Idiognathodus Gunnell, 1931

Hogancamp, Nicholas J., Barrick, James E. & Strauss, Richard E., 2016, Geometric morphometric analysis and taxonomic revision of the Gzhelian (Late Pennsylvanian) conodont Idiognathodus simulator from North America, Acta Palaeontologica Polonica 61 (3), pp. 477-502 : 490

Genus Idiognathodus Gunnell, 1931

Type species: Idiognathodus claviformis Gunnell, 1931 ; Moscovian (Late Desmoinesian), Coal City Limestone Member , Pawnee Limestone , Marmaton Group; Lexington, Missouri, USA .

Idiognathodus simulator (Ellison, 1941) group Remarks.—The Idiognathodus simulator group includes species with asymmetrical P 1 element pairs and typically with an eccentric groove running down the length of the platform. There has been some disagreement whether or not species of the Idiognathodus simulator group should be assigned to the genus Streptognathodus Stauffer and Plummer, 1932 (e.g., Chernykh 2005, 2012) or Idiognathodus Gunnell, 1931 (e.g., Barrick et al. 2008). We assign all these species to Idiognathodus for the same reasons discussed by Barrick et al. (2008). There is additional confusion about whether the P 1 elements (platforms) of the species were strongly asymmetrical, where the sinistral and dextral elements differ in morphology (class IIIb symmetry, Lane [1968]; e.g., Barrick et al. 2008), or whether the P 1 element pairs were near mirror images of each other (class II symmetry, Lane [1968]; e.g., Chernykh 2005, 2012). For these reasons, we include a complete description and diagnosis for each species of the I. simulator group based on the material collected from the Heebner Shale.

Species of the Idiognathodus simulator group are distinguished from one another primarily by the degree of development of lobes and secondarily by the shape of the platform and adcarinal ridge. Members of the I. simulator group are I. auritus , I. lateralis sp. nov., I. luganicus , I. praenuntius , I. simulator , and I. postsimulator . Possible synonymous species include I. gravis , and I. sinistrum , which are discussed under I. auritus and I. simulator , respectively. Idiognathodus luosuensis (Wang and Qi, 2003) may belong to the I. simulator group, but requires additional study.

The dominantly Gzhelian Idiognathodus simulator group is characterized by distinct asymmetry between the dextral and sinistral P 1 elements, in contrast to most other species of Idiognathodus in which the P 1 element pairs are approximately mirror images. A similar degree of P 1 element asymmetry, as well as an eccentric groove, can be seen in the Kasimovian species I. eudoraensis Barrick, Heckel, and Boardman, 2008 , which was proposed to be the ancestor of I. simulator (Barrick et al. 2008) . The beginning of the I. eudoraensis – I. simulator lineage, and the origin of the eccentric groove and P 1 element asymmetry are not known. Rosscoe (2008) reported that the somewhat older Kasimovian species I. magnificus Stauffer and Plummer, 1932 , appears to show moderately asymmetrical variation between the dextral and sinistral P 1 elements, and it may be a possible ancestral form. Idiognathodus magnificus has very large lobes ornamented with abundant nodes and lacks an eccentric groove. In many specimens, the transverse ridges appear notched, and aligned, forming a visible line down the platform, but the transverse ridges remain complete and no groove exists. Because of the likely origin from a typically ungrooved Idiognathodus species, I. simulator group species have been assigned to the genus Idiognathodus (Barrick et al. 2008) .