Idiognathodus auritus ( 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 : 493-495

Idiognathodus auritus ( Chernykh, 2005)

Fig. 14 .

1959 Streptognathodus eccentricus Ellison, 1941 ; Jennings 1959: 995, pl. 124: 9.

1972 Streptognathodus eccentricus Ellison, 1941 ; Von Bitter 1972: pl. 3: 1d.

1975 Streptognathodus simulator Ellison, 1941 ; Barksov and Alekseev 1975: pl. 2: 19.

1978 Streptognathodus eccentricus Ellison, 1941 ; Kozitskaya et al. 1978: 91–92, pl. 30: 6–7.

1983 Streptognathodus eccentricus Ellison, 1941 ; Kozitskaya 1983: pl. 1: 27, 28.

1983 Streptognathodus simulator Ellison, 1941 ; Kozitskaya 1983: pl. 1: 33?

1984 Streptognathodus eccentricus Ellison, 1941 ; Alekseev et al. 1984: pl. 1: 12, 14, 18?

1984 Streptognathodus simulator Ellison, 1941 ; Alekseev et al. 1984: pl. 1: 20, 21.

1986 Streptognathodus simulator Ellison, 1941 ; Alekseev et al. 1986: 134, pl. 34: 9, 12.

1987 Streptognathodus eccentricus Ellison, 1941 ; Barskov et al. 1987: 87, pl. 21: 17, 19.

2005 Streptognathodus auritus sp. nov.; Chernykh 2005: 125, pl. 3: 2 (holotype), 3, 6, 7.

2005 Streptognathodus aff. auritus ; Chernykh 2005: pl. 3: 1.

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

2005 Streptognathodus aff. simulator ; Chernykh 2005: pl. 2: 5.

2008 Streptognathodus auritus Chernykh, 2005 ; Davydov et al. 2008: 130, fig. 11F?, G (holotype re-illustration).

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

2009 Idiognathodus ex gr. simulator ; Alekseev et al. 2009: pl. 5: 2, 3, 7, 8, 10, 12, 13; pl. 3: 1?, 2, 4–6, 7?, 9, 10?, 13, 14?

2009 Idiognathodus simulator (Ellison, 1941) ; Alekseev et al. 2009: pl. 5: 7, pl. 3: 8, 11.

2009 Idiognathodus sinistrum ( Chernykh, 2005) ; Alekseev et al. 2009: pl. 5: 4.

2010 Idiognathodus simulator (Ellison, 1941) ; Barrick et al. 2010: pl. 9: 5, 11–13.

2010 Idiognathodus simulator (Ellison, 1941) ; Goreva and Alekseev 2010: pl. 1: 11, 12?

2012 Streptognathodus simulator Ellison, 1941 ; Chernykh 2012: 81– 83, pl. 6: 3–7, 8?, 11, 12?, 13, 14, 15?, 17, 18, 20, 21.

2012 Streptognathodus aff. auritus ; Chernykh 2012: 81–83, pl. 6: 17, 18, 20.

Material.—156 sinistral P 1 and 29 dextral P 1. Illustrated specimens repository numbers SUI 141042– SUI 141054. 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. Sinistral elements with flared adcarinal ridges and caudal and rostral lobes. Dextral elements with a flared rostral adcarinal ridge and a caudal lobe.

Description.— Sinistral element: Rostral platform margin is broadly curved, with the maximum curvature located near the midpoint or on the ventral half of the margin. Caudal platform margin is curved along its entire length with its maximum curvature near the midpoint or just dorsal of the midpoint the margin. Both adcarinal ridges flare ventrally away from the carina and are concave along their entire length. The concavity formed by the adcarinal ridges and the ventral portion of both platform margins forms a rostrum, with lobes occurring outside the concavity. The lobe and margin concavity extend farther dorsally along the caudal side than along the rostral side. The caudal lobe typically bears one to four nodes, with the number of nodes increasing as the specimen increases in size. The rostral lobe is smaller and usually bears just one or two nodes. The carina is straight and terminates at the point of maximum concavity of the rostrum. The eccentric groove is typically well developed and often terminates along the caudal margin just before reaching the dorsal tip. The groove is shifted to the caudal side resulting in the rostral platform comprising roughly two-thirds of the overall platform area. Transverse ridges intersect the groove at an oblique angle, and are usually aligned with each another on opposite sides of the groove. Development of the rostral lobe occurs slightly later in growth than the caudal lobe, but both can consistently be seen at a platform length of approximately 0.5 mm.

Dextral element: Rostral platform margin is broadly curved and the maximum curvature is located along the ventral half of the margin. Caudal platform margin is curved, with maximum curvature near the midpoint or along the dorsal half of the margin. Both caudal and rostral adcarinal ridges flare away from the carina and the caudal ridge is the longer of the two. The caudal lobe usually bears one to three nodes, with the number of nodes increasing with size. A rostral lobe is rare and only occurs in larger specimens. When a rostral lobe is present, it is small, with just one node. The rostral adcarinal ridge flares ventrally outwards even in the absence of a rostral lobe. Transverse ridges are sub-perpendicular to the eccentric groove and are aligned with each other on opposite sides of the groove. Groove development is variable in dextral elements, but is often complete, and caudally shifted such that the rostral platform area composes approximately two-thirds of the total platform area. The carina terminates at the location of maximum curvature of the rostrum formed by the two adcarinal ridges.

Remarks.—The flaring adcarinal ridges and the rostral lobe are the primary diagnostic characters of Idiognathodus auritus and the species includes sinistral and dextral elements with both caudal and rostral lobes, and dextral elements with only a caudal lobe and a flared rostral adcarinal ridge. The asymmetry in P 1 elements is more than just overall shape variation, but includes the development of a lobe on the rostral side. Both sinistral and dextral elements have a caudal lobe at sizes exceeding a platform length of 0.4 mm. However, the rostral lobe is commonly seen on sinistral elements and rarely on dextral elements. Dextral elements with a rostral lobe are typically large, but have the diagnostic flared rostral adcarinal ridge as seen in smaller specimens. Because two-lobed sinistral elements were more common relative to dextral elements and the co-occurrence of two-lobed sinistral elements with one-lobed dextral elements that have flared adcarinal ridges, it is inferred that some two-lobed sinistral elements were paired with one-lobed dextral elements. In this study, dextral specimens without a rostral lobe that could not be confidently classified as I. auritus based on their shape were classified as I. lateralis sp. nov. based on their lobe character state.

Chernykh (2012) described the two-lobed species Streptognathodus gravis , which appears to differ from I. auritus because it does not show rostral ridge flaring, has a larger rostral lobe, and because the platform tends to taper dorsally without as much curvature along the platform margins.

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

Figures

Fig. 14. Conodont P1 elements of Idiognathodus auritus (Chernykh, 2005) from the Heebner Shale, Gzhelian (Late Pennsylvanian). A. SUI 141042 (TT0314); Sedan 17. B. SUI 141043 (TT0311); Sedan 24. C. SUI 141044 (TT0251); Sedan 11. D. SUI 141045 (TT0222); Clinton 12. E. SUI 141046 (TT0251); Sedan 11. F. SUI 141047 (TT0235); Sedan 11. G. SUI 141048 (TT0256); Sedan 11. H. SUI 141049 (TT0095); I229-83. I. SUI 141050 (TT0093); Clinton 21. J . SUI 141051 (TT0094); I229-83. K. SUI 141052 (TT0221); Clinton 12. L. SUI 141053 (TT0169); Sedan 23. M. SUI 141054 (TT0167); Sedan 23.