Dolocythere amphistiela, 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 : 340-342

Dolocythere amphistiela sp. nov.

Figs. 5B, 11D, 19, 20.

1964 Dolocythere rara Mertens ; Kaye 1964: 322; pl. 55: 12.

Zoobank LSID: urn:lsid:zoobank.org:act:3987244A-FA91-48F1-983B-C098CD0E358A

Etymology: From Ancient Greek ἀμφί (amphi), on both sides and stiela, in reference to ancient Greek coins.

Type material: Holotype, SMF Xe 23761, LV male ( Fig. 20B ) . Paratypes: SMF Xe 23760, RV male ( Fig. 20A ) ; SMF Xe 23762, LV female ( Fig. 20C ); SMF Xe 23763, RV female ( Fig. 20D ); SMF Xe 23764, RV female ( Fig. 20E ); SMF Xe 23765, LV male ( Fig. 20F ); SMF Xe 23766, LV male ( Fig. 20G ); from type locality and horizon.

Type locality: Borehole Rodewald WA 4, 187 m depth, Germany .

Type horizon: Leymeriella tardefurcata Zone , Lower Albian .

Other material.— 140 V, 74 V juvenile, collective number SMF Xe 23767, NW Germany: Borehole Rodewald WA 4, 187 m ; Borehole Rodewald WA6, 203– 236 m; Borehole Rodewald WA7, 245– 311 m; Borehole Rodewald WA11, 184– 222 m; Borehole Rodewald WA13, 211– 227 m; Leymeriella tardefurcata Zone , Lower Albian .

Diagnosis.—A species of Dolocythere characterized by a strongly reticulate ornament in which an irregular rib runs diagonally from near the posterior cardinal angle to the mid antero-ventral area.

Dimensions (in mm).— Females: L = 0.370 –0.400, H = 0.190 –0.210 ( SMF Xe 23762, L = 0.390, H = 0.210; SMF Xe 23763, L = 0.380, H = 0.210; SMF Xe 23764, L = 0.370, H = 0.200). Males: L = 0.360 –0.400, H = 0.180 –0.200 ( SMF Xe 23761, L = 0.380, H = 0.200; SMF Xe 23760, L = 0.380, H = 0.190; SMF Xe 23765, L = 0.400, H = 0.200; SMF Xe 23766, L = 0.370, H = 0.190) .

Description.— Exterior: Elongate in lateral view, greatest height at anterior cardinal angle, greatest length at mid-height; anterior margin broadly and symmetrically rounded with wide marginal rims on both valves, dorsal margin concave medianly on both valves which coincides with a ventral margin concavity giving the valves a “waisted” appearance in lateral view especially in males; posterior symmetrically rounded in LV or slightly inclined ventrally in RV; ventral margin weakly concave; small marginal rim on postero-ventral margin; LV and RV similar in shape and size, overlap difficult to determine as only valves found. Sexual dimorphism present and strongly expressed, with males longer and less high in lateral view than females (Fig. 19), sexes similar in dorsal view. Juveniles weakly ornamented compared to adults. Surface of adults with a strong reticulum with rounded sola containing small granules, ornament strongly developed in mid-valve area but weakens towards anterior and posterior margins with smaller sola; an irregular rib runs diagonally from near the posterior cardinal angle to the mid antero-ventral area.

Normal pores occur on the muri: small StPC-m ( Fig. 20D 2 , D 3 ) display a diameter of about 3 μm. The sieve is minimally visible. In Fig. 20D 3 the StPC-m displays a small peripheral plate with 4 tubuli visible. The diameter of the setal pore is 1.6μm and those of tubuli 0.2 μm. Smaller pores of a diameter in the range of 1.5 μm represent simple NPC ( Fig. 20D 4 ). The SI for the pore of Fig. 20D 3 belonging to a valve of 0.380 mm ( Fig 20D 1 ) is 0.008 and the SeP-SI is 0.5. The density and the dispersion of the pores could not be counted with accuracy, because of poor preservation of the apertures. We recognized 12 pores, this number compared to those identified on the inner side of the valve underestimates the real number.

Interior: Internally the valves show depressions that match the positions of StPC externally. Marginal zone well developed anteriorly and posteriorly, inner margin and line of concrescence coincide; marginal pore canals straight, 12 anteriorly and 6 posteriorly. AMS (Fig. 5B) consist of a vertical row of four rounded-elongate scars, with a U-shaped frontal scar that may comprise two scars one large and U-shaped with an accompanying small rounded scar anteriorly, with ventrally two rounded mandibular scars; between the vertical row of scars and the frontal scars there is an elongate depression that represents the articulation point median depression = fulcral point) of the mandible. Hinge tripartite, relatively strongly developed lophodont: LV short terminal sockets and long smooth median bar, RV short smooth terminal peg-like teeth that merge distally into the free margin and a smooth median groove.

