Benedetto—The strophomenide brachiopod Ahtiella in Gondwana
diductor scars extending to near the valve midlength, its deeply impressed mantle canals, and its high, septum-like dorsal med- ian ridge. As far as can be judged from the incomplete material, A. quadrata resembles A. famatiniana n. sp. in its shell outline and morphology of its ventral muscle field, but the Welsh spe- cies differs in its coarser radial ornamentation. Ahtiella concava clearly differs in having a strongly impressed and longer dorsal muscle field and impersistent (much finer? smooth?) ribbing.
Ahtiella coloradoensis (Benedetto, 1998b) Figure 11.1–11.15
?1980 Valcourea sp.; Havlíček and Branisa, p. 23, pl. 2, fig. 7.
1998b Monorthis coloradoensis Benedetto, p. 11, pl. 2, figs. 15–28.
2003a Monorthis coloradoensis; Benedetto, p. 211, pl. 24, figs. 5–9.
Holotype.—CEGH-UNC 13780, internal mold of ventral valve from the ‘Green Member of the Sepulturas Formation,’ north- west of Los Colorados village, Cordillera Oriental, Jujuy Pro- vince, Argentina.
Diagnosis.—Shell semielliptical, convexoplanar to gently con- vexoconcave (mean length/width ratio 0.63) with right-angled cardinal extremities in adult specimens; ventral valve almost planar with low median fold and well-defined corresponding dorsal sulcus. Ornamentation equally multicostellate, occa- sionally ramicostellate in ventral valve; ribs 10–12 per 2mm. Ventral muscle field subrectangular to subpentagonal, slightly wider than long, extending to ~31% of valve length, bounded by strong ridges in adult specimens; variably developed plat- form in dorsal valve. Vascular system indistinct excepting on periphery of each valve.
Occurrence.—‘Green Member of the Sepulturas Formation’ (sensu Astini et al., 2004a), Los Colorados area, western slope of Cordillera Oriental, Jujuy Province, Argentina. Middle-late? Darriwilian.
Description.—See Benedetto (1998b).
Materials.—In addition to the type material listed by Benedetto (1998b), new specimens CEGH-UNC 27187–27190 were col- lected from Quebrada Chamarra, northwest of Los Colorados village, Jujuy Province, Argentina.
Remarks.—The overall morphology of the Los Colorados spe- cies, in particular its convexoplane shell profile, carinate ventral fold, equally multicostellate ornamentation, acute cardinal extremities, and orthoid cardinalia led Benedetto (1998b) to refer it to the hesperonomiid genus Monorthis. This assignment was further supported by the lack of evidence of pseudopunctae and pseudodeltidium, both considered diagnostic features of the plectambonitoids. As discussed previously, however, the dif- ferences between Monorthis and the basal species of Ahtiella are subtle and exhibit transitional characteristics, so that assigne- ment to one or the other genus depends largely on the weight accorded to each character. The species M. coloradoensis is
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reassigned here to Ahtiella mainly because of the incipient internal geniculation in the ventral valve defining a low disc and a trail, the large subquadrate ventral muscle field bounded by widely splayed dental plates, and the presence of a faint dis- continuous platform in the dorsal valve. In addition, a revision of the type material and newly collected specimens of M. col- oradoensis revealed an incipient apical delthyrial cover, like some specimens of M. cumillangoensis (Figs. 6.15 and 11.9). Havlíček and Branisa (1980) referred to Valcourea sp. a
few ventral valves from supposedly Darriwilian sandstones exposed along the road Sucre-Potosí at Tambo Acachila, Bolivia. The single illustrated interior is nearly indistinguishable from Ahtiella coloradoensis but without dorsal valves and the lack of information of external ornamentation, assignment to this species must remain provisional. Ahtiella coloradoensis resembles A. famatiniana n. sp. in
its large subrectangular (sometimes subpentagonal) ventral muscle field extending anteriorly to near one-third of the valve length, its well-defined peripheral ventral thickerning, and its incipient dorsal platform. Ornamentation is multicostellate in both species but in A. coloradoensis it is more uniform tending to be ramicostellate in the ventral valve. Ahtiella coloradoensis mainly differs from the Famatinan species in its planar to gently concave ventral valve profile, its less transverse shell outline, its rectangular cardinal extremities, and its indistinct pseudodelti- dium. It can be distinguished from A. zarelae by its lower and rounded ventral fold, less pronounced geniculation, larger ventral muscle field, and the presence of a conspicuous peripheral rim in the ventral valve. In addition, internal ribbing in the Peruvian species is strongly marked.
Acknowledgments
I amespecially indebted tomy colleagues F.Dávila and R. Astini who collected material from the Famatina Range (Las Pircas anticline), and B. Waisfeld, who collected specimens of Ahtiella coloradoensis, making them available for this study. The helpful comments of the journal reviewer D.A.T. Harper and the Associate Editor R.-b Zhan greatly improved the original manu- script. The TNT program for phylogenetic analysis was made available with sponsorship of theWilli Hennig Society. Research has been financially supported by CONICET (grant PIP 112- 201101-00803). This paper is a contribution to the International Geoscience Programme (IGCP) Project 653 ‘The onset of the Great Ordovician Biodiversification Event.’
References
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Albanesi, G.L., and Astini, R.A., 2000, New conodont fauna from Suri Forma- tion (Early−Middle Ordovician), Famatina System, western Argentina: Reunión Anual de la Asociación Paleontológica Argentina, Mar del Plata, Abstracts, Ameghiniana, v. 37, no. 4, supplement, p. 68.
Albanesi, G.L., and Astini, R.A., 2002, Faunas de conodontes y Sacabambaspis janvieri (Vertebrata) en el Ordovícico Medio de la Cordillera Oriental Argentina: Implicancias estratigráficas y paleobiogeográficas: Congreso Argentino de Paleontología y Bioestratigrafía, 8th, Corrientes, Argentina, Abstracts, p. 17.
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