 An additional species, ""T. fredericksoni"", was previously thought to occur in Kansas and Nebraska, but its existence is dubious, and may be attributable to "Narceus americanus", a spirobolid common in the eastern U.S.Since no specimens of ""T. fredericksoni"" have been discovered since 1949 and the holotypes are highly fragmented and equivocal, the species is considered a "nomen dubium" or doubtful name.