marine myalinidae (bivalvia: pterioida) from the permian...

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Number 3469, 15 pp., 9 figures, 3 tables March 24, 2005

Marine Myalinidae (Bivalvia: Pterioida) from thePermian of West Texas

CHRISTOPHER A. McROBERTS1 AND NORMAN D. NEWELL2

ABSTRACT

Marine bivalves of the family Myalinidae are an important benthic constituent of the Perm-ian Reef Complex of West Texas and New Mexico. We describe and summarize the myalinidsfrom Lower and Middle Permian reef and near-reef settings and infer living habits as eitherepifaunal or semi-infaunal byssally attached suspension feeders. The six myalinid speciesdescribed are exceptionally preserved as silica pseudomorphs. Included in the fauna are twonew taxa: Myalina lamellosa, a species with distinctive commarginal lamellae, and Myalinaplicata, the only known myalinid with prominent radial plicae. The family Myalinidae isplaced in the Ambonychioidea (Order Pterioida) and an emended diagnosis incorporates lig-ament characters and details of shell ultrastructure.

INTRODUCTION

In terms of taxonomic richness and mor-phologic innovation, the marine Myalinidaeis one of the most successful of Late Paleo-zoic–Early Mesozoic bivalve families. Al-though myalinids are known from as far backas the early Carboniferous, and perhaps evento the Devonian if Myalina squamosa Sow-erby is considered a true myalinid, they didnot reach their glory until the Permian,where richness exceeds 10 genera and 25

1 State University of New York at Cortland, P.O. Box 2000, Cortland, New York 13045 ([email protected]).2 Division of Paleontology (Invertebrates), American Museum of Natural History ([email protected]).

species. Following the end-Permian mass ex-tinction, the marine Myalinidae were greatlydiminished in taxonomic and ecological di-versity but persisted until possibly the Mid-dle Triassic, when the group became extinct.Their resilience, however, is underscored bythe fact that several myalinid species com-prise a significant component of the EarlyTriassic recovery fauna (e.g., Schubert andBottjer, 1995).

Marine myalinids are an abundant andconspicuous component in near-reef and lev-

2 NO. 3469AMERICAN MUSEUM NOVITATES

Fig. 1. Generalized Wolfcampian to Dzhulfian Permian stratigraphy of Glass Mountains region WestTexas. Modified from Rohr and others (2000).

el-bottom paleocommunities associated withthe spectacular Permian reef complex ofWest Texas and southern New Mexico. Al-though several myalinid taxa have been de-scribed from the Permian reef complex in re-cent years (McRoberts and Newell, 1997,2001), this contribution represents a morecomplete account of the taxonomic and mor-phological richness of the family from thePermian of West Texas. Furthermore, weprovide a revised account of the morphologyof the family, with a revised diagnosis whichwill complement Newell’s monograph on theMyalinidae published more than a half cen-tury ago (Newell, 1942).

MATERIAL AND COLLECTIONS

Most of the material described hereincomes from the Glass Mountains of WestTexas situated on the south margin of thePermian Delaware Basin. The bulk of thespecimens come from the Leonardian-Guad-alupian Cathedral Mountain, Road Canyonand Word Formations (fig. 1). They were col-lected and processed by G.A. Cooper as abyproduct of his work on Permian brachio-pods (e.g., Cooper and Grant, 1972). Morecomplete geographic and stratigraphic detailsof the listed U.S. Geological Survey localitynumbers can be found in Cooper and Grant(1972).

2005 3McROBERTS AND NEWELL: MARINE MYALINIDAE

Fig. 2. Generalized internal morphologic fea-tures. DL, duplivincular ligament grooves; BS,byssal sinus; PL, pallial line; PA, posterior ad-ductor insertion scar; AA, anterior adductor in-sertion scar.

The bivalve specimens are preserved assilica pseudomorphs which preserve the finedetails of the outer calcite shell layers andexternal surface ornamentation. Unfortunate-ly, the inner shell layers of the bivalves,which were presumably composed of ara-gonite and would exhibit muscle scars, weremost often not silicified and are typically notpreserved. A few of the better preservedspecimens, however, exhibit coarse silicifi-cation of the inner aragontic shell layers, andtherefore preserve interior adductor musclescars. A more complete description of theunique mode of preservation can be found inCooper and Grant (1972) and Newell et al.(1953).

