Idiognathodus simulator (Ellison, 1941)

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 : 491-493

Idiognathodus simulator (Ellison, 1941)

Fig. 13.

1941 Streptognathodus simulator sp. nov.; Ellison 1941: 133, pl. 22: 25 (holotype), 29.

1959 Streptognathodus simulator Ellison, 1941 ; Jennings 1959: 994– 995, pl. 124: 7.

1972 Streptognathodus eccentricus Ellison, 1941 ; Von Bitter 1972: pl. 3: 1a, 1b, 1c?

1972 Streptognathodus simulator Ellison, 1941 ; Von Bitter 1972: 56, pl. 3: fig. 2a, 2b, 2c?, 2d.

1978 Streptognathodus simulator Ellison, 1941 ; Kozitskaya et al. 1978: 106, pl. 30: 1?, 2.

1981 Streptognathodus eccentricus Ellison, 1941 ; Barskov et al. 1981: pl. 2: 15.

1986 Streptognathodus simulator Ellison, 1941 ; Chernykh 1986: pl. 27: 22.

1987 Streptognathodus simulator Ellison, 1941 ; Barskov et al. 1987: 91, pl. 21: 14, 15.

1987 Streptognathodus eccentricus Ellison, 1941 ; Barskov et al. 1987: 87, pl. 21: 16?, 18.

1987 Streptognathodus simulator Ellison, 1941 ; Cherynkh and Reshetkova 1987: pl. 3:17–20.

1997 Streptognathodus simulator Ellison, 1941 ; Kozitskaya and Nemirovskaya 1997: pl. 10: 26?

2005 Streptognathodus simulator Ellison, 1941 ; Chernykh 2005: 138– 141, pl. 1: 6; pl. 2: 1, 2, 4?, 7.

2005 Streptognathodus sinistrum sp. nov.; Chernykh 2005: 141, pl. 2: 8 (holotype), 9–12.

2009 Idiognathodus simulator (Ellison, 1941) ; Alekseev et al. 2009: pl. 3: 11?

2008 Idiognathodus simulator (Ellison, 1941) ; Barrick et al. 2008: 127–130, pl. 1: 4, 14, 15, 19?, 20, 21, 23.

2008 Idiognathodus simulator (Ellison, 1941) ; Heckel et al. 2008. fig. 1: 5.

2008 Streptognathodus sinistrum Chernykh, 2005 ; Davydov et al. 2008: 124, fig. 11: K, L (holotype re-illustration).

2008 Streptognathodus simulator Ellison, 1941 ; Davydov et al. 2008: 124, fig. 11: A, B?, C?, D.

2010 Idiognathodus simulator (Ellison, 1941) ; Barrick et al. 2010: pl. 9: 14.

2012 Streptognathodus simulator Ellison, 1941 ; Chernykh 2012: 81– 83, pl. 6: 1, 2, 12?, 15?, 16?

2013 Idiognathodus simulator (Ellison, 1941) ; Barrick et al. 2013a: pl. 4: 9 (holotype re-illustration).

Material.—145 sinistral P 1 and 134 dextral P 1 elements. Illustrated specimens repository numbers SUI 141025– SUI 141041. 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 an eccentric groove, and a caudal adcarinal ridge separated from the caudal platform margin by the caudal adcarinal trough.

