphology, supports former views that the holothurian predecessor must have been echinoid‐like. After constitution of a (reliable) early predecessor construction as a model with machine analogies, subsequent steps of structural transformation are explained by functional improvement and economy. Following results are discussed: (i) Holothurians have to be derived from a postlarval precursor; (ii) “Apodida” (as molecular‐genetically derived first holothurians) must originally have been pedate; (iii) ophiocistioids would not be cladistic “holothurians” but a precursor construction of the taxon echinoids plus holothurians; (iv) the Lovenian structure of the calcareous ring of Nudicorona (Middle Devonian), possible radial series in Palaeocucumaria (Lower Devonian), and distribution of the podia in two new holothurian body fossils from the Lower and Middle Devonian (preliminary description as Prokrustia tabulifera n. gen., n. sp. and Podolepithuria walliseri n. gen., n. sp.) obviously corroborate homology of holothurian and other echinoderm radial systems; (v) different extent of podial and body wall skeletonization suggests the existence of respiratory trees by no later than the Middle Devonian.