788
Journal of Paleontology 92(5):768–793
Diagnosis.—Shell transversely semielliptical (mean length/ width ratio 0.48), alate to mucronate; ventral valve with strongly concave posterolateral areas and prominent carinate median fold; corresponding dorsal sulcus narrow and deep posteriorly, broader and wider at anterior margin. Ornamentation subequally multicostellate to incipiently parvicostellate, with primary costellae slightly more accentuated. Ventral muscle field almost pentagonal, extending to ~32% of valve length, with elongate diductor scars bounded laterally by curved ridges and shorter adductor field. Broad rounded dorsal median ridge fading anteriorly. Usually well-defined discontinuos dorsal platform. Vascular system deeply impressed only on disc margin and trail.
Occurrence.—Cerro Morado Group, Las Pircas anticline, cen- tral Famatina Range, La Rioja Province. Northern margin of Chaschuil River (Loma del Médano), Catamarca Province. Dapingian.
Description.—Shell transversely semielliptical, resupinate in lateral profile, up to 18mm in width (mean 13mm, N=26), mean length/width ratio 0.48 (N=20). Cardinal extremities alate, often projecting in rodlike mucrons (usually incompletely preserved). Ventral valve strongly concave with greatest con- cavity at posterolateral areas; carinate median fold stronger posteriorly, becoming wider and rounded anteriorly; anterior commissure broadly sulcate. Ventral interarea planar, procline or occasionally catacline, approximately twice as high as dor- sal. Delthyrium covered in apical two-thirds by semiconical pseudodeltidium. Dorsal valve moderately convex, 30–35% as deep as long, uniformly curved (nongeniculated) in lateral profile,withmedian sulcus initially narrow and deep, becoming broader and wider at anterior margin where it attains ~25% of valve width. Dorsal interarea planar, orthocline to steeply anacline, with triangular notothyrium covered posteriorly by convex chilidium. Radial ornamentation subequally multi- costellate, with rounded primary costellae often slightly more accentuated than second order ones giving ribbing a parvicos- tellate aspect; costellae increasing in number mainly by inter- polation in dorsal valve and by interpolation and dichotomy in ventral valve, typically 9 per 2mm (counted at 5 mm growth
stage). Ribs crossed by prominent, closely and evenly spaced fila (~20 permm) and by fewprominent growth discontinuities. Posterolateral rugae weakly defined or absent. Shell substance apparently impunctate. Ventral interior with strong triangular teeth supported by
short dental plates diverging at ~ 120°; umbonal cavities partially filled by secondary deposits. Muscle field proportion- ally large, subpentagonal in outline, generally wider than long, typically extending 32% of valve length (maximum 36%); diductor scars elongate subtriangular, enclosed laterally and anterolaterally by curved ridges originating in front of dental plates; adductor field shorter and more deeply impressed than diductors, suboval to subrectangular in outline, not enclosed anteriorly by diductors. Valve margin strongly thickened, defining internal disc and trail separated by low, rounded peripheral rim. Vascula media broad, weakly impressed, slightly divergent; distal region of vascular system coinsisting of numerous radially arranged distal branches deeply cutting disc margin and prolonged on trail; small papillae between vascular
trunks. External ribbing weakly impressed only on juvenile interiors. Gonadal areas large, occupying most posterolateral areas, sculptured by roughly radial anastomosing ridges. Dorsal interior with simple bladelike cardinal process
slightly enlarged anteriorly, erected on elevated subtriangular notothyrial platform. Socket ridges strong, slightly thickened and free distally, divergent anterolaterally at ~ 80º, bounding deep semiconical sockets partially excavated under dorsal interarea. Broad, rounded median ridge arising in front of notothyrial platform, fading anteriorly at intersection with peripheral rim. Muscle field weakly impressed, slightly wider than long, extending anteriorly for 30% of valve length, bounded laterally by low ridges; posterior adductor scars subcircular, smaller than suboval anterior pair. Dorsal mantle canal system indistinct or feebly impressed; vascula media broad, divergent, branching toward valve margin. Coarse ridge- like pustules or endospines coalescing to form well-defined discontinuous platform, which extends along entire valve margin except posterolateral extremities.
Etymology.—Named after one of the original localities, the Famatina Range.
Materials.—CEGH-UNC 27131–17159, 29 specimens from the volcanosedimentary unit exposed at Anticlinal Las Pircas, north of Cachiyuyo River, central Famatina Range, La Rioja Province; CEGH-UNC 27163–27167, five specimens from the Chaschuil River, north Famatina Range, Catamarca Province.
Remarks.—In its size and shell convexity, nearly uniform costellate ornamentation, high carinate ventral fold, and well- developed dorsal peripheral rim, this species strongly resembles A. zarelae from the upper Floian of Peru, but differs in having a proportionally larger ventral muscle field reaching approxi- mately one-third of the valve length; broader, rounded, non- septiform dorsal median ridge; and vascular trunks strongly impressed on the margin of the ventral disc and especially on the trail. The Peruvian Ahtiella sp., which is known only by its dorsal valve, can be distinguished from the Famatina species by its finer and uniform ribbing, higher and shorter median ridge, and more continuous, anteriorly excavated platform. Ahtiella argentina can readily be distinguished from A. famatiniana n. sp. by having a more transverse shell outline, unequally parvi-
costellate ornamentation, a smaller triangular ventral muscle field, and a strongly impressed vascular system. The ventral muscle field of A. famatiniana n. sp. is reminiscent of that of A. tunaensis n. sp., but in the latter it is more definitely bilobed. Moreover, the Precordilleran species differs in having sub- rectangular cardinal extremities, a less pronounced ventral median fold, and unequally parvicostellate ornamentation. Ahtiella lirata resembles the Famatinan species in its alate or mucronate cardinal extremities and its large subrectangular ventral muscle field, but can be distinguished by its unequally parvicostellate ornamentation, poorly defined (or absent) dorsal sulcus and corresponding ventral fold, and a strongly convex gibbous dorsal valve. Ahtiella paucirugosa externally resembles A. famatiniana n. sp. in having a well-developed dorsal sinus and ventral fold, moderate shell convexity, and almost indistinct posterolateral rugae, but differs internally in its elongate ventral
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