Myalinella acutirostris

Newell, Christopher A. And Norman D., 2005, Marine Myalinidae (Bivalvia: Pterioida) from the Permian of West Texas, American Museum Novitates 3469, pp. 1-16 : 10-14

Myalinella acutirostris

(Newell and Burma, in Roth et al., 1941) Figure 9d

Myalina squamosa Girty, 1908: 429 , pl. 16, fig. 22, non pl. 29, fig. 15 (= Myalina lamellosa ).

Myalina sp. Newell, 1940: 286 , pl. 2, fig. 1.

Myalina acutirostris Newell and Burma in Roth et al., 1941: 315, pl. 45, figs. 11–15.

Myalina acutirostris Newell et al., 1953 , pl. 22, fig. 14.

TYPE SPECIMEN: Holotype UWM 20846.

DESCRIPTION: Shell moderately small (height = 1.8 cm), triangular; hinge margin dorsal, slightly arched, 0.8 times as long as the greatest shell length and about equal to the shell height; umbo slightly behind ante­ rior­most margin, umbonal ridge rounded in early stages, becoming broad and poorly defined at later growth stages; retrocrescent with α slightly increasing from 42° to 60° during growth; β angle increasing from 35° to 72° during growth; shallow indentation or byssal sinus along anteroventral margin; anterior lobe small, poorly demarcated from main part of shell. Surface of left valve ornamented by six primary commarginal growth squamae, interspaced with numerous finer commarginal growth lines. Body cavity broad, extending deeply to anterior margin; ligament duplivincular, ligament area narrow, possessing two or three posterior deeply incised grooves essentially parallel to the hinge margin; musculature unknown. Shell thin, ultrastructure unknown.

REMARKS AND COMPARISONS: Appears to differ from other Myalinidae in its small size, triangular form, rather smooth surface, small anterior lobe demarcated by shallow sulcus, and straight anteroventral margin.

MATERIAL: Our only specimens from the Glass Mountains are two left valves.

AGE AND OCCURRENCE: Lower Cathedral Mountain Formation (Leonardian), locality USNM 721u; Getaway Member, Cherry Canyon Formation (Guadalupian), locality AMNH 512 (= USNM 728). Additional Guadalupian specimens assigned to this species are known from Quartermaster Formation of Texas (Newell and Burma in Roth et al., 1941) and the Whitehorse Sandstone of Oklahoma ( Newell, 1940).

Genus Novaculapermia McRoberts and Newell, 1997

TYPE SPECIES: Novaculapermia boydi by original designation ( McRoberts and Newell, 1997) .

Novaculapermia boydi McRoberts and Newell, 1997 Figure 9c

Novaculapermia boydi McRoberts and Newell, 1997: 488 , pl. 1, figs. 1–8, text­fig. 1.

TYPE SPECIMEN: Holotype USNM 487771 .

DESCRIPTION: Shells large (max. dimension about 12 cm), relatively flat, lacking umbonal ridge or keel; juvenile specimens retrocrescent becoming infracrescent at about 4 cm from umbo, with an elongated posterior margin and nearly parallel anteroventral and posterodorsal margins; slight anteroventral fold which likely served as a byssal sinus. Surface of both valves equally ornamented by numerous commarginal growth squamae that are somewhat unevenly spaced; lacking radial ornament. Body cavity narrow; umbonal septum absent. Ligament duplivincular, ligament area of left valve broad and flat, incised by eight parallel ligament grooves that are at a steep angle to lower margin of the hinge area, the anterior ends of the ligament grooves abruptly terminate along a steep diagonal groove radiating from the beak to the body cavity. In well­preserved specimens, clear bilobate pallial line extending along internal valve margin to a length of 7 cm in adult specimens, posterodorsal lobe the larger of the two; posterior adductor scar large (9 mm in diameter) and ovate, situated within posterodorsal pallial lobe. Shell moderately thick; ultrastructure unknown.

REMARKS AND COMPARISONS: This species is unlike any known to us, and it is unlikely that that it would be confused with other Permian Bivalvia.

PALEOAUTECOLOGY: Because none of the specimens were recovered in situ, the life orientation for Novaculapermia boydi remains in question. A hypothesized reconstruction in which the bivalves were semi­infaunal with their sagittal plane oriented vertically was illustrated by McRoberts and Newell (1997). This interpretation is further corroborated by the discovery of additional specimens exhibiting encrusting serpulid(?) tubes beginning about 4 cm from the beak and extending to the posterior shell terminus.

MATERIAL: The collection consists of more than 30 valves including several articulated valve pairs.

AGE AND OCCURRENCE: Cathedral Mountain Formation (Leonardian), localities USNM 702, 702un, AMNH 500; Road Canyon Formation (Guadalupian) locality USNM 703c; Word Formation (Guadalupi­ an), locality 706c.

Genus Elversella McRoberts and Newell, 2001

TYPE SPECIES: Elversella rugosa by original designation ( McRoberts and Newell, 2001) .

Elversella rugosa McRoberts and Newell, 2001 Figure 9a,b

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

Elversella rugosa McRoberts and Newell, 2001: 2 , fig. 2.

TYPE SPECIMEN: Holotype USNM 431325 .

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°. 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; both valves bear a small anterodorsal auricle above a rounded sinus and byssal gape. The specimens contain a poorly preserved but simple and continuous pallial line roughly parallel to the posteroventral margin. Further details of the musculature and shell microstructure are unknown due to poor preservation.

REMARKS AND COMPARISONS: Elversella rugosa appears similar in outline and ornamentation to one of the several specimens Girty (1908: 29, fig. 15) attributed to Myalina squamosa Sowerby from the Permian of the Glass Mountains and may therefore be conspecific. However, other Permian specimens attributed by Girty to M. squamosa (e.g., Girty, 1908: 16, fig. 22) bear distinctively different ornamentation and lack an anterior auricle and therefore clearly represent a different species.

MATERIAL: More than 40 specimens, including many articulated valve pairs and nearly equal numbers of left and right valves.

AGE AND OCCURRENCE: Uddenites ­bearing Shale Member, Gaptank Formation (Wolfcampian), locality USNM 701e; Neal Ranch Formation (Wolfcampian) localities USNM 701, USNM 701a, USNM 701c, USNM 701d, USNM 701k; Taylor Ranch Member, Hess Formation (Leonardian), localities USNM 702d, USNM 702e; Cathedral Mountain Formation (Leonardian), localities USNM 702, USNM 702a; Road Canyon Formation (Guadalupian) locality USNM 702c.