Girtyoceras meslerianum ( Girty, 1909 )

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 : 88

Girtyoceras meslerianum ( Girty, 1909) ( Figs. 6 and 7 )

1909 Adelphoceras meslerianum . – Girty, p. 66, pl. 12, figs. 1–3c.

1918 Girtyoceras meslerianum . – Wedekind, p. 140.

1964 Girtyoceras meslerianum . – McCaleb et al., p. 13 (part), pl. 1, figs. 4–6.

1965 Girtyoceras meslerianum . – Gordon (part), pl. 24, Figs. 17–22, 30–32, text-figs. 63A, B, 64D.

Holotype: Specimen USNM 119594 ; illustrated by Girty (1909, pl. 12, figs. 1–1c).

Type locality and horizon: USGS locality 2083, Sec. 14, T. 2 S., R. 7 E., northwest of Wapanucka, Johnston County, Oklahoma; Delaware Creek Member of Caney Shale, Goniatites multiliratus Biozone.

Material: Three immature specimens ( NPL 68364 through NPL 68366 ) from bed 25 (sample 00TXCU-25) .

Diagnosis: Species of Girtyoceras with a thickly discoidal, widely umbilicate conch at 6–8 mm diameter (ww / dm = 0.50–0.60; uw / dm = 0.45–0.55), and a thinly discoidal, narrowly umbilicate conch at 20 mm diameter (ww / dm = 0.45; uw / dm = 0.20). Whorl cross section strongly depressed in juveniles (5 mm diameter) with rounded trapezoidal whorl cross section, followed by an intermediate stage with rounded venter and angular umbilical margin (above 7 mm diameter), and a discoidal stage (above 20 mm diameter) with acute venter. Ornament with sharp riblets in the juvenile stage; coarse prorsiradiate growth lines with concavo-convex course in the intermediate stage and fine biconvex growth lines in the adult stage. Strong shell constrictions in intermediate growth stages.

Description: The three specimens are all mostly testiferous. No sutures are visible. The specimen that represents the largest diameter ( NPL 68366) is only a whorl fragment and poorly preserved. However, it represents a specimen that is clearly advanced over NPL 68364 (at least 12 mm in diameter) and shows the prominent tongue-shaped salient formed in the ribs and constrictions that are typical of this stage.

The better preserved of the two larger specimens ( NPL 68364) is nearly 10 mm in diameter ( Fig. 6 ). By 8 mm diameter the umbilical margin forms a strong right angle and the ribs and constrictions (which number four per whorl, are moderately deep and visible on both the internal moulds and external shell) are very strongly deflected anteriorly. The ribs and constrictions also form a distinct ventral sulcus at this stage. Faint longitudinal lines can be seen in the ventral area.

The smallest specimen ( NPL 68365) is transitioning from the widely umbilicate acutely marginated form to the almost parallel-sided thickly discoidal form with a 90 ◦ umbilical shoulder. The flanks and venter are gently rounded. Fine thread-like riblets originate from the umbilical margin and deflect anteriorly at the ventrolateral margin. The ribs are straight across the venter.

Discussion: Girtyoceras meslerianum is a rare taxon in the Sierra Diablo sections mostly because well-preserved three-dimensional material is difficult to obtain from the concretions that bear its assemblage. Although interpreted rather broadly in the past, in our view, G. meslerianum is a morphological intermediate between G. hamiltonense and the stratigraphically younger taxon Sulcogirtyoceras limatum . It bears the more compressed conch form, strongly developed anterior deflection of the ribs and constrictions on the ventrolateral shoulder, and wider umbilicus of the latter (uw / dm = 0.31), but has the weaker ornament and lack of ventrolateral sulcus of the former. Thus G. meslerianum might be confused with G. hamiltonense and small specimens (smaller than 12 mm) of S. limatum . However, G. meslerianum can be readily differentiated from G. hamiltonense because the latter has a consistently narrower umbilicus (uw / dm = 0.25 at 16 mm diameter). Early members of the genus Sulcogirtyoceras develop a prominent ventrolateral sulcus at approximately 12–15 mm diameter, but also have slightly stronger ribs and constrictions at comparable stages (less than 20 mm diameter). Smaller specimens of more advanced species of Sulcogirtyoceras have more depressed whorls.

Species that are stratigraphically older than G. hamiltonense in North America have less sinuous constrictions and narrower umbilici at comparable diameters (e.g. G. gordoni ), while younger Serpukhovian species (e.g. Girtyoceras jasperense Gordon ) are more narrowly umbilicate and more robust (higher-value ww / wh ratios). Girtyoceras meslerianum appears to be limited to the Goniatites multiliratus Biozone in both the mid-continent USA and the western Cordillera (Chainman Shale). It is rather uncommon outside of its type area, with the only other definitive occurrences we know of at San Saba, Texas in the base of the Barnett Formation and in the Chainman Shale at the classic Duckwater locality (not mentioned by Korn and Titus, 2011). It should occur in the Goniatites multiliratus Biozone of the upper Pool Creek Member of the Moorefield Formation, but specimens from this interval are all crushed ( Gordon, 1965).

Figures

Figure 6. Girtyoceras meslerianum (Girty, 1909) from the Figure 2 Ranch section, Sierra Diablo. Specimen NPL 68364 from bed 17 (sample 00TXCU-25); × 4.0.

Figure 7. Girtyoceras meslerianum (Girty, 1909), conch cross sections and ontogenetic trajectories of material from the Caney Shale of Jackfork Creek near Ada, Oklahoma (a–d). (a) Specimen MB.C.25466; × 2.5. (b) Specimen MB.C.25467; × 2.5. (c) Specimen MB.C.25468; × 2.5. (d) Specimen MB.C.25469; × 2.5. (e–g), ontogenetic development of the conch width index (ww / dm), umbilical width index (uw / dm), and WER of the sectioned specimens and type material.