Minyocythere macroporosa, 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 : 324-328

Minyocythere macroporosa sp. nov.

Figs. 5E, 6A, 7, 8, 9C, D, 11A, 21.

1962 “ Lophodentina ”? sp. 99; Brand and Fahrion 1962: 136–137, pl. 20: fig. 6; table 9.

2012 Dolocythere tuberculata sp. nov.; Luppold 2012: pl. 4: 5–8.

non 2012 Dolocythere tuberculata sp. nov.; Luppold 2012: text-fig. 6a,?b; pl. 4: 9–12, pl. 6: 15, 16.

ZooBank LCID: urn:lsid:zoobank.org:act:1D8AC905-2F98-418C-9467-E5CC136F984F

Etymology: In reference to the prominent StPC visible both externally and internally.

Type material: Holotype, SMF Xe 23721, LV male (Figs. 5E, 7D) . Paratypes: SMF Xe 23718, C female ( Fig. 7A ) ; SMF Xe 23719, C male Fig. 7B ); SMF Xe 23720, C female ( Fig. 7C ); SMF Xe 23722, RV female ( Fig. 7E ); SMF Xe 23723, RV juvenile A-1? ( Fig. 7F ). All from the type locality, different depths.

Type locality: Borehole Hambühren WA 2 (166 m depth), NW Germany .

Type horizon: Lower Bajocian , Witchellia laeviuscula Zone (Braun Jura γ) .

Other material. —10 C, 97 V, 11 V juvenile, collective number SMF Xe 23767.

Diagnosis. —A species of Minyocythere characterized by its carapace tapering posteriorly with a postero-ventral swelling and surface of moderately developed foveolae and/or punctae within which NPC are sometimes located.

Dimensions (in mm).— Females: L = 0.550 –0.624, H = 0.300 –0.375 ( SMF Xe 23718, L = 0.525, H = 0.324; SMF Xe 23720, L = 0.575, H = 0.350; SMF Xe 23722, L = 0.550, H = 0.350). Males: L = 0.525 –0.588, H = 0.286 –0.336 ( SMF Xe 23721, L = 0.575, H = 0.350; SMF Xe 23719, L = 0.550, H = 0.324). Juvenile: SMF Xe 23723, L = 0.476, H = 0.300 .

Description.— Exterior: Subtriangular to subrectangular in lateral view, greatest height at anterior cardinal angle, tapering posteriorly, greatest length at mid-height; anterior margin broadly rounded with marginal rims on both valves; dorsal margin medianly weakly concave; posterior low and symmetrically rounded in LV, ventrally inclined in RV; ventral margin weakly concave with slight postero-ventral swelling on both valves; LV and RV almost identical in shape and size, LV slightly larger than RV on all margins except dorsal. Sexual dimorphism present but relatively weakly expressed, with males appearing relatively elongate in lateral view than females, however, in dorsal view apparent males are slightly more inflated posteriorly whereas apparent females show greatest width at mid-length. Juveniles tend to be more triangular in lateral view. Dimorphism is recognisable in “pre-adult” moult stages (Fig. 8). Ocular structures not evident externally or internally. Surface rugose with weakly to moderately developed foveolae and punctae within which normal pore canals are located, the pores range from simple holes ( Fig. 7D 4 ) to simple sieve plates with 2–3 rows of tubuli ( Fig. 7C 5, D 3, E 5, F 3) and to large sieve plates (with many tubuli, Fig. 7C 3 , D 2 , E 4 , F 2 ) present in one animal; ornament tends to be more strongly developed near the anterior margin especially in males. Juveniles appear relatively smoother than adults, but also with StPC ( Fig. 7F ).

Simple NPC seldom observed at valve periphery ( Fig. 6A 1 ) and are of smaller diameter than StPC ( Fig. 7D 4; 4.5 μm diameter). StPC-M located in valve surface depressions. Mostly round ( Figs. 6A , 7C 3 , D 2 , E 4 ). Size varies, with those towards margins generally smaller than those located in centre of valve. Median diameter c. 11 μm and SI 0.02 ( Table 1). Setal pore always round with diameter c. 1 μm (SeP-SI 0.09). StPC-M have a high number of tubuli, each with a diameter 0.3–0.4 μm, but difficult to count for preservational reasons, we estimate tubuli density in “large” class ( Table 1). Maximum number of StPC-M counted is 53 in Fig. 6A 1 , “low” class. Pore dispersion on the valve is “wide” type ( Figs. 6A 1 , 7B 1, D 1 , E 3 ). DI varies between 20–50 μm. On antero-dorsal area, the two diagnostic StPC-M round or elongate ( Figs. 6A 1 , 7A, B 1 , B 2 , C 1, D 1 , E 3 ) seem to occupy a permanent position. StPC-m round with small diameter (c. 6 μm) and SI 0.01 ( Table 1) ( Figs. 6A , 7C 5, D 3, E 5). Setal pore large with diameter c. 2 μm (SeP-SI c. 0.3). Number of tubuli visible about 20 in 2–3 rows. Number StPC-m counted 12, much fewer than StPC-M.

