Myalina De Koninck 1842

Newell, Christopher A. And Norman D., 2005, Marine Myalinidae (Bivalvia: Pterioida) from the Permian of West Texas, American Museum Novitates 3469, pp. 1-16 : 5-10

Genus Myalina De Koninck 1842

TYPE SPECIES: Myalina goldfussiana DeKoninck, 1842 , subsequently designated by Stoliczka, 1871 .

Myalina lamellosa McRoberts and Newell , new species

Figure 4

TYPE SPECIMENS: Holotype: USNM­ 431335; Paratypes: USNM­431336, USNM­ 431344, USNM­431348.

DIAGNOSIS: Thin­shelled and triangularshaped Myalina possessing distinct irregular commarginal overlapping growth lamellae, a broad anteroventral fold in left valve and corresponding sulcus in right valve, and lacking a distinct anterior auricle.

DESCRIPTION: Shell medium sized (max. height = 3.7 cm), triangular in outline; hinge­line straight, less than total length of shell; small posterior auricle present in most specimens, especially in later growth stages; anterior margin slightly concave, indicating small byssal sinus; posterior margin broadly convex, nearly parallel to anterior margin; broad fold in left valve along anteroventral margin corresponds to broad sulcus in right valve; beaks terminal, arcuate, and projecting inwards, shell retrocrescent, with maximum growth vector α moderately steep (~65°) at early growth stages, becoming steeper (~75°) at later growth stages (see fig. 5) but never infracrescent; left valve slightly larger than right. Surface of both valves ornament­ ed by numerous commarginal overlapping growth lamellae which do not terminate in hemispherical spines. Lamellae becoming more pronounced and at wider spaced intervals by curling up and outwards in later growth stages. Body cavity extending far into the umbones, umbonal septum or deck absent. Ligament opisthodetic, duplivincular; between 4 and 12 parallel grooves, slightly acute, intersecting hinge­line at less than 10°. Musculature characterized by moderate­sized ovate posterior adductor. Anterior adductor or other musculature unknown. Thin­shelled, ultrastructure unknown. See table 1 for measurements.

REMARKS AND COMPARISONS: This species shows similarities to forms regarded by Newell (1942) as Septimyalina burmai Newell 1942 , yet differs from that species in the absence of spinelike projections of growth lamellae and thickness of shell. Additionally, Myalina lamellosa lacks a prominent umbonal septum or deck which presumably occurs in S. burmai although none is illustrated. Differs from Myalina plicata , n.sp., in the presence of weak posteroventral fold and sulcus and absence of radial plicae.

PALEOAUTECOLOGY: Several features lead to the conclusion that Myalina lamellosa was byssally attached in an edgewise position resting on the anterior end (fig. 6a). Although the two valves differ in their size, the equality in the inflation of both valves and of commarginal lamellae suggests the animal was oriented with its commissure plane nearly vertical and slightly resting on its left valve.

AGE AND OCCURRENCE: Cathedral Mountain Formation (Leonardian), localities: USNM­702, USNM­721­u; Road Canyon Formation (Guadalupian), localities:?USNM­721­j, USNM­721­t, USNM­721­z, USNM­724­b, USNM­726­d, USNM­726­z; Word Formation (Guadalupian), localities: USNM­706­e. To these from the USNM, add Girty’s (1908, pl. 29, fig. 15) specimen from Delaware Mountain Formation, southern Delaware Mountains (station 2935).

MATERIAL: The collection consists of 10 articulated valves, 23 left valves, and 8 right valves.

ETYMOLOGY: Specific name refers to prominent growth lamellae.

Myalina plicata McRoberts and Newell , new species Figure 7

TYPE SPECIMENS: Holotype: KU­310505; Paratypes: USNM­431351, USNM­431353.

DIAGNOSIS: Medium to thin­shelled Myalina possessing distinct commarginal growth squamae, broad radial plicae, and distinct byssal sinus.

