Minyocythere angulata, Lord & Cabral & Danielopol, 2020

Lord, Alan R., Cabral, M. Cristina & Danielopol, Dan L., 2020, Sieve-type normal pore canals in Jurassic ostracods: A review with description of a new genus, Acta Palaeontologica Polonica 65 (2), pp. 313-349 : 328-332

Minyocythere angulata sp. nov.

Figs. 4B , 5G, 6B, 9A, B, 12, 13, 21.

Zoobank LCID: urn:lsid:zoobank.org:act:3462B034-52DC-45C7-AB35-0E42983B7F42

Etymology: From Latin angulus, angled.

Type material: Holotype, SMF Xe 23737, LV male ( Fig. 12F ) . Paratypes: SMF Xe 23732, C female ( Fig. 12A ) ; SMF Xe 23733, RV female Fig. 12B ); SMF Xe 23734, LV female ( Fig. 12C ); SMF Xe 23735, C female ( Fig. 12D ); SMF Xe 23736, C female ( Fig. 12E ); SMF Xe 23738, LV female ( Figs. 4B , 12G ); SMF Xe 23739, RV juvenile, A-2? Fig. 12I ); SMF Xe 23740, C male ( Fig. 12H ); SMF Xe 23741, LV male (Fig. 5G).

Type locality: Borehole Hambühren WA 2, NW Germany .

Type horizon: Borehole Hambühren WA 2, 203 m, Upper Aalenian (Braun Jura β) .

Other material. — 21 C, 59 V, 7 C juvenile, 40 V juvenile, plus 3 C, 2 C [cf.], collective number SMF Xe 23767, from Borehole Oldenzaal 1, 246– 251 m depth, The Netherlands; Upper Aalenian (Braun Jura β) .

Diagnosis. —A species of Minyocythere characterized by its strongly tapering almost triangular carapace with strong cardinal angles and weak ornament of foveolae and punctae within which NPC are located.

Dimensions (in mm).— Females: L = 0.476 –0.575, H = 0.324 –0.350 ( SMF Xe 23732, L = 0.475, H = 0.324; SMF Xe 23733, L = 0.500, H = 0.300; SMF Xe 23734, L = 0.475, H = 0.324; SMF Xe 23735, L = 0.500, H = 0.324; SMF Xe 23736, L = 0.500, H = 0.324; SMF Xe 23738, L = 0.500, H = 0.350). Males: L = 0.476 –0.550, H = 0.300 –0.324 ( SMF Xe 23737, L = 0.525, H = 0.324; SMF Xe 23740, L = 0.476, H = 0.274; SMF Xe 23741, L = 0.490, H = 0.310). Juvenile: SMF Xe 23739, L = 0.400, H = 0.274 .

Description.— Exterior: Quadrate to triangular in lateral view, greatest height at anterior cardinal angle, tapering strongly posteriorly, greatest length just below mid-height; anterior margin broadly rounded with weak marginal rims on both valves, slight concavity on anterior margin below anterior cardinal angle on RV; dorsal margin straight to weakly concave medianly especially on LV; posterior low and symmetrically rounded in LV, slightly inclined ventrally in RV; ventral margin straight to convex; LV and RV almost identical in shape and size, LV slightly larger than RV especially at anterior and posterior margins; in dorsal view the female valves are uniformly curved around mid-length. Sexual dimorphism present but relatively weakly expressed, with males less high and appearing relatively longer in lateral view than females. Juveniles tend to be more triangular in lateral view and relatively smoother than adults. Dimorphism is recognisable in “pre-adult” moult stages ( Fig. 13 ). Ocular structures not evident externally or internally. Surface relatively smooth with shallow foveolae and punctae within some of which StPC are located ( Fig. 12B 2 , B 3 , C 2, F 2 , F 3 ). Simple NPC rarely visible, round, diameter c. 2 μm ( Fig. 6B 1 ). StPC-M located in shallow foveolae, most oblong ( Figs. 6B , 12C 2 ), a few round ( Fig. 12B 3 ), eight oblong and four round analysed. Median diameter c. 10 μm and SI 0.02 ( Table 2) for round pores. Setal pore round with diameter median value 1.5 μm. Round StPC-M the SePSI c. 0.2. Number of tubuli in both oblong and round forms high, median value 61, tubuli diameter in range 0.3–0.4 μm. Density “low” ( Fig. 6B 1 ), 48 entities counted, from which 87.5% oblong. The pore dispersion is “wide” ( Fig. 6B ). The DI varies between 30–60 μm. On the antero-dorsal area presence of the two StPC-M typical of the genus ( Figs. 6B 1 , 12B 1, D, F 1 ). StPC-m round ( Fig. 12B 2 , F 3 ) and with small diameter of 5 μm and SI of 0.01 ( Table 2). Setal pore large, c. 3 μm and SePSI around 0.6. Number of tubuli visible c. 16 disposed in 2–3 rows. Density of StPC-m pores similar to the one in M. macroporosa sp. nov. (12 entities, Fig. 6B 1 ).

