Protanastrophia repanda, 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 : 224-225

Protanastrophia repanda gen. et sp. nov.

Figs. 1–3 .

Derivation of the name: Latin repandus, curved, deflected, referring to the warped, asymmetrical, anterior shell margin.

Type material: Holotype, ROM 57734 ( Fig. 1A ) and four figured paratypes, ROM 57735 , 57736 , 57737 , ( Fig. 1B–D ), and 57738 (the last serially sectioned, see Figs. 1E , 3 ).

Type locality and type horizon: AK2c is a shoreline outcrop on the north side of Akimiski Island , UTM 17 E0502916, N5883663, Hudson Bay region, Nunavut, Canada .

Type horizon: Attawapiskat Formation (uppermost Telychian , Lower Silurian), UTM 17 E0502916, N5883663, Hudson Bay region, Nunavut, Canada .

Material.—AK2a (5 specimens), AK2b (21), AK2c (21, including illustrated types), AK2−01a (2), AK4b (1), AK8−01e

JIN AND POPOV—A NEW PARASTROPHINID BRACHIOPOD GENUS 225

(1). All are from the Attawapiskat Formation, uppermost Telychian, Akimiski Island, Hudson Bay region, Canada. Repository: Royal Ontario Museum.

Diagnosis.—Shell small, transversely elliptical to subpentagonal, dorsibiconvex, smooth posteriorly, weakly costate anteromedially; dorsal valve notably larger, deeper, with higher and more prominent umbo than ventral valve. Anterior commissure uniplicate, variably asymmetrical or sigmoidal. Ventral sulcus and dorsal fold developed anteriorly, broad and variously asymmetrical. Ventral median septum short, very low; inner hinge plates high, discrete, subparallel to each other; alate plates well developed.

Description.—Shell small, transversely subelliptical to pentagonal, strongly dorsibiconvex, with average length 6.5 mm, width 8.4 mm, and thickness 4 mm ( Fig. 2 ; Table 1). Hinge line one−third to one−half of shell maximum width; maximum width of shell attained near midlength of shell. Ventral valve weakly convex posteromedially, becoming flattened towards lateral and anterior margins ( Fig. 1A–D ); ventral umbo low, inconspicuous; sulcus originating near midlength of shell, widening rapidly towards anterior margin, symmetrical in smaller shells, becoming asymmetrical in relatively large forms. Dorsal valve three to four times deeper than ventral, with larger and more convex umbo extending posteriorly beyond that of ventral valve; fold broad, gentle, developed corresponding to sulcus in position and in symmetrical or asymmetrical contours. Posterior and lateral portions of shell surface smooth; anteromedial part of shell varying from smooth, faintly costate, to moderately costate ( Fig. 1A 1, B 1, C 1, D 1 ). Coarse, concentric growth lamellae usually present, best developed at late growth stage.

Spondylium broadly triangular in transverse cross section ( Figs. 1E and 3 ), sessile apically, raised slightly above valve floor anteriorly by short, low median septum. Spondylial comb structure (sensu Jin and Copper 2000) not observed. Inner hinge plates slender, high, subparallel to each other along their entire length (except at their apical origin); outer hinge plates rudimentary; alate plates present but not prominent ( Fig. 3 ). Muscle field poorly impressed.

Discussion.—The new species is similar to Protanastrophia lepida ( Thomas, 1926) in its small, dorsibiconvex shell that is quasi−smooth or has gentle anteromedial costae (see Amsden 1968). Protanastrophia lepida , however, tends to have an anteromedially protruding tongue, with a notably higher ventral median septum and more prominent alate plates. The strongly asymmetrical sulcus, which is common in relatively large forms of P. repanda , is not obvious in P. lepida . The two forms described by Rozman (1970a, 1981) from the Upper Ordovician of Siberia and Mongolia resemble the new species in their asymmetry, but these forms have considerably stronger shell costae.

Figures

Fig. 1. Pentameride brachiopod Protanastrophia repanda gen. et sp. nov.; Attawapiskat Formation, uppermost Telychian, Akimiski Island, Hudson Bay region, Nunavut, Canada. A. ROM 57734, holotype, dorsal (A1), ventral (A2), lateral (A3), posterior (A4), and anterior (A5) views. B. ROM 57735, paratype, dorsal (B1), ventral (B2), lateral (B3), posterior (B4), and anterior (B5) views of strongly asymmetrical, anteriorly costate shell. C. ROM 57736, paratype, dorsal (C1), ventral (C2), lateral (C3), posterior (C4), and anterior (C5) views of asymmetrical shell without costae. D. ROM 57737, paratype, dorsal (D1), ventral (D2), lateral (D3), posterior (D4), and anterior (D5) views of relatively small, largely symmetrical shell. E. ROM 57738, paratype, micrograph of transverse serial section, showing low ventral median septum, broad V−shaped spondylium, smooth alate plates, and discrete inner hinge plates, 0.7 mm from apex (refer to Fig. 3).

Fig. 2. Shell measurements of Protanastrophia repanda gen. et sp. nov. Sample AK2c, Attawapiskat Formation, Akimiski Island, Hudson Bay region, Nunavut, Canada.

Fig. 3. Sketches of selected serial sections of Protanastrophia repanda gen. et sp. nov. ROM 57738, paratype, Attawapiskat Formation, locality AK2c, Akimiski Island, Nunavut, Canada. Numbers denote distance from apex.