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Journal of Paleontology 92(5):768–793


with orthoids a simple, not undercut cardinal process, the absence of side septa, and the absence of a bema, a structure interpreted as a lophophore support and/or for providing muscle attachment. A more elaborated trifid, often undercut cardinal process, a variably elevated and bilobed bema, paired dorsal valve septa, hinge-line denticles, and more or less prominent internal papillae and/or septules are all features that gradually appear in different combinations in younger, more derived plectambonitoids. Among taffiids, the ahtiellins—exemplified by the type


genus Ahtiella—display such a combination of internal features that, in absence of pseudopunctae, they are virtually indis- tinguishable from certain basal orthoids (e.g., hesperonomiids). For instance, the inclusion among the ahtiellins of Schedophyla potteri, whose shell structure remains unknown, was cast in doubt by Cocks and Rong (1989, p. 97) who stated that “it is possible that the genus is an orthid.” Laurie (1991) also noted the orthoid dorsal cardinalia and musculature of Schedophyla. Recently Harper et al. (2017, p. 624) noted that “…questions remain regarding the placement of a number of groups such as the toquimiids, that apparently possess orthoid characters.” Perhaps the best example of such a difficulty is the conflicting taxonomic position of ‘Monorthis’ coloradoensis, from the Central Andean Basin, which is hardly differentiable from unquestionable species of Ahtiella such as A. zarelae Villas in Gutiérrez-Marco and Villas, 2007 and A. famatiniana n. sp. (described herein). Originally, the species ‘coloradoensis’ was ascribed to the hesperonomiid Monorthis, on the basis of its convexoplanar, slightly resupinate profile, carinate ventral fold, multicostellate ornamentation, and orthoid cardinalia and mus- cle scars (Benedetto, 1998b). Such an assignment was further supported by the lack of evidence of pseudopunctae and pseu- dodeltidium (Benedetto, 2003b), both considered apomorphic features of plectambonitoids. The shell structure, however, could not be verified because available shells are entirely dec- alcified, so that its nonpunctate condition was inferred from the lack of evidence of pseudopunctae on internal molds, even though many undisputable strophomenoids do not show internal traces of them.


Comparative morphology of Monorthis and Ahtiella


The orthoid ancestor hypothesis of Ahtiella finds empirical support from the comparison of the species Monorthis trans- versa (Fig. 6.1–6.12) and Ahtiella famatiniana n. sp. (Fig. 10.9– 10.24), which occur in successive strata of the Famatina Basin.


Shell shape and ornament.—Overall, the slightly resupinate, carinate, transversely elongate, alate shells of Monorthis trans- versa and Ahtiella famatiniana n. sp. are closely comparable. Shells of M. transversa are rather smaller and more transverse than A. famatiniana, resembling the juvenile to medium-sized specimens of the latter. Although ornamentation in M. transversa is slightly coarser than in A. famatiniana n. sp., it is subequally multicostellate in both species.


Pseudodeltidium.—Specimens of Monorthis transversa from Chaschuil—originally referred to as Monorthis aff.M. menapiae (Davidson, 1868) by Benedetto, 1994—and from Central


Famatina (Benedetto, 2003b) were described as having awidely open delthyrium. However, a slightly arched pseudodeltidium can be observed in a few conjoined shells (Fig. 6.1). Its absence in most specimens is likely due to postmortem breakage by hydrodynamic action, but could also reflect ecophenotypical variation within a population, ranging from absent to nearly complete according to specific (but unknown) environmental constraints.Acareful revision of the Precordilleran specimens of M. cumillangoensis Benedetto, 2001, perhaps the better-known species of Monorthis worldwide (Benedetto, 2001), revealed the presence of an apparently imperforated pseudodeltidium cover- ing the apical region of the delthyrium (Fig. 6.15). A closely comparable structure is also present in Ahtiella coloradoensis (Fig. 11.9). Ahtiella zarelae possesses a small apical pseudo- deltidium that is hardly visible in the illustrated specimens (Gutiérrez-Marco and Villas, 2007, fig. 4E, F), but in Ahtiella sp. fromslightly older strata of the same formation of Peru, it is better developed, almost attaining the delthyrium midlength. Ahtiella famatiniana n. sp. always possesses a well-developed pseudo- deltidium covering one-half to two-thirds of the delthyrium(Fig. 10.9, 10.14). In the younger species A. argentina (Fig. 9.3) and A. tunaensis n. sp. (Fig. 10.5), the pseudodeltidium covers the pedicle opening almost entirely at all growth stages, suggesting a general trend from nearly absent to fully developed.


Muscle scars.—The ventral muscle field of Monorthis trans- versa is subtriangular and confined to the delthyrial cavity (Fig. 6.7) as in younger individuals of Ahtiella famatiniana n. sp. (Fig. 10.15). In mature specimens of the latter species (Fig. 10.16, 10.18), as well as in A. coloradoensis (Fig. 11.3, 11.8) and the Peruvian species A. zarelae (Fig. 11.18), the ventral muscle field becomes larger and more or less sub- pentagonal in outline. The Precordilleran A. tunaensis n. sp. is unique in having a large bilobed muscle field (Fig. 10.4). The dorsal muscle field of M. transversa is quadripartite, with anterior and posterior scars nearly equal in size. A persistent feature in most specimens of M. transversa is the presence of a pair of slightly divergent ridges bounding laterally or bisecting longitudinally the adductor field (Fig. 6.4, 6.5). Remarkably, these ridges are also present in A. famatiniana n. sp., A. color- adoensis, and A. zarelae (Figs. 10.23, 11.14). In the Welsh species A. concava, such ridges extend along the entire length of muscle scars (Bates, 1968, pl. 7, figs. 16, 19).


Cardinalia.—There are only minor differences between the cardinalia of Monorthis (Fig. 6.5, 6.12) and Ahtiella (Figs. 9.17, 10.20, 10.24, 11.10, 11.15). Monorthis transversa shows some degree of intraspecific variation in the cardinal process, ranging from bladelike to a ridge moderately enlarged anteriorly, occu- pying the entire length of a raised subtriangular notothyrial platform. The cardinalia of M. transversa are nearly identical to those of A. famatiniana n. sp. and A. coloradoensis, whereas in the Peruvian A. zarelae and Ahtiella sp., the cardinal process tends to be more robust and ovoid in outline. Ahtiella argentina is characterized by a bladelike to slightly enlarged cardinal process erected on a gently convex notothyrial platform (Fig. 9.20, 9.22). The subtriangular to suboval, anterolaterally open sockets excavated on the valve floor and partially under the


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