NPC can be observed in Figs. 5B, 11D, and 20F. In Fig. 11D the white arrows indicate the apertures of the pores. Importantly, no apical pores exist at the extremity of the AMS, namely the pores observed in D. rara are not present in D. amphistiela ( Figs. 20F 1 , F 3 , 11D ). Small pores (n = 31) occur on the inner side of the valve in Fig. 11D . Therefore, pore density for D. amphistiela is of the “low” type. The diameter of a NPC inner aperture is 2.86 μm ( Fig. 20F 2 ), SI for this pore is 0.007. These values correspond closely with those measured on the exterior of the valve, and even if we cannot see the sieve plate we can consider the aperture as belonging to a StPC-m. The DI values are typical for “widely” spaced pores, between about 25 and 45 μm with an average of 33.4 μm (n = 5). To compare the degree of dispersion of NPC on D. amphistiela with D. rara , we expressed the data as a ratio of the average of the values for the DI to the length of the valve on which we measured the pore dispersion. For D. amphistiela valve length is 400 μm (cf. Fig. 20F 1 ) and the scaled value is 0.0835 (33.4/400). For D. rara the average of the DI is 42 μm (n = 5) and the valve length is 500 μm Fig. 11B 1 ), hence the scaled value is 0.084. Therefore the degree of dispersion of the NPC on the interior of the valves of the two Dolocythere species is surprisingly similar. This result is a good example of the value of analysing data using both interval and ratio scales.

Remarks.— D. amphistiela sp. nov. differs from D. rara in strength and arrangement of the surface reticulate ornament, in lateral outline and in the strength of the postero-ventral marginal rim. Both D. amphistiela and D. rara have postero-ventral rims or keels which seems to be a generic feature (cf. D. cristata Colin, 1974 ). Additionally, D. amphistiela lacks the two apical points at the extremities of the AMS in the valve interior.

Borehole material in the Senckenberg Forschungsinstitut suggests that a number of Dolocythere morphotypes occur in the Albian. No carapaces observed which is probably a taphonomic feature.

Stratigraphic and geographic range.—Lower Albian, Lower Cretaceous; NW Germany.

Figures

Fig. 20. Cytherid ostracod Dolocythere amphistiela sp. nov. from Borehole Rodewald WA4, 187 m, NW Germany Leymeriella tardefurcata Zone, Lower Albian. A. Paratype, SMF Xe 23760, male right valve in external view. B. Holotype, SMF Xe 23761, male left valve in external view (B1), dorsal-posterior area (B2), ornamentation (B3). C. Paratype, SMF Xe 23762, female left valve in external view. D. Paratype, SMF Xe 23763, female right valve in external view (D1), small StPC (cf. micro sieve-type normal pore canals StPC-m) (D2, D3), simple pore (D4). E. Paratype, SMF Xe 23764, female right valve in internal view. F. Paratype, SMF Xe 23765, male left valve in internal view (F1), small StPC (cf. StPC-m) (F2), muscle scars (F3). G. Paratype, SMF Xe 23766, male left valve in internal view (G1), hinge (G2).

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. 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. 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. 11. Distribution of macro sieve-type normal pore canals (StPC-M), micro sieve-type normal pore canals (StPC-m), and simple normal pore canals NPC) on the inner side of cytherid ostracods Minyocythere gen. nov. and Dolocythere Mertens, 1956. A. Minyocythere macroporosa sp. nov., holotype, SMF Xe 23721 from Borehole Hambühren WA2, 166 m, NW Germany, Witchellia laeviuscula Zone (Braun Jura γ), Lower Bajocian; male left valve. B. Dolocythere rara Mertens, 1956, SMF Xe 23756 from Borehole Rodewald WA6, 206 m, NW Germany, Lower Albian; male left valve (B1), detail (B2). C. Minyocythere sp. cf. M. macroporosa sp. nov., SMF Xe 23724 from Borehole Rodewald WA12, 386 m, NW Germany, Upper Aalenian (Braun Jura β upper); male right valve. D. Dolocythere amphistiela sp. nov., paratype, SMF Xe 23765 from Borehole Rodewald WA4, 187 m, NW Germany, Leymeriella tradefurcata Zone, Lower Albian; male left valve. In A and C, black dots refer to StPC-M, white arrows point to StPC-m; in B1, B2, D white arrows point to NPC; black arrows in B1, B2 indicate the peculiar pores of D. rara (which do not exist in D. amphistiela). Scale bars 100 μm.