Institutional repository abbreviations fortype and figured specimens and localities are:AMNH, American Museum of Natural His-tory; USNM, Museum of Natural History,Smithsonian Institution; KU, University ofKansas Paleontological Collections; UWM,Geological Museum of the University ofWisconsin, Madison.

MORPHOLOGY

The shell morphology of myalinid bi-valves has been the subject of much discus-sion which has led to greatly different viewson the structure and significance of manymorphologic features. Because descriptiveterms for myalinid morphology are mostlyavailable elsewhere, the reader is referred toNewell (1942) for a summary.

Although many morphologic features, in-cluding shell outlines, have proven useful indiscriminating myalinid genera and species,it is noteworthy that shape is often controlledby environmental factors such as the grainsize and stability of substrate, water chem-istry, and water temperature (e.g., Hickey,1987) and should not in itself be a singlecriterion for species definition. While rec-ognizing the value of shell shapes and sizesin discriminating between different popula-tions, our approach is to maximize the use ofdiscrete character traits in defining new taxa.A summary of internal features used in thedescriptions is provided in figure 2 and gen-eral conventions of measurement axes andorientation are shown in figure 3.

SYSTEMATIC PALEONTOLOGY

ORDER PTERIOIDA NEWELL, 1965

SUPERFAMILY AMBONYCHIOIDEA MILLER, 1877

FAMILY MYALINIDAE FRECH, 1891emended McRoberts and Newell

REVISED DIAGNOSIS: Mostly inequivalvedAmbonychiacea with right valve slightly lessconvex than left valve; inequilateral; eden-tulous or with cardinal tooth or boss beneathbeak of right valve and corresponding furrowin left valve; pallial line entire, generally pit-ted; anisomyarian; ligament duplivincularcontinuous, not extending to calcitic shelllayer; inner shell layer in both valves nacre-ous aragonite; outer shell layers either pris-matic calcite in both valves or prismatic cal-cite in right valve and homogeneous calciteor homogeneous calcite with mosaic struc-ture in outer shell layer in left valve.

REMARKS: Newell (1942: 44) revised thefamily as: ‘‘Shell inequivalve, the right valveslightly less convex and slightly smaller atthe margin than the left; beaks at or near thesmall and projecting anterior end; posteriormargin sub-ovate, quadrate, or extended in aposterodorsal auricle; ligament external, du-plivincular, mainly opisthodetic, but withamphidetic remnant before the beaks in some


Fig. 3. Measurement axis conventions and orientations for extreme forms a and b. H, height; L,length; HL, hinge length; LG, number of ligament grooves; MGV, maximum growth vector (umbonalridge); PAV, posterior auricle vector; a, angle subtended by the posterio-ventral terminus of maximumgrowth vector and hinge line; b, angle subtended by hinge line and posterior margin; g, angle subtendedby PAV and MGV.

genera; hinge edentulous or with weak car-dinal teeth 1/2a, 2b at the front end of hinge;inner shell layer lamellar, probably aragonite;outer layer composed of calcite, either ho-mogeneous or finely prismatic, the prismsand optic axes nearly normal to the shell sur-face; musculature characteristically aniso-myarian, but monomyarian in a few species.The genera that unquestionably belong heredo not possess well-defined radial ornamen-tation, and all pass through a form stage inthe early ontogeny similar to adult Modiol-opsis.’’

Later, Newell (1969: N289) defined thefamily as: ‘‘Inequivalved, with RV slightlyless convex than LV; edentulous, or with car-dinal tooth or boss beneath beak of RV andcorresponding furrow in LV; pallial line en-tire, generally pitted.’’

To this discussion, Carter (1990: 201) add-ed that myalinids have a simple prismaticcalcite or both simple prismatic and homo-geneous calcite outer shell layer with the ho-mogeneous structure commonly restricted tothe left valve; the middle and inner shell lay-ers are nacreous and aragonitic prismatic.Carter (1990) also noted that the ligament inMyalinidae is duplivincular with one fibrous

sublayer per ligament ridge, and with eachfibrous sublayer initially spanning the twovalves.

