Exaptation of pelmatozoan oral surfaces: constructional pathways in tegmen evolution

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. Examples of tegmens that evolved many times include tegmens with an appearance of oral surface plates cemented in place; tegmens with fixed ambulacral cover plates and primary peristomial cover plates disproportionately enlarged; tegmens composed exclusively, or nearly so, of greatly enlarged primary peristomial cover plates; tegmens with tessellate plating but presumably with some flexibility; and tegmens constructed of innumerable undifferentiated plates. Most tegmens have all ambulacral cover plates fixed; but in some instances, the abaxial ambulacral cover plates remain moveable. Additionally, some lineages that possessed a tegmen evolved an oral surface secondarily, likely as an atavism. Based on this restricted definition of a tegmen, the hemicosmitid blastozoan Caryocrinites also evolved a tegmen. As known, tegmens dominated among camerate crinoids; and oral surfaces were more common among cladids, hybocrinids, disparids, flexibles, and articulates. However, oral surfaces evolved in some camerate lineages; tegmens evolved in some cladid, disparid, and articulate lineages; and pseudo-tegmens evolved in some flexible and articulate lineages.The iterative evolution of tegmens in crinoids and blastozoans is thought to be an adaptive response to cover the mouth and proximal ambulacra to protect this portion of the digestive tract from predation, scavenging, parasites, and disease causing agents.