Benedetto—The strophomenide brachiopod Ahtiella in Gondwana
Holotype.—CORD-PZ 8283, a conjoined specimen from the upper San Juan Formation, Quebrada Los Gatos, Cerro Viejo, San Juan Province, Argentina (Benedetto and Herrera, 1986).
Diagnosis.—Shell transversely semielliptical (mean length/ width ratio 0.39) with acute cardinal extremities; profile variably resupinate with carinate ventral median fold and corresponding subangular dorsal sulcus reaching anterior margin. Radial ornamentation unequally parvicostellate. Ventral muscle field short, triangular, extending up to 25% of valve length. Ventral valve strongly thickened marginally, defining internal disc and trail. Dorsal valve with strong rounded median ridge originating in front of notothyrial platform. Internal surface with elongated pustules coalescing to form irregular, roughly radial ridges. Both valves with deeply impressed saccate mantle canal system.
Occurrence.—Uppermost levels of the San Juan Formation, Precordillera, San Juan Province, Argentina. Darriwilian (Dw1– Dw2).
Description.—Shell to 23mmwidth, transversely semielliptical in outline; mean length/width ratio 0.39 (N=18); profile strongly concavoconvex, mature specimens up to 45% as deep as long. Maximum width at hinge line. Cardinal extremities auriculate or alate in juvenile to medium-sized specimens, becoming acute (40–45°) in mature specimens. Ventral valve resupinate, gently convex at umbonal area, becoming broadly concave anteriorly; ventral median fold initially carinate, becoming broader and rounded toward anterior margin where it attains 15–20% of valve width, flanked by strongly concave areas. Ventral interarea planar, steeply apsacline to almost catacline, attaining height equivalent to 15% of shell length. Pseudodeltidium well developed, semiconical, covering near two-thirds of delthyrium. Dorsal valve moderately to strongly convex with deep angular median sulcus becoming wide and shallow anteriorly. Dorsal interarea anacline, planar, three to four times lower than ventral. Notothyrium covered almost entirely by large convex chilidium. Ornamentation unequally parvicostellate, with 6–8 first-order rounded costellae originat- ing at umbonal area and 4–8 subequal finer parvicostellae intercalated between them, some of which can attain size equal to primaries on anterior third of valve; costellae 10–12 per 2mm
(counted at 5mmgrowth stage). Fine, evenly spaced, concentric fila (~18 per mm). Posterolateral margins with up to six faint rugae strongly oblique to hinge line. Ventral interior with small subtriangular teeth bearing well-
developed, slit-like crural fossettes on medial faces, supported by short thick dental plates slightly converging to valve floor and diverging from each other at 80–90°. Lateral delthyrial
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cavities of adult specimens partially to completely filled with secondary deposits. Ventral muscle field short, triangular in outline, extending ~ 20–25% of valve length, enclosed poster- olaterally by dental plates, delimited anterolaterally by low arcuate ridges; diductor scars elongate, almost three times longer than wide, not clearly differentiated from wider subtriangular adductor field. Mantle canal system saccate, deeply impressed in mature individuals; vascula media broad, straight, diverging forward at 60°, extending from anterior ends of diductors to approximate valve midlength where they bifurcate, with one branch directed posterolaterally and the other directed medially, each becoming repeatedly dichoto- mized at disc margin (Fig. 9.10, 9.18). Surface between vascular trunks covered by pustules. Gonadal areas subtriangular to suboval in outline, faintly striated radially, variably pitted. Valve margin strongly thickened, geniculated, with well- defined disc and trail deflected dorsally, forming angle of ~ 40° with respect to disc surface. Low rounded to subangular subperipheral rim occasionally present along entire disc margin. External ribbing not reflected on valve interior except in juvenile individuals. Dorsal interior with small cardinalia extending ~10% of
valve length. Cardinal process high, bladelike, slightly enlarged anteriorly, erected on moderately elevated slightly convex notothyrial platform merged anteriorly with strong, rounded median ridge, usually extending to valve midlegth or slightly beyond, but reaching disc margin in some specimens. Socket ridges rodlike, free distally, diverging from each other at 95– 100°, bounding deep semiconical sockets excavated below and partially on interarea. Muscle field faintly impressed, quad- ripartite, with subcircular posterior adductor scars and larger suboval anterior adductor scars. Saccate dorsal mantle canal system with broad, well-impressed vascula media divided immediately in front of anterior extremity of muscle field in two branches, one of them running subparallel to median ridge, the other directed anterolaterally, both originating numerous small canals perpendicular to valve margin. Valve interior with elongated papillae, in large specimens coalescing to form irregular roughly radial ridges, which are more conspicuous at valve midlength on both sides and at end of median ridge. Small, closely spaced papillae between peripheral vascular trunks.
Materials.—In addition to the material listed and illustrated by Benedetto and Herrera (1986), the following new specimens have been included in the present study: CEGH-UNC 21960– 21965, 21981, and 21994–21995 from Cerro Viejo (Quebrada Honda); CEGH-UNC 10561–10564, 16319–16321, 22070– 22075, 27104–27130, and 27160–27162 from Cerro Viejo (Quebrada Los Gato).
Figure 9. Ahtiella argentina Benedetto and Herrera, 1986; Upper San Juan Formation, Cerro Viejo, Precordillera: (1) latex cast of ventral valve exterior, CEGH-UNC 27121; (2, 7) latex cast of dorsal valve exterior (2) and profile view (7), CEGH-UNC 27119; (3) posterior view of conjoined specimen showing presudodeltidium and chilidium, CEGH-UNC 16328; (4) early juvenile ventral internal mold, CEGH-UNC 27118; (5) juvenile ventral internal mold, CEGH- UNC 27153; (6) internal mold of small adult ventral valve, CEGH-UNC 27072; (8, 9) internal mold (8) and latex cast (9) of ventral valve, CEGH-UNC 27111; (10) internal mold of ventral valve, CEGH-UNC 27181; (11, 12) external (11) and internal (12) views of silicified ventral valve, CEGH-UNC 21963; (13, 14) ventral (13) and dorsal (14) views of silicified conjoined specimen, CEGH-UNC 21964; (15) internal mold of juvenile dorsal valve, CEGH-UNC 27160; (16, 17) internal mold (16) and latex cast (17) of dorsal valve, CEGH-UNC 27108; (18) internal mold of ventral valve, CEGH-UNC 21995; (19, 20) internal mold (19) and latex cast (20) of dorsal valve, CORD-PZ 8295; (21, 22) internal mold (21) and latex cast (22) of dorsal valve, CEGH-UNC 27075; (23) dorsal internal mold, CEGH-UNC 21118b. All specimens dusted with ammonium chloride. Scale bars=3mm (3, 4); 5mm (remainder).
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