Iapetonudus (N. gen.) and Iapetognathus Landing, Unusual Earliest Ordovician multielement conodont taxa and their utility for biostratigraphy

The Early Ordovician (Tremadocian) multielement conodont genus Iapetognathus is one of the oldest denticulate euconodont genera known. The ramiform-ramiform apparatus structure of Iapetognathus is not similar morphologically to other Late Cambrian to Earliest Ordovician denticulate multielement taxa, such as Eodentatus or Cordyloduts, because the major denticulate process has a lateral rather than a posterior orientation as it is in the other two examples. For this reason the genus is believed to have developed from the coniform-coniform apparatus Iapetonudus ibexensis (N.gen., n.sp.) through the development of the denticulate lateral processes. The two genera have a number of morphologic features in common and appear in stratigraphic succession. Iapetognathus aengensis (Lindström) is redefined as a multielement taxon using topotype material and Ig. preaengensis Landing is placed in synonymy with it. Iapetognathus sprakersi, recently described by Landing in Landing and others (1996), is recognized as a multielement species and the new multielement species, Ig. fluctivagus, Ig. jilinensis and Ig. landingi n. spp. are described herein, based on type specimens from Utah (U.S.A.), Jilin (China) and Colorado (U.S.A.) respectively. Iapetonudus and Iapetognathus are important genera in defining the level of the Cambrian-Ordovician boundary. Iapetonudus is currently recognized only from Utah, Texas and Oklahoma, but Iapetognathus is cosmopolitan in its distribution.