Choctawites cumminsi ( Hyatt, 1893 )

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 : 96-100

Choctawites cumminsi ( Hyatt, 1893) ( Figs. 11a, b and 12 )

1893 Goniatites cumminsi Hyatt , p. 467, pl. 47, figs. 33–43.

1965 Goniatites choctawensis . – Gordon, pl. 19, figs. 1–12, text-fig. 44E, F [only].

Lectotype: Specimen UT 12031 (designated by Cloud and Barnes, 1948); illustrated by Gordon (1965, pl. 19, figs. 1– 6).

Type locality and horizon: 5 miles west of Lampasas (Lampasas County, Texas); Barnett Shale .

Diagnosis: Choctawites with a thickly pachyconic to globular conch between 2 and 12 mm diameter (ww / dm = 0.85– 0.95) and thickly pachyconic conch (ww / dm = 0.70) at 20 mm diameter. Umbilicus moderately narrow to moderately wide in early ontogeny (uw / dm = 0.25–0.35 at 1.5– 2 mm diameter) and very narrow in all stages larger than 4 mm diameter (uw / dm 0.05–0.08). Umbilical wall convexly rounded in all stages, very short early juvenile stage with slightly trapezoidal whorl cross section. Aperture low ( WER = 1.65–1.70 throughout ontogeny). Ornament with about 120 spiral lines and crenulated, slightly biconvex, and rectiradiate growth lines; external sinus shallow. Falcate ornament begins at 40 mm dm.

Material: 43 specimens ( NPL 68497 through NPL 68539 ), all from sample 00TXCU-27. The material consists of brown to black coloured phosphatic steinkerns, which have varying amounts of shell preserved on them. Shell occasionally exhibits iridescent colour in small areas. Most of the inner phragmocones (diameter less than 10 mm) were not filled with minerals and weathered out rapidly or were not preserved, leaving a hollow space in the specimen. Some of these small interior phragmocones are exceptionally well preserved when freshly exposed and show the fine details of the septa and shell. The largest intact specimens were about 20 mm in diameter. Only fragments of larger specimens were collected .

Description: The conch cross section of the small specimen NPL 68499 (10 mm conch diameter) shows a globular shape (ww / dm = 0.86) and a very narrow umbilicus (uw / dm = 0.06) with broadly rounded flanks and venter ( Fig. 12e ). The inner whorls show crescent-shaped whorl profiles and a moderately wide umbilicus; the highest uw / dm ratio with 0.34 occurs at 1.2 mm conch diameter. In the growth interval between 5 and 10 mm conch diameter, the conch is widest at some distance from the umbilicus.

The conch cross section from the Sierra Diablo specimen is very similar to four cross sections (specimens 26-13, 26- 2, 26b, 26-10) produced by R. Kant (Tübingen) of material from San Saba (original specimens identified by M. Gordon in the USNM; acetate peels in the collections of the GPI Tübingen). The San Saba topotypes only differ in the slightly more pronounced trapezoidal whorl profile and the more subparallel flanks ( Fig. 12a–d ).

Conchs are thickly pachyconic at 20 mm (e.g. specimen NPL 68497; Fig. 11a ), with a very narrow umbilicus and moderately depressed whorls (ww / dm = 0.74; ww / wh = 1.36). Flanks and venter are continuously rounded and the aperture is low ( WER = 1.64). The specimen is an internal mould without shell remains; the body chamber has nearly the length of a volution. The internal mould shows shallow constrictions with irregular distances; they extend linearly across the flanks and turn forward for a low and wide ventral projection.

The slightly smaller specimen NPL 68497 (17 mm dm) largely resembles the previous specimen. The internal mould has four prominent constrictions, which probably follow the growth lines in their course, are almost rectiradiate, bearing only a slight forward inflection or are salient over the ventral area ( Fig. 11b ).

The smallest specimens (e.g. specimen NPL 68501) are 7 mm in diameter. By that stage, the conch is starting to change from sub-trapezoidal (strongly recurved flanks) to rounded whorl profiles. As a result, the trapezoidal phase, which is only weakly developed in this taxon, was only seen on cross-sectioned conchs.

Specimen NPL 68500 shows the suture line of a specimen in the intermediate growth stage (at 14.4 mm dm). It possesses a Y-shaped external lobe with sinuous flanks. The external lobe has, measured at half depth, 0.64 of the external lobe depth and is 1.25 times wider than the ventrolateral saddle. The median saddle reaches a height of 0.42 of the external lobe depth. The ventrolateral saddle is tectiform and subacute ( Fig. 12f ).

Ornament is only commonly preserved on the larger whorl fragments, but smaller specimens show crenistriate ornament up to diameters of approximately 15 mm, after which it changes to strong longitudinal lirae numbering about 19– 20 per 5 mm.

Discussion: Forms we refer to Choctawites have historically been attributed to three species; “ Goniatites choctawensis Shumard, 1863 ”, “ Goniatites cumminsi Hyatt, 1893 ”, and “ Goniatites kentuckiensis Miller, 1889 ”. Gordon (1965) regarded G. cumminsi as a junior synonym of G. choctawensis , a view supported in nearly every subsequent paper. Recently, Work and Mason (2009a) synonymized G. kentuckiensis with G. choctawensis and followed Ruzhencev and Bogoslovskaya (1971) in assigning G. choctawensis to Dombarites , which left G. choctawensis as the only valid species in this group. After reviewing the literature and numerous specimens, we have concluded that all three established species of the “trapezoidal” group ( G. kentuckiensis , G. choctawensis , and G. cumminsi ) are valid and represent a stratigraphic and possibly evolutionary succession.