Interior: Marginal zone well developed anteriorly, inner margin and line of concrescence coincide; marginal pore canals short, straight, widely spaced and arranged in a fan anteriorly, 10–11 anteriorly and 4–5 posteriorly ( Fig. 9A, B ). Muscle scars (Fig. 5G) consist of a semi-vertical curved row of four AMS, the dorsal and ventral ones rounded and the central ones more elongate, with two frontal scars one large and kidney-shaped and one small and round located centrally above, with ventrally two oval mandibular scars; between the vertical row of scars and the frontal scars there is a round depression that represents the articulation point (median depression = fulcral point) of the mandible. Hinge tripartite, relatively weakly developed, modified lophodont: LV ( Fig. 12F 4 , F 5, G 1) short terminal sockets and long smooth median bar, RV short smooth terminal teeth that merge into the free margin and a smooth median groove; the median elements may appear denticulate or locellate in

)

mm

in

(

Height

Length (in mm)

poorly preserved material and in some specimens the posterior element appears loculate or dentate.

Internally the valves show tubular apertures (depressions) that are typical of StPC-M or StPC-m. The apertures were most easily matched with external StPC in the centre of the valve interior.

Remarks.— Minyocythere angulata sp. nov. differs from M. macroporosa sp. nov. in its more triangular lateral outline, smoother surface with weak depressions corresponding to StPC, in number of StPC-M, and a secondary punctate ornament. M. angulata differs from M. maculosa and especially from M. tuberculata in strength of ornament and in lateral shape where M. maculosa is more rectangular and M. tuberculata more quadrate.

Especially interesting is comparison between the high frequency of oblong pores in M. angulata ( Fig. 6B 2 ) and those of M. macroporosa previously described ( Fig. 6A 2 ). For approximately equivalent surface areas we counted in M. angulata eight oblong pores and only four round ones, while in M. macroporosa practically all pores in the equivalent valve area are of round-type.

Stratigraphic and geographic range.—Aalenian–Bajocian, Middle Jurassic; NW Europe.

Figures

Fig. 4. Comparison of muscle scars of Dolocythere Mertens, 1956 and Minyocythere gen. nov. A. Dolocythere rara Mertens, 1956 from Borehole Lingen 196, 697–704 m, NW Germany, Lower Albian. Paratype, BGR T.-K. 1300 (Mertens Collection, Hannover), female left valve in internal view. B. Minyocythere angulata sp. nov. from Borehole Hambühren WA2, 203 m, NW Germany, Upper Aalenian (Braun Jura β). Paratype, SMF Xe 23738, female left valve in internal view. A1, B1, photographs; A2, B2, explanatory drawings.

Fig. 12. Cytherid ostracod Minyocythere angulata sp. nov. from Borehole Hambühren WA2, 177–180 m (A, H), 203–206 m (B–E), 203 m (F, G), NW Germany, Upper Aalenian (Braun Jura β). A. Paratype, SMF Xe 23732, female carapace in right view. B. Paratype, SMF Xe 23733, female right valve in external view (B1), micro sieve-type pore canals (StPC-m) (B2), and round macro sieve-type normal pore canals (StPC-M) (B3). C. Paratype, SMF Xe 23734, female left valve in external view (C1), elongate StPC-M (C2). D. Paratype, SMF Xe 23735, female carapace in dorsal view. E. Paratype, SMF Xe 23736, female carapace in ventral view. F. Holotype, SMF Xe 23737, male left valve in external (F1) and internal (F4) views, ornamentation and StPC-M and StPC-m (F2), StPC-m (F3), hinge (F5). G. Paratype, SMF Xe 23738, female left valve in internal view (G1), muscle scars (G2), StPC-m (G3). H. Paratype, SMF Xe 23740, male carapace in right view. I. Paratype, SMF Xe 23739, A-2? juvenile right valve in external view.

Fig. 13. Length versus height plots for Minyocythere angulata sp. nov. A-1, A-2, A-4, estimated growth (moult) stages.

