Mid-Cretaceous ammonite zones, Western Interior, United States

The Western Interior of the United States contains an important sequence of middle Cenomanian-late Turonian ammonites. Ammonites from the northern part of the Western Interior are mostly North Temperate forms, whereas those from the southern part are a mixture of Tethyan and North Temperate forms. The ammonite succession in the southern part can be more finely zoned than that of the northern part, and offers better possibilities for international correlation. In the zonation, the boundary between the middle and upper Cenomanian is placed at the top of the zone of Plesiacanthoceras aff. P. wyomingense. The top of the Cenomanian is drawn at the top of the zone of Neocardioceras juddii as recommended in a 1981 study by C. W. Wright and W. J. Kennedy. However, Mammites nodosoides, which occurs three zones higher, is usually regarded in Europe as basal Turonian, although Wright and Kennedy have now recognized a zone of Watinoceras coloradoense at the base, which lies below the zone of Mammites nodosoides. In the Western Interior, W. coloradoense is restricted to the zone of Vascoceras (Greenhornoceras) birchbyi, which is separated from the zone of Neocardioceras juddii by a zone of Pseudaspidoceras flexuosum. Inasmuch as the ammonites in the latter zone are closely related to those in the zone of V. birchbyi, a Turonian assignment seems best for the zone of P. flexuosum. Collignoniceras woollgari, which is widely distributed in Europe, Asia, and North America, is usually regarded as middle Turonian in a threefold division and late Turonian in a twofold division. I consider the species to mark the base of the middle Turonian. Where to place the top of the middle Turonian, however, is a problem. Inasmuch as the best fauna! break is at the top of the zone of Prionocyclus hyatti, that is where the boundary is placed in this report. Inoceramids in the zone of P. hyatti are characterized by forms having a radial depression like the middle Turonian Inoceramus hobetsensis of Japan. The top of the Turonian is designated here at the top of the zone of Prionocyclus quadratus. Fossils of this zone include inoceramids of the fiegei-dresdensis group, which in Europe have been considered either very late Turonian in age or very early Coniacian. Prionocyclus quadratus is the youngest species of a lineage of prionocyclids. This species is succeeded by Forresteria (Solgerites) of Coniacian Age.