The family Gomphocystitidae (Echinodermata, Diploporita) and the functional morphology of spiral ambulacra

Abstract The Ordovician to Silurian diploporite family Gomphocystitidae is here defined by the possession of at least some unilateral spiral ambulacra composed of adambulacral plates each of which bears a single facet for unknown erect feeding structures (probably biserial brachioles). Pyrocystites , which also has unilateral spiral ambulacra, is rejected as a gomphocystitid because it lacks a regular ambulacral structure. Accepted genera are distinguished on oral and ambulacral structure. Fungocystites has at least one bilateral ambulacrum with facets on both sides. All other genera have five unilateral spiral ambulacra. Celticystis has a mouth surrounded by four oral plates; Gomphocystites and Fresticystis have five or more orals forming the mouth frame. In Fungocystites , Celticystis , and Fresticystis , the two posterior orals also contribute to the periproct frame; in Gomphocystites , the two plates between the mouth and anus are arranged one above the other. “ Protocrinus ” sparsiporus Bather, 1906, from the Ordovician of Myanmar, is assigned to “ Gomphocystites ?” because it has spiral ambulacra and adambulacral plates identical to other gomphocystitids, but its oral plating is unknown. Gomphocystitids are known from North America, Europe, and Asia. The functional morphology of unilateral spiral ambulacra is reviewed. It is geometrically impossible to maintain both equal spacing and regular left–right alternation of brachioles in a spiral ambulacrum. Arranging brachioles on the outside of the curve allows more to be developed and orients them with the food groove downstream, the preferred orientation for feeding in living crinoids. Unilateral spiral ambulacra arose due to functional constraints. UUID: http://zoobank.org/31f9f4dc-98f5-43fc-8dd1-d2e08dad3650