Girtyoceras hamiltonense Korn and Titus 2011

Girtyoceras hamiltonense Korn and Titus, 2011 (Fig. 5)

1949 Girtyoceras aff. G. meslerianum. – Miller et al., p. 608, pl. 99, figs. 9, 10.

1964 Girtyoceras meslerianum. – McCaleb et al., p. 13 (part), pl. 2, figs. 8, 9.

2011 Girtyoceras hamiltonense Korn and Titus, p. 150, text-figs. 30, 31 [for more synonymy].

Holotype: Specimen UMNH IP 3127; illustrated by Korn and Titus (2011, fig. 30B).

Type locality and horizon: Hamilton Canyon, bed 44; Camp Canyon Member of Chainman Formation, Goniatites eganensis Biozone.

Material: Four specimens, NPL 68361 (sample OOTXCU-21q), NPL 68362 (sample OOTXCU-21b), NPL 68363 (sample OOTXCU-21a), UTSA 07062.

Diagnosis: For a complete diagnosis see Korn and Titus (2011).

Description: The largest specimen, nearly 22 mm conch diameter (Table 1), is almost fully ornamented and has a thinly discoidal, subinvolute conch shape (ww / dm = 0.42; uw / dm = 0.18). At the largest diameter, it possesses an angular umbilical margin, subparallel flanks, and a narrowly rounded venter. The fine growth lines are strongly biconvex with a prominent ventrolateral projection and a deep ventral sinus. The last volution displays four shell constrictions extending parallel to the growth lines. They originate just outside the umbilical margin on the inner flank area; they are deepest on the mid-flank but continue across the venter (Fig. 5).

The next smaller specimen (NPL 68362) has a conch diameter of nearly 15 mm with similar conch proportions (ww / dm = 0.45; uw / dm = 0.25). It is a partly testiferous internal mould with predepositional breakage to the living chamber. There are four constrictions, which are, as in all specimens, visible externally. The fine rib density is about 12 per millimetre.

The smallest specimen (UTSA 07062) is approximately 10 mm in diameter, thickly discoidal, and broken obliquely, but nearly in half, with the other half missing. The umbilical shoulders are angular and about 90 ◦. The umbilical width index is entirely within a narrow range. Ribs and constrictions are moderately sinuous, and just start to develop a ventral sinus at about 8 mm diameter. There are three constrictions.

Discussion: G. hamiltonense is the stratigraphically oldest Girtyoceras species currently known from the Sierra Diablo section. It resembles G. gordoni Korn and Titus, 2011, G. welleri Gordon, 1965, and G. meslerianum (Girty, 1909). However, G. gordoni has less sinuous ribs and constrictions at comparable diameters, while G. welleri has a slightly narrower umbilicus between 6 and 20 mm conch diameter and a more narrowly rounded ventral profile at diameters between 6 and 15 mm. G. hamiltonense is now known from Nevada and Utah (Chainman Shale) and the Sierra Diablo (bed 9). The near time-equivalent species G. welleri occurs in the lower portion of Moorefield Formation near Batesville, Arkansas, and the Caney Shale of the Arbuckle Mountains region. In every case, these eastern occurrences are the oldest known horizons with a Goniatites assemblage at their respective localities.