Idiognathodus lateralis, Hogancamp & Barrick & Strauss, 2016

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 : 495-497

Idiognathodus lateralis sp. nov.

Fig. 15A–M .

1941 Streptognathodus simulator sp. nov.; Ellison 1941: 133, pl. 22: 27, 28, 30 (only).

1978 Streptognathodus simulator Ellison, 1941 ; Kozitskaya et al. 1978: 106, pl. 30: 3.

1983 Streptognathodus simulator Ellison, 1941 ; Kozitskaya 1983: pl. 1: 30.

1987 Streptognathodus simulator Ellison, 1941 ; Cherynkh and Reshetkova 1987: pl. 2: 19, 20.

1996 Streptognathodus eccentricus Ellison, 1941 ; Sobolev and Nakrem 1996: 73–74, pl. 5: B.

2003 Idiognathodus simulator Ellison, 1941 ; Wang and Qi 2003: pl. 4: 9.

2005 Streptognathodus luganicus Kozitskaya, 1978 ; Chernykh 2005: pl. 3: 8.

2005 Streptognathodus simulator Ellison, 1941 ; Chernykh 2005: 138– 141, pl. 2: 3.

2006 Streptognathodus simulator Ellison, 1941 ; Boardman et al. 2006: pl. 1: 9, 10.

2008 Idiognathodus simulator (Ellison, 1941) ; Barrick et al. 2008: 127–130, pl. 1: 3?, 9?, 10–12, 18, 22.

2008 Idiognathodus simulator (Ellison, 1941) ; Heckel et al. 2008: fig. 1: 6.

2009 Idiognathodus ex gr. simulator ; Alekseev et al. 2009: pl. 5: 6.

2010 Idiognathodus simulator (Ellison, 1941) ; Barrick et al. 2010: pl. 9: 7–9.

2012 Streptognathodus simulator Ellison, 1941 ; Chernykh 2012: 81– 83, pl. 6: 8?, 10, 19.

2013 Idiognathodus simulator (Ellison, 1941) ; Barrick et al. 2013a: pl. 4: 10.

2013 Idiognathodus simulator (Ellison, 1941) ; Barrick et al. 2013b: fig. 10: 9.

Etymology: From Latin lateralis , side; referring to the side or flank.

Holotype: Sinistral P 1 element SUI 141070 ; TT00039 ( Fig. 15I ).

Type horizon: Early Gzhelian, Late Pennsylvanian.

Type locality: Heebner Shale Member of the Oread Formation, sample

19, Clinton section, Kansas, USA.

Material. —891 sinistral P 1 and 756 dextral P 1 elements. Illustrated specimens repository numbers SUI 141062–SUI 1410474. 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, one lobe on the caudal side, a caudally shifted eccentric groove, the platform margins and adcarinal ridges form continuous boundaries, and the caudal lobe is located outside of the adcarinal ridge.

Description.—Sinistral element: Rostral platform is broadly curved with the point of maximum curvature located near the midpoint of the margin. Rostral adcarinal ridge is either parallel to the carina or follows the curvature of the platform margin, forming a continuous curve from the dorsal tip to the end of the adcarinal ridge. Caudal platform margin can be straight and tapered, or show minor curvature, with the maximum curvature occurring along the dorsal half of the margin. Caudal adcarinal ridge flares away from the carina and is typically concave near the lobe. Specimens with a straighter caudal platform margin show less adcarinal ridge concavity near the lobe than the specimens that are more curved along the platform margin. Carina terminates dorsally near the most ventral transverse ridges. Eccentric groove is usually well developed and is shifted to the caudal margin of the platform. The eccentric groove is shifted closer to the caudal margin in specimens that have a straighter caudal platform margin, and the caudal platform area may represent only about ¼ of the total platform area. Oral platform surface typically dips inwards towards the eccentric groove on both sides of the groove. The eccentric groove either extends to the dorsal tip or terminates on the caudal margin of the element near the dorsal tip. Transverse ridges intersect the eccentric groove at an oblique angle and vary in alignment to each other on opposite sides of the groove. The caudal lobe appears in small specimens with a platform length of 0.3–0.4 mm. The lobe has one node until the platform reaches a length of approximately 0.5 mm. At this size a second or third node may develop, but these are often smaller in size than the original node. The lobe often terminates near the first or second transverse ridge, dorsally to the adcarinal ridge.

