Pachylyroceras cloudi ( Miller and Youngquist, 1948 )

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 : 101

Pachylyroceras cloudi ( Miller and Youngquist, 1948)

( Figs. 15a, b and 16c )

1909 Goniatites newsomi . – Girty, pl. 12, figs. 11, 11a.

1948 Lyrogoniatites cloudi Miller and Youngquist , p. 660, pl. 94, figs. 1–3.

1965 Neoglyphioceras cloudi . – Gordon, pl. 20, figs. 12–14, text-fig. 59D, E, J.

1965 Neoglyphioceras cloudi . – King, p. 42.

1971 Lyrogoniatites newsomi cloudi . – Furnish and Saunders, pl. 2, fig. 5.

2009b Pachylyroceras cloudi . – Kullmann, p. 65, text-fig. 38.5a–d.

Holotype: Specimen USNM 113011 ; illustrated by Miller and Youngquist (1948, pl. 94, figs. 1–3).

Type locality and horizon: 3.5 km south of San Saba, Texas (Chappel Hill locality); lower portion of Barnett Shale, probably Choctawites cumminsi Biozone.

Diagnosis: Species of Pachylyroceras with rounded umbilical shoulder and 23–30 spiral lines.

Material: A total of 22 specimens; 21 specimens from sample 00TXCU-27 ( NPL 68540 through NPL 68560 ) and 1 specimen from NMC 1/03-14 ( UTSA 07023 ). All of the specimens are from the same starved interval and are nearly all black to dark brown coloured phosphatic steinkerns with rare phosphatized shell remains preserved. All show various states of corrosion and dissolution .

Description: The earliest whorls are not preserved in the material, but in specimen NPL 68540 the conch has achieved a discoidal shape with moderately depressed whorls and a medium width umbilicus (ww / dm = 0.54; uw / dm = 0.30) by 21 mm diameter ( Fig. 15a ). The whorl cross section is slightly depressed (ww / wh = 1.41) with a broadly rounded umbilical shoulder and broadly rounded ventral region. The internal mould shows rather prominent constrictions, being rectiradiate at the umbilical margin, and bending rather sharply forward at the ventrolateral shoulder forming a pronounced ventral salient and a shallow ventral sinus. Longitudinal lirae are relatively coarse; 28 can be counted from umbilical shoulder to umbilical shoulder.

The suture line of specimen NPL 68542 (phragmocone whorl width of 13.2 mm, whorl height of 8.0 mm) shows a Vshaped external lobe with sinuous flanks and slightly asymmetric prongs. The median saddle has a height of one-third of the external lobe depth. On the flanks follow a bell-shaped ventrolateral saddle and a symmetric adventive lobe with slightly convex flanks ( Fig. 16c ).

Discussion: The Sierra Diablo material of Pachylyroceras appears to represent only a single species, with all specimens being very close to each other in ornament and morphology. They are also close, in their conch width/diameter and umbilical width ratios, to several of the paratypes in the type lot from San Saba. As has been pointed out by Gordon (1965), the holotype is atypical for the type material in that it possesses five fewer spiral lines and a much wider umbilicus, rather approaching the morphology of “ Neoglyphiceras hyatti Gordon, 1960 ” than the other types. However, the holotype is the largest specimen in the type lot by about 4 mm, and these differences may be the result of later ontogenetic changes or strong variation rather than of taxonomic value. Unfortunately, because the type lot was collected from ex situ nodules and has no detailed stratigraphic context, it is possible that the type lot is from a different horizon.

Occurrence: Ruzhencev and Bogoslovskaya (1971) reported the species from the South Urals, but we consider this assignment erroneous because the conch shape, suture, and ornament of the Uralian forms are fundamentally different from the type of P. cloudi . We propose the new name Uralyroceras arquatum (named after the arched course of the constrictions) for the Uralian species (see below). Pachylyroceras cloudi appears to be characteristic of the Choctawites cumminsi Biozone of the eastern USA and equivalent age strata in Nevada and Utah.

Figures

Figure 15. Pachylyroceras cloudi (Miller and Youngquist, 1948) from bed 25 (sample 00TXCU-27) of the Figure 2 Ranch section; both × 2.5. (a) Specimen NPL 68540. (b) Specimen NPL 68541.

Figure 16. Suture lines of representatives of Pachylyroceras from localities in Utah, Nevada, and Texas; all × 6.0. (a) Pachylyroceras utahensis (Miller et al., 1952), specimen NPL 68561 from Skunk Spring, Utah, at 11.6 mm diameter, 8.3 mm ww, 5.6 mm wh. (b) Pachylyroceras utahensis (Miller et al., 1952), specimen MB.C.25472 from Hamilton Canyon, Nevada, at 9.3 mm ww, 6.1 mm wh. (c) Pachylyroceras cloudi (Miller and Youngquist, 1948), specimen NPL 68542 from bed 25 of the Figure 2 Ranch, Texas, at 13.2 mm ww, 8.0 mm wh.