Description.— Sinistral P 1 element: Rostral platform margin forms a broadly curved, convex edge. Caudal platform margin is slightly curved and convex, with the point of maximum curvature located near the midpoint of the margin. Typically, the caudal platform margin does not extend to the dorsal tip of the element, and is often separated from the tip by the eccentric groove. The eccentric groove is well developed and may intersect the caudal margin before reaching the dorsal tip of the platform. Rostral adcarinal ridge is either parallel to the carina or dips in towards it. The caudal adcarinal ridge is truncated by the adcarinal trough and separated from the caudal platform margin. Generally, the ventral end of the caudal platform margin and dorsal end of the caudal adcarinal ridge overlap one another. This is a diagnostic feature of the species. In smaller specimens with a platform length less than ~ 0.5 mm, a tall solitary node lies outside the caudal platform and extends laterally. This juvenile state is morphologically similar to juveniles of I. lateralis sp. nov., but differs in having a higher node and the lateral direction of its peak. In larger specimens, the peak of the node fuses with the caudal adcarinal ridge and rises higher than the caudal platform margin, forming a parapet. The separation between the parapet and the platform margin becomes more apparent as the dorsal-ventral growth of each begins to overlap, accentuating the adcarinal trough between them. The transverse ridges on the platform typically intersect the eccentric groove at an oblique angle and are frequently aligned with one another on opposite sides of the groove. In the largest specimens a rostral lobe with one or two nodes may develop. This lobe is small in all dimensions of growth and has little effect on the curvature of the rostral platform margin and adcarinal ridge. Larger specimens typically have a wider platform relative to length than smaller specimens.

Dextral element: Rostral platform margin is widest near the ventral end and tapers inwards to the dorsal tip, giving the platform a broader, more triangular appearance than the sinistral element. Caudal platform margin is slightly convex with maximum curvature occurring near the midpoint of the margin. Rostral adcarinal ridge is short and is typically parallel to the carina, but may dip in towards it. Caudal adcarinal ridge is subparallel to the carina and its separation from the platform margin may not be obvious until the platform is approximately 0.4 mm long. This results in morphological similarity between juvenile dextral P 1 elements of I. simulator and I. luganicus . The isolated caudal adcarinal ridge appears to be more parallel to the carina in dextral elements than in sinistral elements where the ridge flares outwards. A rostral lobe on large, dextral elements is rare and less common than on sinistral elements. The eccentric groove is shifted to the caudal edge of the platform and is usually well developed. The transverse ridges on the dextral elements are subperpendicular to the eccentric groove and typically aligned with each other on opposite sides of the platform. Carina typically extends dorsally to the point of the first or second ventral-most transverse ridge in most specimens.

Remarks.—The disconnected adcarinal ridge is diagnostic of Idiognathodus simulator andoccursontheholotypeofEllison (1941). Chernykh (2005) diagnosed S. sinistrum Chernykh, 2005 , as having a high, isolated, parapet. Specimens of I. sinistrum illustrated by Chernykh (2005) and Davydov et al. (2008) appear to be small sinistral specimens of I. simulator . Illustrations of I. simulator in Davydov et al. (2008) are larger specimens that have a rostral lobe. Most of these specimens ( Davydov et al. 2008: fig. 11A–D) appear to have the diagnostic isolated and outward flaring adcarinal ridge of I. simulator , and also have a broader platform than the illustrated specimens of S. sinistrum . Widening of the platform and development of a rostral lobe are characteristics that develop during growth in I. simulator and are typically only seen in larger specimens. Davydov et al. (2008) also record that I. simulator and S. sinistrum have the same stratigraphic occurrence. We interpret S. sinistrum to be a junior synonym of I. simulator . Davydov et al. (2008) stated that North American specimens have a more elongate and asymmetrical platform than specimens from the Urals, which are more isometric in platform shape. Both elongate forms ( Fig. 13B–D, K, L) and more isometric forms ( Fig. 13E, F, H–J, Q) with the diagnostic adcarinal ridge are present in the Heebner Shale.

The specimen illustrated as Streptognathodus simulator by Jennings (1959: pl. 124: 7) appears to have a slight separation between the adcarinal ridge and the platform margin, and we consider it to belong to Idiognathodus simulator , even though the separation is faint. In the specimen illustrated as I. tersus by Zagorodnyuk et al. (1979: pl. 35: 7) the caudal adcarinal ridge appears to be separated from the platform margin, but it difficult to tell from the illustration. Specimens illustrated as I. simulator by Chernykh (1986: pl. 27: 20, 21) belong to the I. simulator group. However, the photo quality makes it difficult to be certain whether these specimens belong to I. simulator or I. lateralis sp. nov.

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