There has been much discussion concern-ing the taxonomic content and systematicplacement of the Myalinidae within highercategories of the Bivalvia. Contrary to earlierworkers who have aligned them with thePterioidea, Newell (1942) was impressedwith the similarities, especially in muscula-ture, with the Mytiloidea. However, it waslater determined that similarities in ligamentindicate placement into the superfamily Am-bonychoidea (Newell, 1965; Pojeta, 1966).Furthermore, it is likely that based on simi-larities in ligament, the Alatoconchidae, agroup of aberrant giant clams, are closely re-lated to, or descended from, the Myalinidae(Yancey and Boyd, 1983).

Currently, the following genera and theirranges are here considered to be marine My-alinidae: Myalina (?Devonian, Carbonifer-ous–L. Triassic,? M. Triassic), Septimyalina(Carboniferous–L. Permian), Orthomyalina(U. Carboniferous–L. Permian), Myalinella(U. Carboniferous–L. Triassic), Arctomyali-na (U. Carboniferous), Elversella (M. Perm-ian), Pseudomyalina (L. Permian), Seleni-


TABLE 1Myalina lamellosa McRoberts and Newell, new species

(Measurements in millimeters; see fig. 3 caption for abbreviations.)

Specimen Valve H L HL MGV a b LG

USNM 431335a

USNM 431336USNM 431337a

USNM 431338a

USNM 431339

LVLVRVLVLV

17.321.316.813.710.5

22.126.319.716.511.5

18.319.917.217.211.6

23.728.123.118.813.6

4241363842

6963808592

4545

—USNM 431340USNM 431341a

USNM 431342a

USNM 431343a

USNM 431344a

LVLVLVLVLV

19.214.112.317.228.6

16.917.81523.330.5

22.416.314.719.128.9

28.421.517.125.235.2

3638373541

6779715297

—4

——

7USNM 431345a

USNM 431346a

USNM 431347USNM 431348a

USNM 431348a

LVLVRVLVRV

13.58.8

15.612.116.8

16.210.123.216.719.2

14.39.3

20.715.818.5

17.110.923.816.523.2

4349384144

8089

1028969

12————

USNM 431349a

USNM 431349a

USNM 431350a

USNM 431350a

LVRVLVRV

37.435.317.517.6

41.337.621.620.3

36.233.317.917.8

45.243.423.423.9

37363442

969297

103

———na

a Measurements taken on commarginal growth lamellae.

myalina (U. Carboniferous), Novaculaper-mia (L. Permian), Promyalina (L. Triassic),and less certainly, Liebea (U. Permian) andAviculomyalina (M. Triassic).

Genus Myalina De Koninck 1842

TYPE SPECIES: Myalina goldfussianaDeKoninck, 1842, subsequently designatedby Stoliczka, 1871.

Myalina lamellosa McRoberts and Newell,new species

Figure 4

TYPE SPECIMENS: Holotype: USNM-431335; Paratypes: USNM-431336, USNM-431344, USNM-431348.

DIAGNOSIS: Thin-shelled and triangular-shaped Myalina possessing distinct irregularcommarginal overlapping growth lamellae, abroad anteroventral fold in left valve and cor-responding sulcus in right valve, and lackinga distinct anterior auricle.

DESCRIPTION: Shell medium sized (max.height 5 3.7 cm), triangular in outline;hinge-line straight, less than total length ofshell; small posterior auricle present in most

specimens, especially in later growth stages;anterior margin slightly concave, indicatingsmall byssal sinus; posterior margin broadlyconvex, nearly parallel to anterior margin;broad fold in left valve along anteroventralmargin corresponds to broad sulcus in rightvalve; beaks terminal, arcuate, and projectinginwards, shell retrocrescent, with maximumgrowth vector a moderately steep (;658) atearly growth stages, becoming steeper(;758) at later growth stages (see fig. 5) butnever infracrescent; left valve slightly largerthan right. Surface of both valves ornament-ed by numerous commarginal overlappinggrowth lamellae which do not terminate inhemispherical spines. Lamellae becomingmore pronounced and at wider spaced inter-vals by curling up and outwards in latergrowth stages. Body cavity extending far intothe umbones, umbonal septum or deck ab-sent. Ligament opisthodetic, duplivincular;between 4 and 12 parallel grooves, slightlyacute, intersecting hinge-line at less than 108.Musculature characterized by moderate-sizedovate posterior adductor. Anterior adductoror other musculature unknown. Thin-shelled,


