TEGMEN EVOLUTION IN CAMERATE CRINOIDS

The crinoid tegmen is comprised of plates that support and protect the mouth. Ambulacra run from the arms into the mouth, which is usually exposed on the surface in crinoids. In camerate crinoids, the mouth is almost always internalized and the tegmen has developed into a rigid surface comprised of interlocking plates. Following the revised nomenclature for the tegmen, based on homology with other pelmatozoans, the plates that support the mouth are the orals, whereas the plates that cover the mouth are the peristomial cover plates, with both sets of plates interradial in position. In camerates the orals are usually not exposed. Starting from the peristomial cover plates, it is possible to identify other plates on the tegmen, such as fixed ambulacral cover plates, including axillaries, as well as interambulacral plates.

The tegmens of monobathrid camerates can more readily be homologized with cladid crinoid tegmens and the oral surfaces of other pelmatozoans. The Silurian monobathrid Allocrinus has open ambulacra with moveable cover plates, but fixed peristomial cover plates. Whereas the Silurian monobathrid, Marsupiocrinus, has fixed ambulacral cover plates, clearly derived from biserial moveable plates, as well as fixed peristomial cover plates. The Devonian monobathrid Cytarrocrinus is atypical in retaining a primitive open mouth with exposed orals. All these genera are key to understanding tegmen plate arrangement in later Paleozoic monobathrid crinoids.

The tegmens of diplobathrid camerates are typically more complex, less regular, and harder to homologize with other crinoid tegmens. A notable exception is the Silurian Gazacrinus, which preserves orals beneath five fixed peristomial cover plates. However, many diplobathrids developed a complex pavement of tessellate plates that are often difficult to homologize with the fixed ambulacral and peristomial cover plates of monobathrids. Comparison of monobathrid and diplobathrid tegmens indicates distinctly unique evolutionary patterns.