Fig. 2. Sieve-type normal pore canals (StPC) in Phraterfabanella spp. A, B. Phraterfabanella tridentinensis Whatley and Boomer in Boomer et al., 2001, from Rotzo Member, Calcari Grigi Formation, Trento Platform, Venetian Prealps, NE Italy, Upper Sinemurian, Lower Jurassic. A. SMF Xe 23715, female left valve, in external view (A1). Detail of antero-dorsal area (A2), macro StPC (StPC-M) (A3), micro StPC (StPC-m) (A4), and large StPC (A5). B. SMF Xe 23716, male? fragment of right valve, in internal view (B1), StPC-M in general view (B2) and details (B3, B4). C. Phraterfabanella boomeri Cabral and Colin, 2015, from Praia da Concha, Coimbra Formation, sample PCR-318, Portugal, Upper Sinemurian, Lower Jurassic. SMF Xe 23717, male carapace, in external left view (C1), StPC-M in general view (C3) and details (C2, C4, C5).

Fig. 3. Sieve-type normal pore canals (StPC) in Aphelocythere and Camptocythere spp. A. Aphelocythere perforata Plumhoff, 1963, from Borehole Wesendorf 51a, 1031–1035 m, NW Germany, Upper Aalenian. SMF Xe 5608 (Plumhoff Collection), carapace in external right view (A1), macro StPC (StPC-M) (A2–A4). B, C. Camptocythere praecox Triebel, 1950 from Borehole Hambühren WA 2, 341 m, NW Germany, Leioceras opalinum Zone, Lower Aalenian (Braun Jura α). B. Paratype, SMF Xe 1448, male right valve in external view (B1), mid-anterior area behind marginal rim, ornamentation and pores (large and small) (B2), StPC-M with a peripheral large pore (B3), micro StPC (StPC-m) (B4). C. Paratype, SMF Xe 1448, male right valve in internal view (C1), hinge (C2), StPC-m (C3). D. Camptocythere media Triebel, 1950 from Borehole Altencelle 1014, 212/16 m, NW Germany, Upper Aalenian (Braun Jura β). Paratype, SMF Xe 1513, female left valve in external view (D1), detail, antero-dorsal area, ornamentation and pores (large and small) (D2), round StPC-M (D3), elongate StPC-M (D4), StPC-m (D5).

Fig. 6. Distribution of sieve-type normal pore canals (StPC) and simple normal pore canals (NPC) on the outer side of left valves of cytherid ostracod Minyocythere gen. et spp. nov. A. Minyocythere macroporosa sp. nov. from Borehole Hambühren WA2, 166 m, NW Germany, Witchellia laeviuscula Zone (Braun Jura γ), Lower Bajocian. Holotype, SMF Xe 23721, male left valve (A), detail (A), total area of A, c. 45 μm2. B. Minyocythere angulata 1 2 2 sp. nov. from Borehole Hambühren WA2, 203 m, NW Germany, Upper Aalenian (Braun Jura β). Holotype, SMF Xe 23737, male left valve (B1), detail (B), total area of B, c. 44 μm2. C. Minyocythere sp. cf. M. macroporosa sp. nov. from Borehole Rodewald WA12, 404.5 m, NW Germany, Witchellia 2 2 laeviuscula Zone (Braun Jura γ upper), Lower Bajocian. SMF Xe 23725, male left valve (C1), detail (C2). D. Minyocythere maculosa (Bate, 1963) from Hambühren WA2, 171–174 m, NW Germany, Upper Aalenian (Braun Jura β). SMF Xe 23750, male left valve. E. Minyocythere tuberculata (Luppold, 2012) from Borehole Rodewald WA12, 404.5 m, NW Germany, Witchellia laeviuscula Zone (Braun Jura γ upper), Lower Bajocian. SMF Xe 23754, female left valve. Black dots refer to macro StPC (StPC-M); white arrows point to micro StPC (StPC-m); simple NPC underlined; black arrows indicate position of the two StPC-M characteristic of Minyocythere species. Scale bars 100 μm.

Fig. 1. Schematic cross-sections of ostracod valves, exterior to top, interior to bottom. A. NPC classification of Puri and Dickau (1969). B. StPC with sieve plate flush with external surface (1) and sieve plate in funnel-shaped depression (2).

Fig. 9. Marginal pore canals of cytherid ostracods Minyocythere macroporosa sp. nov. and Minyocythere angulata sp. nov. A, B. Minyocythere angulata sp. nov. from Borehole Hambühren WA2, 203 m, NW Germany, Upper Aalenian (Braun Jura β). A. SMF Xe 23742 (specimen broken), female right valve in internal view. B. Unnumbered (specimen missing), male right valve in internal view. C, D. Minyocythere macroporosa sp. nov. from Hambühren WA2, 166 m, NW Germany, Upper Aalenian (Braun Jura β). C. Holotype, SMF Xe 23721, male left valve in external view. D. Paratype, SMF Xe 23722, female right valve in external view. Light micrographs by Erich Triebel. Scale bars 100 μm.