Fig. 4. Myalina lamellosa McRoberts and Newell, new species. Scale bar is 1 cm. a, holotype,USNM 431335 (USNM loc. 702), left valve interior (a1), exterior (a2); b, paratype, articulated valvepair, USNM 431348 (USNM loc. 702), right valve (b1), left valve (b2), dorsal view (b3); c, paratype,left valve interior USNM 431336 (USNM loc. 702), (c1), exterior (c2); d, paratype, USNM 431344(USNM loc. 702), left valve interior (d1), exterior (d2).


Fig. 5. Ontogenetic series showing increasing a angle in Myalina lamellosa (a) and M. plicata (b).

ultrastructure unknown. See table 1 for mea-surements.

REMARKS AND COMPARISONS: This speciesshows similarities to forms regarded byNewell (1942) as Septimyalina burmai New-ell 1942, yet differs from that species in theabsence of spinelike projections of growthlamellae and thickness of shell. Additionally,Myalina lamellosa lacks a prominent umbo-nal septum or deck which presumably occursin S. burmai although none is illustrated. Dif-fers from Myalina plicata, n.sp., in the pres-ence of weak posteroventral fold and sulcusand absence of radial plicae.

PALEOAUTECOLOGY: Several features leadto the conclusion that Myalina lamellosa wasbyssally attached in an edgewise positionresting on the anterior end (fig. 6a). Althoughthe two valves differ in their size, the equal-ity in the inflation of both valves and of com-marginal lamellae suggests the animal wasoriented with its commissure plane nearlyvertical and slightly resting on its left valve.

AGE AND OCCURRENCE: Cathedral Moun-tain Formation (Leonardian), localities:USNM-702, USNM-721-u; Road CanyonFormation (Guadalupian), localities:?USNM-721-j, USNM-721-t, USNM-721-z,USNM-724-b, USNM-726-d, USNM-726-z;Word Formation (Guadalupian), localities:

USNM-706-e. To these from the USNM, addGirty’s (1908, pl. 29, fig. 15) specimen fromDelaware Mountain Formation, southernDelaware Mountains (station 2935).

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

ETYMOLOGY: Specific name refers to prom-inent growth lamellae.

Myalina plicata McRoberts and Newell,new species

Figure 7

TYPE SPECIMENS: Holotype: KU-310505;Paratypes: USNM-431351, USNM-431353.

DIAGNOSIS: Medium to thin-shelled Myali-na possessing distinct commarginal growthsquamae, broad radial plicae, and distinctbyssal sinus.

DESCRIPTION: Shell moderately large (max.height 5 5.2 cm); triangular in outline;hinge-line straight, less than total length ofshell; small posterior auricle present in mostspecimens, especially in later growth stages;anterior margin slightly concave indicatingsmall byssal sinus; posterior margin broadlyconvex, nearly parallel to anterior margin;beaks terminal, arcuate, and projecting in-wards; shell retrocrescent with angle a about


Fig. 6. Hypothesized living habits of Permian Myalinidae. a, Myalina lamellosa McRoberts andNewell, n.sp.; b, Myalina plicata McRoberts and Newell, n.sp.; c, Myalina copei; d, Elversella rugosa;e, Novaculapermia boydi; f, Myalinella acutirostris.

408 in earlier growth stages, becoming nearlyinfracrescent in adult stages. Surface of bothvalves ornamented by numerous commargin-al growth lamellae. Lamellae becoming morepronounced and at wider spaced intervals bycurling up and outwards in late growth stag-es; strong radial plications in both valves,more pronounced on valve exterior, becom-ing fainter on posterodorsal margin. Bodycavity extending far into umbones; dentitionconsists of single linear tooth on right valveformed by sharp fold in cardinal area, fittinginto a narrow furrow in left valve. Ligamentopisthodetic, duplivincular; between 5 and10 grooves slightly acute to nearly parallel,

intersecting hinge-line at less than 108,grooves becoming fainter posteriorly along abroadening hinge plate. Pallial line faint butcontinuous, with a single large ovate raisedposterior adductor scar nearly 1 cm in di-ameter. Shell moderately thin; ultrastructureunknown. See table 2 for measurements.