Interior: Marginal zone well developed anteriorly, inner margin and line of concrescence coincide; marginal pore

Length (in mm)

Fig. 8. Length versus height plots for Minyocythere macroporosa sp. nov. (A) and Minyocythere sp. cf. M. macroporosa sp. nov. (B), comparison (C). A-1–A-4, estimated growth (moult) stages.

canals short, straight, widely spaced and arranged in a fan anteriorly, 9–10 anteriorly and c. 5 posteriorly ( Fig. 9C, D ). Muscle scars (Figs. 5E, 7D 7) consist of a semi-vertical curved row of four adductor muscle scars (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 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 fulcral point (= median depression) of the mandible. Hinge tripartite, relatively weakly developed, modified lophodont: LV ( Fig. 7D 8) short terminal sockets and long smooth median bar, RV ( Fig. 7E 2 ) short smooth terminal teeth that merge into the free margin and a smooth median groove; the median elements may appear denticulate or locellate in poorly preserved material and in some specimens the posterior element appears loculate or dentate. Internally the valves show depressions that match the positions of StPC externally and the numerous tubuli are clearly seen ( Fig. 7D 6 –D 8, E 2). StPC-M show values similer to exterior view ( Table 1). Number of tubuli counted on one StPC ( Fig. 7D 6 ) 57, certainly an under estimate because of preservation. StPC-m diameter smaller than on exterior and in one pore 12 tubuli counted ( Table 1).

normal pore canals (StPC-m) (C 5). D. Holotype, SMF Xe 23721, male left valve in external (D 1) and internal (D 5) views, StPC-M (D 2, D 6), StPC-m (D 3), simple pore (D 4), muscle scars (D 7), hinge (D 8). E . Paratype, SMF Xe 23722, female right valve in internal (E 1) and external (E 3) views, hinge (E 2), StPC-M (E 4), StPC-m (E 5). F . Paratype, SMF Xe 23723, A-1?, juvenile right valve in external view (F 1), “large type” StPC (F 2), “small type” StPC (F 3) .

Remarks. —The Brand and Fahrion material of “ Lophodentina ”? sp. 99 contains both Minyocythere macroporosa sp. nov. ( Brand and Fahrion 1962: pl. 20: 6) and M. maculosa ( Brand and Fahrion 1962: pl. 20: 25). Luppold (2012: pl. 4: 5–8) interpreted our M. macroporosa sp. nov. as juveniles of his new species Dolocythere tuberculata , which is understandable given that M. macroporosa has relatively thinly calcified valves compared to the well calcified and strongly ornamented M. tuberculata ; herein M. tuberculata Luppold, 2012 ) is considered to belong to Minyocythere .

The material includes a few specimens that are similar in shape to juveniles of M. macroporosa sp. nov. but which appear to be adults in size (female: L = 0.525 –0.600, H = 0.313 –0.375; male: L = 0.550 –0.575, H = 0.313 –0.324) with a developed marginal zone and hingement and we have designated them as M. sp. cf. M. macroporosa sp. nov. (Figs. 5F, 6C, 8, 10, 11C, 21). Compared to M. macroporosa , M. sp. cf. M. macroporosa has a less irregular surface and the postero-ventral swelling is less developed; the growth series of juveniles and females of both forms is similar (regression lines in Fig. 8A, B) as opposed to the males (Fig. 8C). M. sp. cf. M. macroporosa occurs in Hambühren WA 2 in the Upper Aalenian (9 C, 7 V) and in Rodewald WA 12 in the Lower Bajocian (1 C, 18 V). M. macroporosa and M. sp. cf. M. macroporosa occur together in Hambühren WA2 but not in Rodewald WA12. As sizes and gender overlap (Fig. 8) and M. sp. cf. M. macroporosa is generally less well-preserved, there is no justification for recognising it as sp. nov. or subsp. nov.

Comparing NPC of M. macroporosa sp. nov. and M. sp. cf. M. macroporosa for completeness, we note that internally the former shows smaller StPC-M than the latter; median diameter 10.84 μm (n = 14) as opposed to 12.27 μm (n = 11) in the latter. This difference appears to be preservational.

The difference between the StPC-M of M. macroporosa sp. nov. and M. angulata sp. nov. is also of comparative value, as for equivalent surface areas in M. macroporosa and M. angulata almost all pores were round in the former whereas in the latter eleven were oblong and only four round Fig. 6A 2, B 2 ).

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

Figures

Fig. 7. Cytherid ostracod Minyocythere macroporosa sp. nov. from Borehole Hambühren WA2, 166–167 m (A–C), 166 m (D, E), 168–169 m (F), NW Germany, Witchellia laeviuscula Zone (Braun Jura γ), Lower Bajocian. A. Paratype, SMF Xe 23718, female carapace in right view. B. Paratype, SMF Xe 23719, male carapace in right (B1) and dorsal (B2) views, antero-dorsal margin (B3). C. Paratype, SMF Xe 23720, female carapace in dorsal (C1) and ventral (C6) views, pores (large and small) (C2), macro sieve-type normal pore canals (StPC-M) (C3), antero-dorsal margin (C4), and micro sieve-type →

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. 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. 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. 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.