Exaptation of pelmatozoan oral surfaces: constructional pathways in tegmen evolution

Abstract. The adoral surface of a crinoid theca has traditionally been called the tegmen, despite a wide range of morphologies; and, unfortunately, this has obscured the potential to recognize homologies between blastozoans and crinoids. With present recognition of these homologies, the constructional morphology of crinoid oral regions is explored, herein. Two major types of oral regions exist among crinoids: (1) an oral surface with the mouth exposed; and (2) a tegmen, in a restricted definition, with the mouth covered beneath solid plating. A tegmen is constructed by exaptation of oral surface plating and, commonly, other thecal plating. A pseudo-tegmen is an exaptation of aboral cup plates (i.e., radial plates). Tegmens or pseudo-tegmens evolved in all major crinoid clades at least once as an exaptation of oral surface plating. Tegmens evolved iteratively both between and within clades. In some cases, tegmen plates can be homologized with oral surface plates, but in other cases this is not apparent. Ex...