REMARKS AND COMPARISONS: This beingthe only known Myalina with radial plicae,it is unlike any other species known to us. Inother characteristics, however, this speciesresembles, especially in early growth stages,Myalina lamellosa in its position of raisedgrowth squamae. However, M. plicata differsfrom M. lamellosa in having greater a angles


Fig. 7. Myalina plicata McRoberts and Newell, n.sp. Scale bar is 1 cm. a, holotype KU 310505(KU loc. 27), left valve interior (a1), exterior (a2); b, paratype, USNM 431351 (USNM loc. 706c), leftvalve interior (b1), exterior (b2); c, paratype, USNM 431353 (USNM loc. 706c), right valve exterior(c1), interior (c2).

and tending to become more infracrescent inlater growth stages.

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

AGE AND OCCURRENCE: Word Formation(Guadalupian), localities: USNM-706c,USNM-721j, ?USNM-706e, University ofKansas Loc. 27.

ETYMOLOGY: Specific name refers to prom-inent and radial plications.

Myalina copei Whitfield, 1902Figure 8

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


TABLE 2Myalina plicata McRoberts and Newell, new species


Specimen Valve H L HL MGV a b LG

KU 310505USNM 431351USNM 431352USNM 431353

LVLVLVRV

37.652.826.143

38.967.928.5a

42.1

32.94123.7a

38.6a

48.775.732.5a

53.2

72586882

74429281a

610

58

USNM 431354b

USNM 431355b

USNM 431356b

RVLVLV

18.327.128.1

25.63334.1

25.630.129.7

26.939.138.9

655961

908692

6nana

a Estimated from broken shell.b Measurements taken on commarginal growth lamellae.

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

TYPE SPECIMEN: Lectotype AMNH-8364/2designated by Newell (1942).

DESCRIPTION: Shell large (max. heightnearly 7 cm), retrocrescent becoming infra-crescent later in ontogeny. Posterior alationmoderately to strongly developed and rela-tively flattened; anterior lobe small devel-oped in both valves; byssal sinus formed bybroad invagination on anterior-ventral mar-gin. Shell surface relatively smooth exceptfor faint, evenly spaced commarginal growthlines in some specimens. Body cavity broad,extending deeply to anterior margin; liga-ment duplivincular, ligament area broad, pos-sessing numerous deeply incised grooves es-sentially parallel to the hinge margin or in-tersecting at a very low angle (, 58); mus-culature unknown. Shell ultrastructureunknown. See table 3 for measurements.

REMARKS AND COMPARISONS: The outline,angular dimensions, hinge features, and es-pecially the extended posterior auricle seemto agree with previously illustrated speci-mens of M. copei. Although Newell (1942)described Myalina copei as inequivalved asthe right valve was somewhat smaller andfitting within the left valve, we have no con-joined or disarticulated valve pairs from thesame individual to confirm this observation.Newell (1942) commented on the similarityin shape of Myalina copei and Myalina plio-petina Newell. Myalina copei can be distin-guished from M. pliopetina Newell in pos-sessing a somewhat thicker shell and havinga more upright form (greater angle a in adult

specimens. This difference in angle a maybe difficult to discern in immature or frag-mented specimens.

MATERIAL: The collection consists of fiveleft valves and two right valves.

AGE AND OCCURRENCE: Lower Hueco For-mation (Wolfcampian), locality AMNH 48.Although this Formation is not listed in fig-ure 1 as it is known from the Hueco Moun-tains near El Paso, Texas, according to Coo-per and Grant (1972), it is likely age-equiv-alent to either the lower part of the NealRanch Formation or the underlying ‘‘Uden-ites-bearing shale member’’ of the GaptankFormation. According to Newell (1942), M.copei is known from the Hueco Mountainsand other localities in Texas (including thetype specimens from Shackelford County) aswell as Kansas and Nebraska.

