Benedetto—The strophomenide brachiopod Ahtiella in Gondwana
Remarks.—This species clearly differs from Ahtiella lirata in its more transverse shell outline and less convex, uniformly curved shell profile lacking abrupt geniculation. Moreover in A. lirata, the dorsal sulcus is very shallow and confined to the posterior region of the valve and the corresponding ventral fold is inconspicuous, whereas in A. argentina, the sulcus is deeper and always attains the anterior margin, and the ventral fold is well defined and carinate posteriorly. Internally, the Precordilleran species can be distinguished by having smaller subtriangular ventral muscle field, which in the type species tends to be sub- quadrate and extends to approximately one-third of the valve length; there are deeply impressed vascular trunks on the inter- nal surface of both valves, but especially in the ventral one. The slightly older species A. baltica, from the Rögo Sandstone of Estonia, resembles A. argentina in its more transverse shell
outline but differs in having a longer bilobed ventral muscle field of lirata-type. The dorsal interior, even though exhibiting features of the genus, shows a quite atypical continuous, ante- riorly free dorsal platform (Öpik, 1933, pl. 4, fig. 6). Ahtiella arenaria Öpik, 1933 is a poorly known Estonian species (only a dorsal exterior and a ventral interior have been illustrated, by Öpik, 1933, pl. 4, figs. 7, 8) recovered from the same beds and localities as A. baltica to which it strongly resembles. Of the species from Sweden (Island of Öland) described by Hessland (1949), only A. jaanussoni is known from dorsal interiors (ventral interiors are unknown). Its dorsal valve is reminiscent of A. lirata, but the shell is somewhat more transverse, the posterolateral rugae are more pronounced, and the dorsal inter- ior possesses a long prominent median ridge. As Cocks and Rong (1989) stated, the rest of the Ahtiella species described by Hessland (1949) can only be questionably assigned to the genus because internal morphology remains unknown. Among them, A.? oelandica Hessland, 1949 is the most similar to A. argentina in its transverse shell outline, uniformly curved dorsal valve profile, and especially in the presence of a well-developed dorsal median sulcus reaching the anterior margin. The main external difference lies in the coarser and more prominent rugae in the Swedish species. It is unclear whether such differences in size, shell outline, definition of disc and trail, and strength of pos- terolateral rugae are taxonomically significant because these features show gradation between species and are quite variable even in individuals of the same species. Clearly, a revision of the Baltic species is needed to clarify this issue. Ahtiella concava from the Bob Deiniol Formation of
Anglesey (Wales) resembles A. argentina in its moderately convex and uniformly curved dorsal valve profile, acute cardinal angles, and well-developed, carinate ventral fold, but differs from the Precordilleran species in having a catacline to procline ventral interarea and a less transverse shell; the width/length
785
ratio in the specimens measured by Bates (1968, p. 168) is 0.63. According to Bates (1968, p. 167), the ornamentation in the Welsh specimens is “too fine to be observed,” and posterolateral rugae are not evident. Internally, A. concava has a subquadrate rather than subtriangular ventral muscle field. The dorsal valve shares with A. argentina a high rounded median dorsal ridge extending beyond the valve midlength, but in the Welsh species, the muscle field is strongly elongated and bounded by a pair of conspicuous ridges, a feature atypical of the genus. Ahtiella quadrata from the Torllwyn Formation of Anglesey is represented by fragmentary material, but judging from the two illustrated complete ventral valves (Bates, 1968, pl. 8, figs. 2, 3), the shell is slightly wider than long and coarsely costellate (~6 costellae per 2mm) than A. argentina. Ahtiella zarelae from the San José Formation of Peru can
readily be distinguished from A. argentina by its less transverse, nonauriculate shell, and its ramicostellate radial ornamentation. Internally, the Peruvian species possesses a subpentagonal rather than triangular ventral muscle field, and a higher, septum- like dorsal median ridge. In addition, the inner surface of both valves is almost entirely covered by radial ribs lacking vascular impressions. Most similar to the Precordilleran species is Ahtiella
paucirugosa from the volcaniclastic Summerford Group of Newfoundland. Given that the North American specimens are tectonically distorted, shell outline is difficult to compare (A. paucirugosa seems to be approximately twice as wide as long). However, they have in common such external features as a well-defined dorsal sinus and ventral fold, and moderate shell convexity, although posterolateral rugae are less marked in the North American species. Internally, the dorsal valve resembles that of A. argentina in having a broad median ridge almost reaching the margin, rows of elongate pustules and septules forming radial crests, and deeply marked mantle canals of the saccate type. The main difference lies in the ventral muscle field of A. paucirugosa, which is bilobed with longer diductor scars extending approximately to the valve midlength.
Ahtiella tunaensis new species
urn:lsid:
zoobank.org:act:C9CF5CB8-E169-4777-8C0B- ADEF34A52181 Figure 10.1–10.8
2003a Ahtiella n. sp.; Benedetto, p. 201, pl. 9, figs. 16–18.
Type specimens.—Holotype, CEGH-UNC 21083, a ventral valve. Paratypes: CEGH-UNC 27171, an internal mold of a dorsal valve; CEGH-UNC 21084 and 27172, two conjoined specimens.
Figure 10. (1−8) Ahtiella tunaensis n. sp.; Las Chacritas Formation, Sierra de la Trampa, Precordillera: (1 −3, 5) paratype, conjoined specimen, CEGH-UNC 21084, in ventral (1), dorsal (2), and profile (3) views, and detail of interarea (5) showing pseudodeltidium and chilidium; (4) holotype, interior of ventral valve, CEGH-UNC 21083; (6) paratype, ventral view of conjoined specimen, CEGH-UNC 27172; (7) detail of ventral valve, CEGH-UNC 27174, showing pseudopunctae; (8) paratype, dorsal internal mold, CEGH-UNC 27171. (9−24) Ahtiella famatiniana n. sp.; Punta Pétrea Member of the ‘Suri’ Formation (Chaschuil), and volcaniclastic beds exoposed at Las Pircas anticline (Central Famatina Range): (9) paratype, latex cast of ventral valve exterior, CEGH-UNC 27149; (10, 11) external mold (10) and latex cast (11) of ventral valve, CEGH-UNC 27131; (12− 14) ventral (12) and dorsal (13) views of conjoined specimen, CEGH-UNC 27163, and detail of pseudodeltidium (14); (15) juvenile ventral internal mold, CEGH-UNC 27158; (16, 17) paratype, internal mold (16) and latex cast (17) of ventral valve, CEGH-UNC 27135a; (18, 21) holotype, internal mold (18) and latex cast (21) of ventral valve, CEGH-UNC 27137; (19, 20) internal mold (19) and latex cast (20) of dorsal valve, CEGH-UNC 27135b; (22) internal mold of ventral valve, CEGH-UNC 27140; (23, 24) internal mold (23) and latex cast (24) of dorsal valve, CEGH-UNC 27141, showing detail of cardinalium. All specimens dusted with ammonium chloride. Scale bars=1mm (7); 3mm (5, 14); 5mm (remainder).
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