Elversella rugosa McRoberts and Newell, 2001

Elversella rugosa McRoberts and Newell ,

new species

Figure 2 View Fig

Myalina squamosa?, Girty, 1908: 429 , pl. 29, fig. 15, not Myalina squamosa Sowerby , which is a Devonian form.

ETYMOLOGY: Specific name refers to rugose ornamentation on the left valve.

DIAGNOSIS: As for the genus.

DESCRIPTION: The valves are moderately small (maximum dimension generally less than 5 cm). In profile, the beaks are conspicuous and extended forward above a broad and shallow anterior sinus. The umbonal ridge, which is poorly defined in later growth stages, curves down and backward at the margins, forming an angle of less than 45° with the hinge at the rounded posteroventral extremity. The specimens bear five or six duplivincular ligament grooves that are slightly curved and intersect the hinge margin at an angle slightly less than 30° (figs. 2.1, 2.5). The left valve bears as many as 15 coarse commarginal rugae, whereas the right valve is less convex and nearly smooth. Between the coarse rugae of the left valve are numerous fine commarginal growth lines. The right valve margin below the hinge lies well within the edge of left valve; each valve bears a small posterodorsal auricle above a rounded sinus and byssal gape (figs. 2.1, 2.3A). The specimens contain a poorly preserved, but simple and continuous pallial line roughly parallel to the posteroventral margin (fig. 2.5). Further details of the musculature and shell microstructure are unknown due to poor preservation.

MATERIAL AND MEASUREMENTS: Our specimens are few and incomplete around the margins due to predepositional wear and breakage. The inequality in valve size suggests that some dissolution of the outer shell layer of the right valve, which may have been composed of conchiolin, occurred prior to silicification. The measurements ( table 1 View TABLE 1 ) of the five specimens shown in figure 2 indicate variability and approximate size for the species.

DISCUSSION: This myalinid species is unlike any known to us. Elversella rugosa appears similar in outline and ornamentation to one of the several specimens Girty (1908: pl. 29, fig. 15) attributed to Myalina squamosa Sowerby from the Permian of the Glass Mts. and may therefore be conspecific. However, other Permian specimens attributed by Girty to M. squamosa (e.g, Girty, 1908: pl. 16, fig. 22) bear distinctively different ornamentation and lack an anterior auricle; therefore, they clearly represent a different species.

Elversella rugosa superficially resembles the equivalved Septimyalina burmai Newell , (1942: pl. 12, fig. 4) from the Florena Shale, (Wolfcampian); however, the new species lacks the umbonal septum of that form. E. rugosa is similar to several Aviculomyalina species known from Lower and Middle Triassic localities in Europe and North America (e.g., Assmann, 1937; McLearn, 1941). Unlike Elversella rugosa , however, Aviculomyalina is equivalved and possesses a smaller umbonal angle. Considering its inequivalved condition, we interpret E. rugosa to have been pleurothetic, and epibyssate, resting on the right valve.

RANGE AND DISTRIBUTION: Lower and Middle Permian of West Texas. Specimens are from U.S. Geological Survey locality 2930, Middle Permian, Pine Spring, Guadalupe Mts., West Texas, University of Kansas locality 21 (‘‘Leonardian’’), and the following USNM localities (Cooper and Grant, 1972):

702c Road Canyon Formation

702, 702a Cathedral Mountain Formation

701, 701a, c, d, k Neal Ranch Formation

702d, 702e Hess Formation, Taylor Ranch Member

701e Gaptank Formation, Uddenites ­bearing shale member