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

TYPE SPECIES: Myalina meeki Dunbar,1924, by original designation

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 (5 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.


Fig. 8. Myalina copei. Scale bar is 1 cm. Specimens from AMNH locality 48. a, AMNH 43310,left valve exterior (a1), left valve interior (a2); b, AMNH 43320, left valve interior; c, AMNH 433214,right valve interior.

TABLE 3Myalina copei Whitfield


Specimen Valve H L HL MGV PAV a b g LG

AMNH 43310AMNH 43312b

AMNH 43313AMNH 43314

LVLVLVRV

69.727.647.6a

51.4

62.1a

27.842.549a

62.1a

29.548.649.0a

78.943.359.0a

62.9

64.0a

28.959.146a

89848782

10798

11999a

83655132

5135

a Estimated from broken shell.b Measurements taken on commarginal growth lines.


Fig. 9. a,b. Elversella rugosa. a, right valve interior USNM 431322; b, articulated valve pair USNM431323, left valve (b1), right valve (b2); c, Novaculapermia boydi, holotype USNM 487771, left valveinterior (c1), exterior (c2); d, Myalinella acutirostris, USNM 431357 (USNM loc. 721-u), left valveinterior (d1), exterior (d2).

DESCRIPTION: Shell moderately small(height 5 1.8 cm), triangular; hinge margindorsal, slightly arched, 0.8 times as long asthe greatest shell length and about equal tothe shell height; umbo slightly behind ante-

rior-most margin, umbonal ridge rounded inearly stages, becoming broad and poorly de-fined at later growth stages; retrocrescentwith a slightly increasing from 428 to 608during growth; b angle increasing from 358


to 728 during growth; shallow indentation orbyssal sinus along anteroventral margin; an-terior lobe small, poorly demarcated frommain part of shell. Surface of left valve or-namented by six primary commarginalgrowth squamae, interspaced with numerousfiner commarginal growth lines. Body cavitybroad, extending deeply to anterior margin;ligament duplivincular, ligament area narrow,possessing two or three posterior deeply in-cised grooves essentially parallel to the hingemargin; musculature unknown. Shell thin, ul-trastructure unknown.

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

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

AGE AND OCCURRENCE: Lower CathedralMountain Formation (Leonardian), localityUSNM 721u; Getaway Member, CherryCanyon Formation (Guadalupian), localityAMNH 512 (5 USNM 728). AdditionalGuadalupian specimens assigned to this spe-cies are known from Quartermaster Forma-tion of Texas (Newell and Burma in Roth etal., 1941) and the Whitehorse Sandstone ofOklahoma (Newell, 1940).

Genus Novaculapermia McRoberts andNewell, 1997

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

Novaculapermia boydi McRoberts andNewell, 1997

Figure 9c

Novaculapermia boydi McRoberts and Newell,1997: 488, pl. 1, figs. 1–8, text-fig. 1.

TYPE SPECIMEN: Holotype USNM 487771.DESCRIPTION: Shells large (max. dimension

about 12 cm), relatively flat, lacking umbo-nal ridge or keel; juvenile specimens retro-crescent becoming infracrescent at about 4cm from umbo, with an elongated posteriormargin and nearly parallel anteroventral andposterodorsal margins; slight anteroventralfold which likely served as a byssal sinus.Surface of both valves equally ornamented

by numerous commarginal growth squamaethat are somewhat unevenly spaced; lackingradial ornament. Body cavity narrow; um-bonal septum absent. Ligament duplivincular,ligament area of left valve broad and flat,incised by eight parallel ligament groovesthat are at a steep angle to lower margin ofthe hinge area, the anterior ends of the liga-ment grooves abruptly terminate along asteep diagonal groove radiating from thebeak to the body cavity. In well-preservedspecimens, clear bilobate pallial line extend-ing along internal valve margin to a lengthof 7 cm in adult specimens, posterodorsallobe the larger of the two; posterior adductorscar large (9 mm in diameter) and ovate, sit-uated within posterodorsal pallial lobe. Shellmoderately thick; ultrastructure unknown.

