Idiognathodus luganicus ( Kozitskaya, 1978 )

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 : 497-499

Idiognathodus luganicus ( Kozitskaya, 1978)

Fig. 16 .

1978 Streptognathodus luganicus sp. nov.; Kozitskaya et al. 1978: 101–102, pl. 30: 10 (holotype), 11.

1979 Streptognathodus simulator Ellison, 1941 ; Zagorodnyuk et al. 1979: pl. 36: 13.

1987 Streptognathodus luganicus Kozitskaya, 1978 ; Cherynkh and Reshetkova 1987: pl. 3: 16.

1995 Streptognathodus simulator Ellison, 1941 ; Henderson et al. 1995: pl. 1: 9.

1996 Streptognathodus luganicus Kozitskaya, 1978 ; Sobolev and Nakrem 1996: 79, pl. 5: A?

1997 Streptognathodus luganicus Kozitskaya, 1978 ; Kozitskaya and Nemirovskaya 1997: pl. 10: 25.

2006 Streptognathodus luganicus Kozitskaya, 1978 ; Boardman et al. 2006: pl. 1: 7.

2006 Streptognathodus simulator Ellison, 1941 ; Boardman et al. 2006: pl. 1: 8.

2009 Idiognathodus luganicus ( Kozitskaya, 1978) ; Alekseev et al. 2009: pl. 5: 5.

2009 Idiognathodus ex gr. simulator ; Alekseev et al. 2009: pl. 5: 9.

2009 Idiognathodus simulator (Ellison, 1941) ; Alekseev et al. 2009: pl. 5: 11.

2007 Idiognathodus lobulatus Kozitskaya, 1978 ; Sudar et al. 2007: pl. 2: 6?

2008 Idiognathodus simulator (Ellison, 1941) ; Barrick et al. 2008: 127–130, pl. 1: 5, 13, 16, 17.

2008 Streptognathodus simulator Ellison, 1941 ; Davydov et al. 2008: 124, fig. 11E.

Material.—403 sinistral P 1 and 516 dextral P 1 elements. Illustrated specimens repository numbers SUI 141094 – SUI 141104. Collected from the early Gzhelian Heebner Shale from all three outcrops in this study, Sedan, Clinton, and I-229 roadcut, Kansas, USA.

H. SUI 141069 (TT0064); Clinton 29. I. Holotype, SUI 141070 (TT0039); Clinton 19. J. SUI 141071 (TT0049); Clinton 28. K. SUI 141072 (TT0034); Sedan 20. L. SUI 141073 (TT0185); Clinton 37. M. SUI 141074 (TT0170); Sedan 20. N. SUI 141075 (TT0214); Clinton 18. O. SUI 141076 (TT0066); Clinton 27. P. SUI 141077 (TT0055); Sedan 23. Q. SUI 141078 (TT0176); Clinton 19. R. SUI 141079 (TT0188); Clinton 29. S. SUI 141080 (TT0269); Sedan 11. T. SUI 141081 (TT0264); Sedan 11.

Diagnosis.—Asymmetrical P 1 element pair, presence of an eccentric groove, and absence of lobes on caudal and rostral platform margins.

Description.— Sinistral P 1 element: Rostral platform margin is slightly curved along the dorsal half and typically straight along the ventral half. Caudal platform margin is relatively straight and the point of maximum curvature is located on the dorsal half of the margin. The caudal adcarinal ridge is longer than the rostral ridge and extends parallel to the carina. The rostral adcarinal ridge is reduced in length and may appear as a small node extending from the rostral platform margin. The rostral adcarinal ridge is typically parallel to the carina or dips towards it at an angle less than 45°. The carina ends ventrally of the transverse ridges, and is relatively longer in smaller specimens. The eccentric groove is frequently deeper on sinistral elements than dextral elements and is shifted caudally, such that the caudal platform area comprises approximately 1/3 of the total platform area. The groove is developed at all ontogenetic stages, but is occasionally constricted by transverse ridge growth. Platform surface appears in two styles. In one, a flat oral surface with little relief to the eccentric groove occurs. In the other, the oral surface slants inwards towards the eccentric groove. The latter platform is more abundant in smaller elements and in elements with an eccentric groove that is strongly shifted caudally. The platform grows predominately lengthwise until it is approximately 0.5 mm in length. At that point the platform starts to widen and the eccentric groove becomes more weakly developed. Transverse ridges tend to be slightly oblique to the eccentric groove on both sides of the platform. In some cases the transverse ridges on opposite sides of the groove are nearly aligned and in other cases they may dip dorsally from one another at the groove.

