Early Ordovician (Arenig) brachiopods from volcaniclastic rocks of the Famatina Range, Northwest Argentina

Journal of Paleontology, Mar 2003 by Benedetto, Juan L

Material examined.-One ventral valve and three dorsal valves with both internal and external molds, one incomplete external mold of dorsal valve, CEGH-UNC 15897-15901.

Occurrence.-Upper member of the Suri Formation, level CA, Cachiyuyo River section.

Discussion.-The genus Pinatotoechia was first described from Arenig volcaniclastic rocks of the Puna region, northwestern Argentina (Benedetto, 2001b). The external molds and ventral interiors from the Suri Formation are indistinguishable from the type material of P. acantha from the Vega Pinato locality, western Puna of Argentina. Associated dorsal valves in the Famatinian collection (absent in the type material) confirms that internally Pinatotoechia is closer to Tritoechia than any other representative of the Family Tritoechiidae. Pinatotoechia includes tritoechiids possessing a characteristic spinose ornament formed by simple rows of erect, tubular hollow spines on the top of the costellae. In other respects is very similar to the genus Tritoechia. The only other representative of this family bearing spinose ornament is Acanthotoechia Williams and Curry, 1985 from the Arenig Tourmakeady Limestone of Ireland. The latter genus, however, can be distinguished by having parvicostellate radial ornament and a concave dorsal valve that internally bears a well-defined peripheral rim.

Class RHYNCHONELLATA Williams et al., 1996

Order PROTORTHIDA Schuchert and Cooper, 1931

Superfamily SKENIDIOIDEA Kozlowski, 1929

Family SKENIDIIDAE Kozlowski, 1929

Genus SKENIDIOIDES Schuchert and Cooper, 1931

Type species.-Skenidioides billingsi SCHUCHERT AND COOPER, 1931.

SKENIDIOIDES KAYSERI new species

Figure 6.7-6.18

Skenidioides sp., BENEDETTO, 1994, p. 235, pl. 3, figs. 18-20, pl. 4, figs. 1-3.

Diagnosis.-Small, costate Skenidioides with about 14 costae on either flank, two weaker costae along dorsal sulcus, and a stronger, anteriorly dichotomized median costa in ventral valve; bifurcations and intercalations sporadically developed. Spondylium proportionally small, semicircular. Septalium long and narrow.

Description.-Shell small, 4.7 to 6.0 mm in length, ventribiconvex, semielliptical in outline (average length/width ratio: 0.72), maximum width at hingeline. Cardinal angles acute. Ventral valve hemipyramidal, with rounded carina and slightly curved lateral flanks; ventral interarea planar, proportionally high, about one-third as high as wide, strongly apsacline to catacline; delthyrium completely open. Dorsal valve moderately convex, flanks swollen, median sulcus narrow becomes deeper anteriorly. Dorsal interarea low, anacline. Ornament of rounded, subequal costae numbering 5-6 per mm with a mean of 14 costae on each flank; generally two weaker costae along dorsal sulcus and a coarser, anteriorly dichotomized median costa in ventral valve; bifurcations and intercalations sporadically developed on flanks, especially on anterior third of valve. Intercostal spaces covered by very fine, evenly spaced concentric growth lamellae.

Interior of ventral valve with small semicircular spondylium confined to delthyrial chamber, mostly free but supported umbonally by a short septum extending anteriorly for one-sixth to one-eighth of valve length. Teeth small, rounded. Vascula narrow, widely divergent.