Dextral element: Rostral platform margin is widest near the ventral end and tapers straight to the dorsal tip, producing a wide ventral platform region. Caudal platform margin is slightly curved, concave near the lobe, and is usually most curved from near the midpoint to the dorsal half of the margin. In some specimens the caudal platform can be nearly straight and smaller specimens tend to have straighter caudal margins than larger specimens. As the element increase in size, both ventral platform margins increase greatly in width and become straighter, giving the platform a triangular shape. Rostral adcarinal ridge is very short and may appear as a small node above the most ventral transverse ridge. The rostral adcarinal ridge is either parallel to the carina or dips in towards it. The caudal adcarinal ridge is much longer, and flares away from the carina. The eccentric groove is more weakly developed in dextral elements than in sinistral elements, and can vary in its depth and width along the entire length of the groove. Transverse ridges are sub-perpendicular to the eccentric groove and are typically aligned with each other on opposite sides of the groove. Platform surface is generally flat, but may dip in towards the eccentric groove. The caudal lobe can extend dorsally to the midpoint of the platform margin. Often, the junction of the dorsal termination of the lobe and the platform margin is, or is near to, the point of maximum curvature along the platform margin. The concavity along the platform margin and the adcarinal ridge associated with the lobe is typically broad and smooth.

Remarks.— Idiognathodus lateralis sp. nov. is the most abundant species collected from the Heebner Shale. The asymmetric P 1 element pair, single caudal lobe, strongly caudally shifted eccentric groove, the continuous boundary formed by the caudal margin and the caudal adcarinal ridge, and asymmetrical platform margins are the diagnostic features of this species. The restriction of I. simulator to P 1 elements with an isolated caudal adcarinal ridge leaves many specimens previously assigned to I. simulator without a species designation, most of which are placed within I. lateralis sp. nov. Idiognathodus lateralis sp. nov. is similar to I. praenuntius ( Chernykh, 2005) and the two are distinguished by the location of the eccentric groove and the shape of the platform margins. In I. lateralis sp. nov. the eccentric groove is shifted more strongly caudally and the platform margins are more asymmetrical than in I. praenuntius . There is some degree of subjectivity in distinguishing a more medial groove from a caudally shifted one. However, specimens with a more medial groove frequently have more symmetrical platform margins, with curvature occurring at the same length along each margin. Juvenile sinistral elements and dextral elements of I. auritus ( Chernykh, 2005) that only have a caudal lobe can be distinguished from I. lateralis by the outwards flaring of the rostral adcarinal ridge, which can be seen before a rostral lobe is developed.

Asymmetry between the sinistral and dextral P 1 elements of Idiognathodus lateralis sp. nov. increases as the size of the specimens increases, with the sinistral elements being more similar to the smaller elements, and the dextral elements becoming very different in shape at larger sizes. Dextral elements appear to grow to a maximum platform length of approximately 0.75 mm and then expand laterally, forming broad, and flat oral platform surfaces, and triangular platform shapes. Sinistral elements typically have an oral platform surface that dips inwards towards the eccentric groove, and maintain their relative platform length and width dimensions at all growth stages. In large specimens the eccentric groove is typically better developed on the sinistral element than the dextral element. The lobe of the dextral element is more dorsally-ventrally expansive and larger than the lobe on the sinistral element, except for in larger specimens, where the lobe may become wider. Nodes on the dextral element lobe have a greater tendency to be more numerous, fused, or elongated in shape, and sometimes appear as parapets. Sinistral element lobe nodes are typically domal in shape, and the lobe is not as large as the lobe on the dextral element.

Stratigraphic and geographic range.—Early Gzhelian, Idiognathodus simulator Zone. Midcontinent North America, south-central New Mexico, Donets Basin, southern and central Urals, Novaya Zemlya, Arctic Russia, Moscow Basin, south China.

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. →