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Journal of Paleontology 92(5):768–793
dorsal interarea of Monorthis and Ahtiella, as well as the slen- der, distally enlarged socket ridges, are also closely comparable.
Subperipheral rims and platforms.—Despite the redefinition of these terms by Cocks and Rong (1989), some imprecision per- sists in the literature, with the term ‘platform’ having been applied to both dorsal and ventral valves (not only to the dorsal one, as these authors proposed), and the two structures are often not easy to differentiate on the basis of their morphology. To avoid confusion, ‘platform’ (= ‘diaphragm’) is used here to designate a low to high, somewhat undercut elevation of the ventral and/or dorsal valve floor originating at or near the car- dinal angles and not related to external geniculation, the internal disc, or any kind of valve thickening. On the other hand, fol- lowing Cocks and Rong (1989), the term ‘peripheral rim’ (or ‘subperipheral rim’) is applied to a raised rim running at or near the variably thickened valve margin. When a more or less pro- minent peripheral rim has developed in the ventral valve, it can be mirrored in the dorsal valve by a similar structure that is often related to an internal deflection of the valve. According to the original diagnosis of Monorthis (see
Bates, 1968), platforms are absent in both valves. Valve margins of the Famatinan M. transversa are crenulated but not thickened (Fig. 6.9), whereas the large ventral valves of the Precordilleran M. cumillangoensis show variably thickened margins and an internal geniculation, which is nearly identical to that seen in Ahtiella zarelae (cf. Figs. 6.14, 11.18). The ventral valves of A. famatiniana n. sp. and A. coloradoensis exhibit a conspic- uous thickening along the geniculation; in both species, however, it is absent in juvenile individuals (Fig. 10.15), suggesting that this structure developed progressively by peramorphy. In A. argentina, the whole ventral valve margin is geniculated, forming a prominent internal disc deeply incised by the vascula terminalia (Fig. 9.9), like in the Baltic A. lirata. A corresponding discontinuous platform-like structure is usually present in the dorsal valve of A. famatiniana n. sp. (Fig. 10.20). On the contrary, in A. coloradoensis, it is poorly developed or even absent (Fig. 11.10). Unlike other Gondwanan species, the large dorsal valves of A. argentina display a series of curved, roughly radial ridges that can be interpreted as a platform-like structure (Fig. 9.20, 9.22). Ahtiella baltica Öpik, 1932, as can be seen in the specimen figured by Öpik (1933, pl. 4, fig. 6), possesses a continuous undercut platform, but it is faint or absent in A. lirata.
Vascular system.—Vascular trunks are not discernible in Monorthis cumillangoensis in part due to the strong internal impression of the external ornamentation, whereas in M. trans- versa, a series of short anastomosing canals can be present along valve margins (Fig. 6.4, 6.8). In Ahtiella famatiniana n. sp., the
distal portion of mantle canals is well marked on the anterior third of the ventral valve (Fig. 10.22). Adult stages of A. argentina and A. paucirugosa always display a deeply impressed mantle canal system of saccate type, with posteriorly directed branches of vascula media enclosing large gonadal pouches (Fig. 9.10, 9.18).
Trends of morphological change.—From the above compar- isons, the following trends can be recognized through the inferred Monorthis transversa (and its putative ancestor Hesperonomiella arcuata Benedetto, 2003b)–Ahtiella argen- tina lineage (Fig. 7): (1) the nongeniculated ventral valve mar- gin of M. transversa progressively thickens, originating in Ahtiella an internal geniculation, which is low in the species from Wales (A. quadrata, A. concava) and northwestern Argentina (A. coloradoensis), intermediate in the Famatinan (A. famatiniana n. sp.) and Peruvian (A. zarelae) forms, and more prominent in the Darriwilian species from Cuyania and Baltica. In the latter (e.g., A. lirata, A. jaanussoni Hessland, 1949), the main trend is toward a strongly convex gibbous dorsal valve; (2) the pseudodeltidium is absent or incipient in Monorthis, is restricted to the apical region of delthyrium in the oldest known species (A. zarelae) as well as in the Welsh spe- cies, and reaches almost two-thirds in the later species from Precordillera and Baltica; (3) external ornamentation evolved from equally multicostellate in Monorthis (retained in the younger A. coloradoensis) to ramicostellate in the Floian A. zarelae, becoming unequally multicostellate to incipiently par- vicostellate in the Dapingian A. famatiniana, and definitely parvicostellate in the Darriwilian species; (4) the dorsal platform is absent in Monorthis, is variably developed in the early species A. zaleae and A. famatiniana n. sp., and becomes more promi- nent in the younger Cuyanian and Baltic species; (5) the mantle canal system is indistinct or confined to the valve margin in Monorthis, has well-impressed distal branches on the margin of disc and trail in the Floian-Dapingian species of Ahtiella, and culminates in A. argentina and A. paucirugosa with a deeply impressed mantle canal system on the entire surface of adult specimens.
Phylogenetic analysis
Cladistic analysis of Ahtiella species.—Comparative morphol- ogy makes evident that differences between Monorthis and basal species of Ahtiella are subtle, which makes them difficult to distinguish from each other. The question is whether such similarities reflect homologies and therefore reveal phylogenetic affinities or, on the contrary, they can be viewed as cases of extreme morphological convergence along two independent lineages. Given the striking resemblance in multiple external
Figure 6. (1−9) Monorthis transversa Benedetto, 2003b; Loma del Kilometro Member of the Suri Formation (Chaschuil) and Molles Formation, Famatina Range: (1) latex cast of ventral valve exterior, CEGH-UNC 19628a; (2) latex cast of dorsal valve exterior, CEGH-UNC 19628b; (3) latex cast of ventral valve, CEGH-UNC 19628a, showing incipient delthyrium cover; (4, 5) internal mold (4) and latex cast (5) of dorsal valve, CEGH-UNC 19635; (6) internal mold of dorsal valve, CEGH-UNC 19632; (7) internal mold of ventral valve, CEGH-UNC 19625; (8, 9) internal mold of ventral valve (8) and latex cast (9), CEGH-UNC 10962. (10−18) Monorthis cumillangoensis Benedetto, 2001; San Juan Formation, silicified specimens from Cerro Cumillango and Cerro La Chilca, Precordillera: (10) ventral valve exterior, CEGH-UNC 17915; (11) ventral valve exterior, CEGH-UNC 17917; (12) dorsal valve interior, CEGH-UNC 17920; (13) ventral valve interior, CEGH-UNC 17933; (14, 15) ventral valve interior (14) and detail of incipient delthyrium cover (15), CEGH-UNC 17948; (16) ventral valve interior, CEGH-UNC 21152; (17) dorsal valve exterior, CEGH-UNC 21153; (18) dorsal valve interior, CEGH-UNC 17942. All specimens dusted with ammonium chloride. Scale bars=5mm.
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