Dextral P 1 element: Rostral platform margin is not a long, smooth curve like it is on sinistral elements, but is wider in the ventral region. In larger forms the ventral margin forms a “shoulder” that tapers straight to the dorsal tip. This lateral growth is less apparent in smaller specimens. The caudal platform margin is relatively straight and parallel to the carina. In many larger specimens a geniculation point is located at the junction of the platform margin and the adcarinal ridge. This results in the adcarinal trough being shifted outwardly, but it still lies sub-parallel to the adcarinal ridge and the caudal platform margin. Specimens lacking the elongate and laterally shifted adcarinal ridge have an adcarinal ridge that is in line with the platform margin, still longer than the rostral adcarinal ridge, but relatively short. The carina terminates ventrally of the transverse ridges except for occasional rostral transverse ridges. The eccentric groove is shallower than the groove in the sinistral elements and is shifted caudally, resulting in a rostral platform area that makes up approximately ⅔ of the total platform. The platform surface tends to be relatively flat across the oral surface, but can occasionally slant inwards towards the eccentric groove. The transverse ridges are sub-perpendicular to the eccentric groove and are often aligned with each another on opposite sides of the groove.

Remarks.—The outward shifting of the caudal adcarinal ridge in the dextral element of Idiognathodus luganicus creates a caudal margin similar to that in dextral elements of I. simulator . However, the adcarinal trough of I. simulator separates the adcarinal ridge from the platform margin, whereas they are not separated in I. luganicus . Idiognathodus luganicus and I. simulator appear to have a similar juvenile dextral element. Only at a platform length of approximately 0.4 mm does the adcarinal ridge begin to clearly disconnect from the platform margin in I. simulator . Specimens of I. luganicus exhibit one of two platform morphologies. The first has a broader, more asymmetrical platform that is similar to I. simulator . The second is typically a straight, more elongate, and narrower element with a lesser degree of asymmetry than the broader morphology. The holotype of I. luganicus is a dextral element with the more asymmetrical P 1 element pair.

Chernykh (2012) described another grooved, lobeless species, Streptognathodus postsimulator Chernykh, 2012 , which differs from Idiognathodus luganicus on the basis of platform shape. Sinistral and dextral elements of S. postsimulator show the same pattern of asymmetry as I. luganicus , with a broader dextral element and a more slender sinistral element. The sinistral elements of S. postsimulator differ from I. luganicus by having an oblique eccentric groove that is aligned with the carina, a more tapered element along both platform margins, a dorsal tip that is shifted caudally out of alignment with the carina, and shorter adcarinal ridges, with the caudal ridge extending further ventrally than the rostral one. Dextral forms of S. postsimulator are also heavily tapered along both margins, but differ from the sinistral forms in the location of maximum curvature along the caudal platform margin.

Some specimens illustrated as Idiognathodus simulator by Barrick et al. (2010: pl. 9: 6, 10) should be classified as I. luganicus under the taxonomic scheme used here. However, these specimens are atypical from specimens observed from the Heebner Shale and possess a flaring rostral adcarinal ridge. Another atypical specimen illustrated as Streptognathodus luganicus by Sobolev and Nakrem (1996: pl. 5: A) has relatively long adcarinal ridges, with the rostral adcarinal ridge being much longer than the caudal ridge.

Stratigraphic and geographic range.―Early Gzhelian Idiognathodus simulator Zone. Midcontinent North America, Ellesmere Island Arctic Canada, Donets Basin, Novaya ZemlyaArcticRussia,southernUrals,centralUrals,Moscow Basin, Serbia.

Figures

Fig. 16. Conodont P1 elements of Idiognathodus luganicus (Kozitskaya, 1978) from the Heebner Shale (Gzhelian, Late Pennsylvanian). A. SUI 141094 TT0200); Sedan 11. B. SUI 141095 (TT0209); Clinton 12. C. SUI 141098 (TT0285); Clinton 12. D. SUI 141096 (TT0284); Clinton 12. E. SUI 141097 TT0141); Clinton 28. F. SUI 141099 (TT0018); Clinton 15. G. SUI 141100 (TT0026); I229-83. H. SUI14101 (TT0016); Clinton 14. I. SUI 14102 TT0119); Sedan 20. J. SUI 141103 (TT0118); Sedan 20. K. SUI 141104 (TT0102); Sedan 12.