Idiognathodus praenuntius ( Chernykh, 2005 )

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 : 499-500

Idiognathodus praenuntius ( Chernykh, 2005)

Fig. 15N–T .

1983 Streptognathodus simulator Ellison, 1941 ; Kozitskaya 1983: pl. 1: 31, 32.

1987 Idiognathodus aff. lobulatus ; Cherynkh and Reshetkova 1987: pl. 1: 11, 12.

2005 Streptognathodus praenuntius sp. nov.; Chernykh 2005: 135– 136, pl. 1: 1–4, 5 (holotype).

2008 Streptognathodus praenuntius Chernykh, 2005 ; Davydov et al. 2008: 126–128, fig. 10: M (holotype re-illustration), N, O.

2009 Idiognathodus ex gr. simulator ; Alekseev et al. 2009: pl. 3: 3.

2012 Streptognathodus praenuntius Chernykh, 2005 ; Chernykh 2012: 79–80, pl. 2: 5–9.

Material.— 60 sinistral P 1 and 17 dextral P 1 elements. Illustrated specimens repository numbers SUI 141075– 141081 . Collected from the early Gzhelian Heebner Shale from all three outcrops in this study, Sedan, Clinton, and I-229 roadcut, Kansas, USA .

Diagnosis.—Asymmetrical P 1 element pair, presence of a slightly eccentric often interrupted groove, a caudal lobe, and nearly symmetrical platform margins.

Description.— Sinistral element: Rostral and caudal platform margins are broadly curved and the maximum curvature on each lies at nearly the same location on the margin, generally near the midpoint. The oral platform surface dips in towards the eccentric groove, which is complete in most specimens. The eccentric groove is shifted only slightly towards the caudal side. The transverse ridges either intersect the groove at a nearly perpendicular orientation, or they radiate from the center of the platform, changing their orientation along the length of the platform. The rostral adcarinal ridge is short and is either subparallel to the carina or flares inwards towards the carina. The caudal adcarinal ridge is longer than the rostral ridge and flares away from the carina. The caudal adcarinal ridge and platform margin are concave near the caudal lobe. The caudal lobe does not extend far along the length of the platform. The lobe is ornamented with one or two nodes. The carina terminates either ventrally of the transverse ridges, or extends into the platform to the first few ventral transverse ridges.

Dextral element: The platform is relatively short and squat. Rostral platform margin is curved with the point of maximum curvature located along the ventral portion and tapers in a slightly straighter direction to the dorsal tip. Caudal platform margin is curved and reaches maximum curvature near the midpoint of the margin. The rostral adcarinal ridge is typically parallel to the carina or dips slightly inward towards it. The caudal adcarinal ridge can also be subparallel to the carina or dip slightly away from it. The eccentric groove is shifted slightly caudally. A lobe occurs only on the caudal side and on larger elements the number of nodes increases, often resulting in large specimens with chaotic node architecture. Transverse ridges tend to dip at a slightly oblique angle towards the eccentric groove, but are usually more parallel in the ventral half of the platform. Oral platform surface is generally flat, but may dip slightly inward toward the eccentric groove. This dip is far less pronounced in the dextral element than the sinistral element. The eccentric groove is typically thin and is typically truncated by complete transverse ridges. In larger specimens the most ventral rostral transverse ridges flare ventrally.

Remarks.— Idiognathodus praenuntius is the least common of the species of the I. simulator group recovered from the Heebner Shale. Assignment of specimens to I. praenuntius was based on the combination of platform margin symmetry and a more medially located groove. Also, I. praenuntius has an oral platform surface that dips inwards towards the middle of the element, forming a diagnostic bowl-shape. Chernykh (2005, 2012) described I. praenuntius as having a rhombic platform and a nearly medial groove, and his illustrated specimens are dextral elements with one caudal lobe. Davydov et al. (2008) stated that the more medial and less developed groove were diagnostic of I. praenuntius , but added that the near medial position of the groove is variable. In our collections, sinistral elements typically have a better developed groove than the dextral elements. Platform margin symmetry also tends to increase with a more medially positioned groove. In addition to the features listed above, the platform of I. praenuntius is shorter, more squat, and more ovate than the platforms of I. auritus , I. simulator , I. lateralis sp. nov., and I. luganicus .

Small dextral elements of Idiognathodus praenuntius and I. lateralis sp. nov. are similar and difficult to distinguish. Questionable specimens were assigned to I. lateralis sp. nov. In some samples, numerous sinistral specimens of I. praenuntius could be identified, but no truly diagnostic dextral elements. Dextral forms are more diagnostic at larger growth stages, as is shown by the specimens illustrated by Chernykh (2005, 2012). Distinctive larger dextral elements of I. praenuntius are squatter and shorter than dextral elements of I. lateralis sp. nov.

Specimens illustrated as Idiognathodus praenuntius by Barrick et al. (2010: pl. 7: 9–24) from south China appear to have a more medial groove than I. lateralis sp. nov. and I. simulator . However, the platform shape is more elongate and tapered than any specimens of I. praenuntius described by Chernykh (2005, 2012), or recovered in this study. The rostral ridges of the dextral elements also appear to be more elongate than typical I. praenuntius .

Stratigraphic and geographic range.― Early Gzhelian Idiognathodus simulator Zone. Midcontinent North America, Donets Basin, southern and central Urals , Moscow Basin .

Figures

Fig. 15. Conodont P1 elements of Idiognathodus lateralis sp. nov. (A–M) and Idiognathodus praenuntius (Chernykh, 2005) (N–T) from the Heebner Shale, Gzhelian (Late Pennsylvanian). A. SUI 141062 (TT0228); Sedan 11. B. SUI 141063 (TT0233); Sedan 11. C. SUI 141064 (TT0050); Clinton 20. D., SUI 141065 (TT0019); Clinton 22. E. SUI 141066 (TT0168); Sedan 23. F. SUI 141067 (TT0261); Sedan 11. G. SUI 141068 (TT0260); Sedan 11. →