The stratigraphically oldest species is Choctawites kentuckiensis , which is associated with Sulcogirtyoceras limatum and can be differentiated from the other species because it possesses a moderately well-developed trapezoidal shape ( Fig. 13a, b ) and late development of falcate ornament (typically appearing at diameters greater than 50 mm). This taxon was referred to as Dombarites choctawensis by Work and Mason (2009a). However, type specimens of Choctawites choctawensis consistently develop falcations by 40–45 mm diameter. Also, the early ontogenies of Ch. choctawensis and Ch. kentuckiensis are similar, but the former has a much stronger expression of the trapezoidal shape. These two criteria can be used to readily differentiate between the two species. There is an apparent facies difference in the occurrence pattern of the two species, with Choctawites kentuckiensis being known from shallower water deposits including a carbonate platform and pro-deltaic mudstone facies in the Appalachian and Ouachita foreland basins ( Gordon, 1965; Work and Mason, 2009a), and Ch. choctawensis occurring in the deeper water sequences of the Caney and Barnett shales. The exclusion of the latter from shallow water facies may have resulted from the sea level fall recorded by the Batesville Sandstone wedge in Arkansas.

The stratigraphically next species we can recognize in succession is the type species, Ch. choctawensis , which has the unique combination of early appearance of falcations and a strongly developed trapezoidal phase ( Fig. 14a–c ). Ch. choctawensis is associated with a more advanced species of Sulcogirtyoceras , S. . cf. S. ornatissimum . It is known widely from the Caney Shale of Oklahoma, e.g. specimen USNM 119505 from USGS locality 2078A ( Gordon, 1965, fig. 44G, H) and specimen Kant71 (Elias Collection) from 6 miles SE of Ada along Highway 99 (Pontotoc County, Oklahoma), the Barnett Shale of central Texas (specimen Kant26-1), and tentatively, the Batesville Sandstone of Arkansas ( Gordon, 1965).

The stratigraphically youngest species, Ch. cumminsi , is a form whose distinct, but reduced, trapezoidal phase ( Fig. 12a–e ), more narrowly umbilicate early whorls, and early appearance of falcations are highly definitive. The species can still be differentiated from the other species of Choctawites using the brief or even absent trapezoidal stage. Ch. cumminsi is known mostly from the Barnett Shale of Texas, but almost certainly occurs in the Caney Shale of Oklahoma and Arkansas (e.g. specimen USNM 119502; Gordon 1965, fig. 44e, f). This species is associated with Sulcogirytoceras ornatissimum ( Miller and Youngquist, 1948) and species of Pachylyroceras , which together comprise one of the youngest Viséan ammonoid assemblages we can recognize in the eastern USA. Since the Sierra Diablo specimens all exhibit the characteristic reduced trapezoidal stage, we refer to them as Ch. cumminsi .

Figures

Figure 11. Choctawites cumminsi (Hyatt, 1893) from bed 25 (sample 00TXCU-27) of the Figure 2 Ranch section; both × 2.5. (a) Specimen NPL 68497. (b) Specimen NPL 68498.

Figure 12. Choctawites cumminsi (Hyatt, 1893) from San Saba (a–d) and the Figure 2 Ranch section (e, f). (a) Cross section of specimen 26- 13 from San Saba; × 2.5. (b) Cross section of specimen 26-2 from San Saba; × 2.5. (c) Cross section of specimen 26b from San Saba; × 2.5. (d) Cross section of specimen 26-10 from San Saba; × 2.5. (e) Cross section of specimen NPL 68499 from bed 25 (sample 00TXCU27) of Figure 2 Ranch; × 2.5. (f) Suture line of specimen NPL 68500 from bed 25 (sample 00TXCU27) of Figure 2 Ranch, at 14.4 mm diameter, 11.3 mm ww, 7.2 mm wh; × 5.0. (g–i), Ontogenetic development of the conch width index (ww / dm), umbilical width index (uw / dm), and WER of all available specimens (the cross sections of specimens (a–d) from San Saba were produced by R. Kant; the original specimens are stored in the collections of the USNM, acetate peels are stored in the collections of the GPI Tübingen).

Figure 13. Choctawites kentuckiensis (Miller, 1889). (a) Cross section of specimen 57a from Moorefield, Arkansas; × 2.5. (b) Cross section of specimen MB.C.25470 from Moorefield, Arkansas; × 2.5. (c) Suture line of specimen MB.C.25471 from Moorefield, Arkansas, at 14.1 mm diameter, 10.2 mm ww, 6.6 mm wh; × 5.0. (d–f) Ontogenetic development of the conch width index (ww / dm), umbilical width index (uw / dm), and WER of all available specimens (cross section (a) was produced by R. Kant; the original specimen is stored in the collections of the University of Iowa, acetate peels are stored in the collections of the GPI Tübingen).

Figure 14. Choctawites choctawensis (Shumard, 1863). (a) Cross section of topotype USNM 119504 from Brushy Creek, Oklahoma; × 2.5 (from Gordon, 1965). (b) Cross section of specimen 71 from Ada, Oklahoma; × 2.5. (c) Cross section of specimen 26-1 from San Saba; × 2.5. (d–f), Ontogenetic development of the conch width index (ww/ dm), umbilical width index (uw / dm), and whorl expansion rate (WER) of all available specimens (the cross sections of specimens (b) and (c) from Ada and San Saba were produced by R. Kant; the original specimens is stored in the collections of the USNM, acetate peels are stored in the collections of the GPI Tübingen).