The hexamer hypothesis explains apparent irregularities in the plating of early and extant crinoids

de Lussanet de la Sablonière, MHE

Zusammenfassung

In 2011 I proposed a new hypothesis for the evolution and development of the five-rayed structure of echinoderms, the Hexamer Hypothesis (Lussanet, 2011). According to this hypothesis, the five-rayed structure develops by reduction from a six-rayed Bauplan. Crinoids (sea lilies) are unique as the only extant clade with stalked representatives, reaching back at least to the early Ordovician. Their extraordinary rich fossil record and the known embryology of recent forms make them ideal to test the Hexamer Hypothesis. It is concluded that the plating of the calyx and the stalk can be explained by the new hypothesis.