The septate, lamellose, spiriferacean brachiopod genus Tylothyris North, 1920, is considered to be a delthyridid of the subfamily Tylothyridinae Carter, 1972. Herein, the diagnosis of the Tylothyridinae is emended to include forms with micro-ornament, and with a myophragm or median septum on the floor of the pedicle valve.
For the first time, Tylothyris is formally reported from the Traverse Group rocks of Michigan, Givetian through early Frasnian in age. Two species of the genus occur in the Traverse Group. The first, T. subvaricosa (Hall and Whitfield, 1872), previously described by Stainbrook (1943) from the Cedar Valley Limestone of Iowa, is also known from the Milwaukee Formation of Wisconsin and the Mineola Limestone of Missouri. Tylothyris randalia Stainbrook, 1943, also from the Cedar Valley Limestone, is considered to be synonymous with T. subvaricosa. In the Traverse Group, T. subvaricosa occurs in the Bell Shale, Rockport Quarry Limestone, Ferron Point Formation, Genshaw Formation, Alpena Limestone, Gravel Point Formation, Norway Point Formation, and Potter Farm Formation. The second species, T. rockportensis n. sp., occurs only in the Rockport Quarry Limestone, and appears to be endemic to the Michigan Basin.
Tylothyris is considered to have originated in the Illinois area during the late Eifelian, then to have migrated to the Michigan area by the early Givetian. With the onset of Taghanic Onlap during the late Givetian, the genus dispersed from its Illinois–Michigan area of origin, becoming distributed across much of North America during the Frasnian and Famennian, and becoming essentially cosmopolitan in distribution during the Tournaisian. Tylothyris apparently became extinct by the end of the Visean.