Ontogeny of non-muscular brachial articulations in Balanocrininae (Echinodermata, Crinoidea): iterative trajectories or phylogenetic significance?

Ontogeny of non-muscular brachial articulations in extant species of Balanocrininae, i.e., Neocrinus decorus, Neocrinus blakei and Hypalocrinus naresianus (Crinoidea, Isocrinida), is described using SEM observations. All three species share embayed synarthries and symplexies (previously only known in crinoid stalks) showing a radiating crenularium pattern in their proximal arms but differ in several important ways. Neocrinus decorus has a shallow simple symmorphy affecting symplexies, and embayed synarthries. During the latest ontogeny of embayed synarthries, irregular syzygial ridges appear on the aboral segment of the fulcral ridge. Neocrinus blakei and H. naresianus share a peculiar sharp deep symmorphy superimposed on symplexies, and synarthries with a more complete single fulcral ridge that only appears late in ontogeny. Comparison with other crinoid taxa that have more advanced arm axial synarthries shows that this ontogenetic trajectory is restricted to paedomorphic stages in extant balanocrinins. An embayed synarthry seems to be derived from the earliest developmental stage of the radiating symplexial crenularium via hypermorphosis of a single crenula. An embayed synarthry is, therefore, a symplesiomorphy based on paedomorphic stage of development; it thus lacks phylogenetic significance, and should be abandoned as a major character in the classification of Isocrinida. The most advanced brachial synarthries shared by distant crinoid taxa mainly represent a homoplasy under morphofunctional constraints. However, they could result from different ontogenetic trajectories, which have only rarely been investigated. Another distinctive articulation feature, the peculiar sharp deep symmorphy observed in extant balanocrinins is a derived character known in a few fossil isocrinids beginning in the Middle Jurassic. We question its phylogenetic significance and suggest that it has developed repeatedly via iterative evolution in Isocrinida. Therefore, because these three extant balanocrinin species share the same ontogenetic trajectories of arm and stalk ligamentary articulations, and differ only in various states of heterochronic development of a few characters, we treat them as belonging to the same genus. We, therefore, consider Hypalocrinus as a junior synonym of the genus Neocrinus.