DESCRIPTION: Shell moderately large (max. height = 5.2 cm); triangular in outline; hinge­line straight, less than total length of shell; small posterior auricle present in most specimens, especially in later growth stages; anterior margin slightly concave indicating small byssal sinus; posterior margin broadly convex, nearly parallel to anterior margin; beaks terminal, arcuate, and projecting inwards; shell retrocrescent with angle α about 40° in earlier growth stages, becoming nearly infracrescent in adult stages. Surface of both valves ornamented by numerous commarginal growth lamellae. Lamellae becoming more pronounced and at wider spaced intervals by curling up and outwards in late growth stages; strong radial plications in both valves, more pronounced on valve exterior, becoming fainter on posterodorsal margin. Body cavity extending far into umbones; dentition consists of single linear tooth on right valve formed by sharp fold in cardinal area, fitting into a narrow furrow in left valve. Ligament opisthodetic, duplivincular; between 5 and 10 grooves slightly acute to nearly parallel, intersecting hinge­line at less than 10°, grooves becoming fainter posteriorly along a broadening hinge plate. Pallial line faint but continuous, with a single large ovate raised posterior adductor scar nearly 1 cm in diameter. Shell moderately thin; ultrastructure unknown. See table 2 for measurements.

REMARKS AND COMPARISONS: This being the only known Myalina with radial plicae, it is unlike any other species known to us. In other characteristics, however, this species resembles, especially in early growth stages, Myalina lamellosa in its position of raised growth squamae. However, M. plicata differs from M. lamellosa in having greater α angles and tending to become more infracrescent in later growth stages.

MATERIAL: The collection consists of 10 left valves and 5 right valves.

AGE AND OCCURRENCE: Word Formation (Guadalupian), localities: USNM­706c, USNM­721j,?USNM­706e, University of Kansas Loc. 27.

ETYMOLOGY: Specific name refers to prominent and radial plications.

Myalina copei Whitfield, 1902

Figure 8

Myalina copei Whitfield, 1902: 63–66 , text fig. 2 (not fig. 1).

Myalina copei Whitfield, Newell, 1942: 55 , pl. 4, figs. 1a–c, pl.

TYPE SPECIMEN: Lectotype AMNH­8364/2 designated by Newell (1942).

DESCRIPTION: Shell large (max. height nearly 7 cm), retrocrescent becoming infracrescent later in ontogeny. Posterior alation moderately to strongly developed and relatively flattened; anterior lobe small developed in both valves; byssal sinus formed by broad invagination on anterior­ventral margin. Shell surface relatively smooth except for faint, evenly spaced commarginal growth lines in some specimens. Body cavity broad, extending deeply to anterior margin; ligament duplivincular, ligament area broad, possessing numerous deeply incised grooves essentially parallel to the hinge margin or intersecting at a very low angle (<5°); musculature unknown. Shell ultrastructure unknown. See table 3 for measurements.

REMARKS AND COMPARISONS: The outline, angular dimensions, hinge features, and especially the extended posterior auricle seem to agree with previously illustrated specimens of M. copei . Although Newell (1942) described Myalina copei as inequivalved as the right valve was somewhat smaller and fitting within the left valve, we have no conjoined or disarticulated valve pairs from the same individual to confirm this observation. Newell (1942) commented on the similarity in shape of Myalina copei and Myalina pliopetina Newell. Myalina copei can be distinguished from M. pliopetina Newell in possessing a somewhat thicker shell and having a more upright form (greater angle α in adult specimens. This difference in angle α may be difficult to discern in immature or fragmented specimens.

MATERIAL: The collection consists of five left valves and two right valves.

AGE AND OCCURRENCE: Lower Hueco Formation (Wolfcampian), locality AMNH 48. Although this Formation is not listed in figure 1 as it is known from the Hueco Mountains near El Paso, Texas, according to Cooper and Grant (1972), it is likely age­equivalent to either the lower part of the Neal Ranch Formation or the underlying ‘‘Udenites­bearing shale member’’ of the Gaptank Formation. According to Newell (1942), M. copei is known from the Hueco Mountains and other localities in Texas (including the type specimens from Shackelford County) as well as Kansas and Nebraska.

Genus Myalinella Newell, 1942 [nom. transl. Newell, 1969]

TYPE SPECIES: Myalina meeki Dunbar ,

1924, by original designation