Goniatites eganensis Korn and Titus, 2011

Titus, A. L., Korn, D., Harrell, J. E. & Lambert, L. L., 2015, Late Viséan (late Mississippian) ammonoids from the Barnett Shale, Sierra Diablo Escarpment, Culberson County, Texas, USA, Fossil Record 18 (2), pp. 81-104 : 90

Goniatites eganensis Korn and Titus, 2011 ( Fig. 8 )

2011 Goniatites eganensis Korn and Titus , p. 175, text-figs. 51–54 [for more synonymy].

Holotype: Specimen UMNH IP 4021 ; illustrated by Korn and Titus (2011, fig. 51B).

Type locality and horizon: Trough Springs Canyon, bed 6 (sample 93NVLI-3); Camp Canyon Member of Chainman Formation, Goniatites eganensis Biozone.

Material: 127 specimens with conch diameters up to 48 mm from samples 00TXCU-20a–00TXCU-20f; 00TXCU-21a, and 00TXCU-21b.

Diagnosis: Goniatites with a globular conch between 2 and 8 mm diameter (ww / dm 0.85–0.90) and thickly pachyconic conch (ww / dm 0.75–0.80) at 20 mm diameter. Umbilicus moderate in early ontogeny (uw / dm = 0.20–0.30 at 2 mm diameter) and very narrow in all stages larger than 4 mm diameter (uw / dm 0.02–0.10). Umbilical wall convexly rounded in all stages. Aperture low or moderately high ( WER = 1.70–1.80 at 20 mm diameter). Ornament with crenulated, slightly biconvex, and rectiradiate growth lines; external sinus shallow. Suture line at 25 mm conch diameter with narrow external lobe (0.55 of the external lobe depth; 1.25 of the ventrolateral saddle width), and moderately low median saddle (0.40 of the external lobe depth). External lobe V-shaped with narrow, V-shaped prongs; ventrolateral saddle subangular.

Description: Four specimens are selected to outline the morphology of the material from the Sierra Diablo. Unfortunately, inner whorls are largely recrystallized and thus do not contain solid information about the conch ontogeny.

Specimen NPL 68431 is a fully chambered specimen of 47 mm conch diameter; it is representative for a larger growth stage ( Fig. 8a ). The pachyconic conch (ww / dm = 0.67; uw / dm = 0.06) is very narrowly umbilicate and possesses a broadly rounded, wide venter and a steep umbilical wall. Parts of the specimen are covered with shell remains. These show fine and crenulated growth lines, which extend with weakly biconvex course across flanks and venter, forming equally high dorsolateral and ventrolateral projections and shallow lateral and ventral sinuses.

The smaller specimen NPL 68471 (38 mm dm) shows very similar conch proportions but differs in the slightly coarser, irregularly spaced growth lines ( Fig. 8b ). Specimens NPL 68493 (27 mm dm) and NPL 68389 (18.4 mm dm) are thickly pachyconic conchs with stronger depressed whorl cross section ( Fig. 8c, d ). The larger of the two possesses well-preserved shell ornament with biconvex, strongly crenulated growth lines. These form a shallow lateral sinus and a very shallow ventral sinus. The smaller of the two possesses weakly crenulated growth lines standing in wide distances.

Figures

Figure 8. Goniatites eganensis Korn and Titus, 2011 from the Figure 2 Ranch section, Sierra Diablo. (a) Specimen NPL 68431 from bed 9 (sample OOTXCU-21b); × 1.0. (b) Specimen NPL 68471 from bed 9 (sample OOTXCU-21b); × 1.25. (c) Specimen NPL 68493 from bed 11 (sample OOTXCU-22); × 1.75. (d) Specimen NPL 68389 from bed 9 (sample OOTXCU-21a); × 2.5.