Article: Middle and Upper Ordovician conodont and graptolite biostratigraphy of the Marathon, Texas graptolite zone reference standard
Samples from the Marathon Ordovician succession, by many considered the reference standard for the Pacific Province graptolite zone succession in North America, have produced numerous conodonts, which provide new information on the correlation of the graptolite succession and the mutual relations between Pacific Province graptolite zones, North Atlantic Province conodont zones, and European Province graptolite zones. Most of the Marathon Glyptograptus cf. G. teretiusculus Zone is equivalent to the Baltoscandian G. teretiusculus Zone and the lower subzone of the Pygodus anserinus conodont Zone. The base of the Marathon and Baltoscandian Nemagraptus gracilis Zone occupies the same stratigraphic level, but no strata corresponding to the highest part of the Baltoscandian N. gracilis Zone have been found at Marathon, where a large hiatus separates the Woods Hollow Shale from the overlying Maravillas Formation. The basal part of the Maravillas, the reference interval of Berry's Orthograptus trucatus var. intermedius Zone, is of Maysvillian (late Ordovician) age and represents the Amorphognathus ordovicicus conodont Zone and is probably coeval with the Pleurograptus linearis Zone. Its stratigraphic incompleteness and other factors make the Marathon succession a less than ideal reference standard for the North American Middle and Upper Ordovician graptolite zone succession.