Marine onlap of major proportions generally has been attributed to the Late Devonian in the United States west of the Mississippi River, but this onlap ensued early during the Taghanic Stage and thus began as a late Middle Devonian event. Pre-Taghanic Givetian marine areas of eastern and western North America were not connected across the southern United States and were connected only briefly between western Canada and the north-central United States (Indiana and Michigan). Pre-Taghanic Middle Devonian brachiopod faunas of eastern and western North America belong to different faunal provinces, that is, to the Appalachian and Old World provinces respectively. The Taghanic onlap of the continental backbone in the southwestern United States provided shallow-water marine areas for dispersal of bethonic animals and the resulting intermigration brought an end to brachiopod provinciality that had prevailed since the Early Devonian. By analogy, provincial shifts in established faunal successions should provide dates for other sedimentary-tetonic events.
Because Appalachian Province fossils are known to range as far as Colombia and Venezuela in Emsian-Eifelian time, but not at the same time to the American west, a large land barrier is postulated for times of provinciality, that is, during the intervals Ludlow-early Siegenian, and Emsian-mid-Givetian. Other intervals during the Silurian and Devonian were times of breaching of the land barrier by marine seas.
Taghanic brachiopod faunas of Givetian age in western North America are represented by two faunas, a lower one with chonetids of the aurora type and Leiorhynchus of the mesacostale type, and an upper one with Leiorhynchus hippocastanea, Hadrorhynchia sandersoni, and Warrenella occidentalis. Elements of the latter fauna have been reported lower in the Givetian due to confusion with the fauna that includes Leiorhynchus castanea. The Leiorhynchus hippocastanea fauna occurs with hermanni-cristatus zone conodonts above the “C.” aurora fauna and marks the top of the Givetian, above the range of Stringocephalus in North America. The aurora group of chonetids is proposed as a new genus Rhyssochonetes, based on the new subspecies R. aurora solox.