Protanastrophia, Jin & Popov, 2008

Jin, Jisuo & Popov, Leonid E., 2008, A new genus of Late Ordovician-Early Silurian pentameride brachiopods and its phylogenetic relationships, Acta Palaeontologica Polonica 53 (2), pp. 221-236 : 222-224

Genus Protanastrophia nov.

Type species: Protanastrophia repanda gen et sp. nov. Attawapiskat Formation , Akimiski Island, Hudson Bay region, Nunavut, Canada .

Derivation of the name: Greek, proto −, first, early. Protanastrophia implies an early form of parastrophinid similar to Anastrophia .

Diagnosis.—Small, transversely elliptical to subpentagonal, dorsibiconvex shell, smooth posteriorly, faintly to moderately costate anteromedially; dorsal valve usually larger, deeper, with higher and more prominent umbo than ventral valve. Ventral sulcus and anterior commissure commonly asymmetrical. Ventral median septum short, low; inner hinge plates high, discrete, subparallel to each other; alate plates present.

Other species assigned.—Most species assigned to the new genus were described initially in publications that are not easily accessible. Thus a brief note on each species is provided herein.

Parastrophinella asimmetrica Rozman, 1970a (72, pl. 5: 10–13, text−fig. 20), Upper Ordovician (Katian, Padun Stage), Nalchan Formation, Selenyakh Range, Northeast Siberia, Russia. Shell dorsibiconvex, up to 13 mm wide, asymmetrical shell with sigmoidal anterior commissure; ventral sulcus and dorsal fold originating at midlength of mature shells; up to 14 rounded costae in anterior half of shell; discrete, parallel inner hinge plates. Rudimentary strophic shell with a small ventral interarea in this species can be inferred from the transverse serial sections and emended diagnosis of Parastrophinella provided by Rozman (1970a).

Parastrophinella distorta Oradovskaya in Nikolaev et al. 1977: 48, pl. 5: 4. Upper Ordovician (Katian, Tirekhtyakh Stage), lower part of Tirekhtyakh Formation, Member N, Northeast Siberia, Russia. Shells up to 15 mm wide, weakly dorsibiconvex, strongly asymmetrical shell with sigmoidal anterior commissure; rudimentary, apsacline ventral interarea ( Oradovskaya 1983: pl. 9: 2), incipient fold and sulcus originating at 2–3 mm from umbo; up to 24 rounded ribs originating at some distance from umbones of both valves; discrete, subparallel inner hinge plates.

Parastrophinella gracilis Oradovskaya, 1983 (52, pl. 10: 4–7). Upper Ordovician, (Katian, Tirekhtyakh Stage), lower part of Tirekhtyakh Formation, Member Q, Northeast Siberia, Russia. Shells symmetrical, up to 10 mm wide, strongly dorsibiconvex; ventral sulcus and dorsal fold originating at midlength of mature specimens; single low, rounded costa in sulcus and two costae on fold in larger shells; up to two gentle costae on each shell flank in relatively large shells; discrete, subparallel inner hinge plates. Existing descriptions and illustrations make it difficult to determine a strophic or astrophic condition of the shell.

Parastrophinella indistincta Rubel, 1970 , Silurian, lower to middle Rhuddanian, Estonia. Shell up to 12 mm long, dorsibiconvex; anterior commissure uniplicate, symmetrical or slightly asymmetrical; ventral sulcus, dorsal fold, and costae developed anteriorly; one or two costae in sulcus, two or three on fold, one to three on each flank; broad spondylium supported by low median septum extending to midlength of valve; parallel inner hinge plates. Strophic or astrophic condition of the shell cannot be determined from available data.

Parastrophinella lepida ( Thomas, 1926) , St. Clair Limestone , Arkansas, and Clarita Formation (Fitzhugh Member), Wenlock, Oklahoma (see description of Amsden 1968) .

Parastrophinella salairica Severgina, 1978 (see also Rozman 1981: 156, pl. 40: 1–10, pl. 41: 1–4, text−figs. 47, 48), Upper Ordovician (Katian), upper Chakyr and lower Dietken regional stages of Salair and Gornyi Altai, southwestern Siberia, Russia; Tsagandel, Sairin, and Uregnur beds of Mongolia. Shells up to 12 mm long, strongly dorsibiconvex; anterior commissure uniplicate, symmetrical or slightly asymmetrical; ventral sulcus and dorsal fold originating at midlength; costae in anterior half of shell, one to five in sulcus, two to six on fold, and three to seven on each flank; narrow spondylium supported by median septum extending to midlength of valve; parallel inner hinge plates. Strophic or astrophic condition of the shell cannot be determined from available data.

Species questionably included.—? Parastrophinella asimmetrica longiplicata Rozman, 1981 , Upper Ordovician (Katian), middle Tsagandel beds, Mongolia. Known from a single specimen about 10 mm long. Externally similar to Parastrophinella asimmetrica but differs in having 18 costae originating in umbonal area. Interior of both valves unknown.

Parastrophinella sp. ( Nikiforova, 1978), Silurian (Llandovery), Minkuchar beds, Zeravshan Range, Uzbekistan. Known from one complete shell and three dorsal valves. Smooth, symmetrical, dorsibiconvex shell with ventral sulcus and dorsal fold originating anterior of umbo, subparallel inner hinge plates, and small alate plates.

Occurrences.—Late Ordovician (Ashgill, middle to late Katian)–Early Silurian (Wenlock), North America, Northeast Siberia, Gornyi Altai, Estonia.

Discussion.—The new genus is most similar to Anastrophia Hall, 1867 and Grayina Boucot, 1975 in having consistently discrete, subparallel inner hinge plates, but the latter two genera of Wenlock–Pragian age have a symmetrical shell with strong costae that extend from the apex to the margin ( Havlíček 1990; Carlson 2002). In the new genus, asymmetrical fold and sulcus tend to become common in larger shells (greater than 8 mm long and 10 mm wide). Such shell asymmetry is rare in other genera of the family Parastrophinidae . In all other parastrophinid genera, the inner hinge plates are united basomedially to form a septalium, which may be posteriorly sessile or supported by a median septum along its entire length ( Jin and Copper 1997).