REMARKS AND COMPARISONS: This speciesis unlike any known to us, and it is unlikelythat that it would be confused with otherPermian Bivalvia.

PALEOAUTECOLOGY: Because none of thespecimens were recovered in situ, the life ori-entation for Novaculapermia boydi remainsin question. A hypothesized reconstruction inwhich the bivalves were semi-infaunal withtheir sagittal plane oriented vertically was il-lustrated by McRoberts and Newell (1997).This interpretation is further corroborated bythe discovery of additional specimens exhib-iting encrusting serpulid(?) tubes beginningabout 4 cm from the beak and extending tothe posterior shell terminus.

MATERIAL: The collection consists of morethan 30 valves including several articulatedvalve pairs.

AGE AND OCCURRENCE: Cathedral Moun-tain Formation (Leonardian), localitiesUSNM 702, 702un, AMNH 500; Road Can-yon Formation (Guadalupian) localityUSNM 703c; Word Formation (Guadalupi-an), locality 706c.

Genus Elversella McRoberts and Newell,2001

TYPE SPECIES: Elversella rugosa by origi-nal 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 aDevonian form.

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

TYPE SPECIMEN: Holotype USNM 431325.DESCRIPTION: The valves are moderately

small (maximum dimension generally lessthan 5 cm). In profile, the beaks are conspic-uous and extended forward above a broadand shallow anterior sinus. The umbonalridge, which is poorly defined in later growthstages, curves down and backward at themargins, forming an angle of less than 458with the hinge at the rounded posteroventralextremity. The specimens bear five or six du-plivincular ligament grooves that are slightlycurved and intersect the hinge margin at anangle slightly less than 308. The left valvebears as many as 15 coarse commarginal ru-gae, whereas the right valve is less convexand nearly smooth. Between the coarse rugaeof the left valve are numerous fine commar-ginal growth lines. The right valve marginbelow the hinge lies well within the edge ofleft valve; both valves bear a small antero-dorsal auricle above a rounded sinus and bys-sal gape. The specimens contain a poorlypreserved but simple and continuous pallialline roughly parallel to the posteroventralmargin. Further details of the musculatureand shell microstructure are unknown due topoor preservation.

REMARKS AND COMPARISONS: Elversellarugosa appears similar in outline and orna-mentation to one of the several specimensGirty (1908: 29, fig. 15) attributed to Myali-na squamosa Sowerby from the Permian ofthe Glass Mountains and may therefore beconspecific. However, other Permian speci-mens attributed by Girty to M. squamosa(e.g., Girty, 1908: 16, fig. 22) bear distinc-tively different ornamentation and lack ananterior auricle and therefore clearly repre-sent a different species.

MATERIAL: More than 40 specimens, in-cluding many articulated valve pairs andnearly equal numbers of left and right valves.

AGE AND OCCURRENCE: Uddenites-bearing

Shale Member, Gaptank Formation (Wolf-campian), locality USNM 701e; Neal RanchFormation (Wolfcampian) localities USNM701, USNM 701a, USNM 701c, USNM701d, USNM 701k; Taylor Ranch Member,Hess Formation (Leonardian), localitiesUSNM 702d, USNM 702e; Cathedral Moun-tain Formation (Leonardian), localitiesUSNM 702, USNM 702a; Road Canyon For-mation (Guadalupian) locality USNM 702c.

ACKNOWLEDGMENTS

We thank Gillian W. Newell and Jacque-line Scallan (Cortland) for assistance on var-ious aspects of this manuscript. Bushra M.Hussaini (AMNH), Jann Thompson(USNM), and Alice Hart (KU) assisted withcuration of specimens. Jason Belknap draftedfigure 6. Donald Boyd and Thomas Wallerprovided helpful reviews that improved themanuscript.

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Complete lists of all issues of the Novitates and the Bulletin are available at World Wide Website http://library.amnh.org/pubs. Inquire about ordering printed copies via e-mail [email protected] or via standard mail from: American Museum of Natural History, Library—Scientific Publications, Central Park West at 79th St., New York, NY 10024. TEL: (212) 769-5545. FAX: (212) 769-5009.

a This paper meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper).

marine myalinidae (bivalvia: pterioida) from the permian...

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