Two Coils in the Morphology of Myelodactylids (Crinoidea, Disparida): the Morphogenetic Basis of Their Formation and Adaptation Potential

In myelodactylids, the coil of the mesostele and the crown with a proxistele coiled in the same plane in the opposite direction result from the two fundamental processes in the evolution of echinoderms, which happened when their free-floating ancestor settled at the bottom and became attached to the ground by the ventral side of the anterior end of the body. The coil of the mesostele emerged due to the tendency of the stem to curve from the ventral to the dorsal side in the larvae, after they became attached, by the ventral side of the preoral lobe, and the growth vector changed from horizontal to vertical. The curvature of the proxistele, together with the crown in the same larval plane but in the opposite direction, from the dorsal side to the ventral, appeared during the paedomorphic delay in the process of elevation (torsion) in the ontogeny of myelodactylids. The coiling of the mesostele and the curvature of the proxistele with the crown in the E-BC plane determine the conformity of this plane to the larval plane in pentaradiate echinoderms, and the approximate conformity of the E ray to the dorsal side of the larva, and the interray BC to its ventral side. The morphology of myelodactylids shows that their stem in the feeding position stretched along slightly compacted ground, resting on cirri, and the crown extended at a slight acute angle over the stem downstream. In cases of danger, the stem coiled with its lower side inward, so that the crown was protected inside the coil by cirri. The coil of Zuravlicrinus milicinae gen. et sp. nov. from the Silurian of the Urals, and Valimocrinus terentyevi gen. et sp. nov. from the Ordovician of the Leningrad Region, coiled while growing around stems of other crinoids, and could not uncoil. Both genera, as well as Musicrinus Donovan, are known only from stem fragments and are tentatively assigned to myelodactylids. The finding of the genus Eomyelodactylus in China shows its spread far beyond North America. Findings of myelodactylids in Siberia show significant differences from other myelodactylids, allowing them to be assigned to a new genus Imagdacrinus gen .nov