Rhomboentomozoe rhomboidea ( Barrande, 1872 )

Legiot, Nicolas, 2019, Silurian myodocope ostracods from Poland, Acta Palaeontologica Polonica 64 (2), pp. 379-397 : 389-390

Rhomboentomozoe rhomboidea ( Barrande, 1872)

Figs. 7 , 8.

1872 Cryptoc. ? rhomboidea Barr. ; Barrande 1872: 464, pl. 31: 14, 15.

1936 Entomis (Richteria) rhomboidea ( Barrande, 1872) ; Bouček 1936: 58, pl. 6: 3, 4.

1951 Rhomboentomozoe rhomboidea (Barrande) ; Přibyl 1951: 15, pl. 1: 2, pl. 2: 5.

1954 Rhomboentomozoe rhomboidea (Barrande) ; Pokorný 1954: 462, text-fig. 680.

1958 Rhomboentomozoe rhomboidea (Barrande) ; Pokorný 1958: 311, text-fig. 1035.

1960 Rhomboentomozoe rhomboidea (Barrande) ; Zanina and Polenova 1960: 331, text-fig. 834.

1961 Rhomboentomozoe rhomboidea (Barrande) ; Sylvester-Bradley 1961: 390, text-fig. 315.2.

1962 Rhomboentomozoe ; Gründel 1962: 1193, 1198, text-fig. 9.

1988 Rhomboentomozoe rhomboidea (Barrande) ; Přibyl 1988: 114, text-fig. 5.2.

1991 Rhomboentomozoe rhomboidea (Barrande) ; Stupnicka et al. 1991: 391.

1994 Rhomboentomozoe rhomboidea ( Barrande, 1872) ; Whatley et al. 1994: 351.

Holotype: A carapace in butterfly position (NM-L 23322; CE 1222, lnv.

No. 1648); Barrande (1872: pl. 31: 14, 15) and Bouček (1936: pl. 6:

4); herein Fig. 8G.

Type locality: Kozel, 3 km east of Beroun, Bohemia, Czech Republic.

Type horizon: Lower part of the Kopanina Formation, Ludlow Series,

Silurian, horizon with the trilobite “ Cromus ” beaumonti .

Material.— Type specimens plus tens of valves from two localities (Widełki and Niestachów), Holy Cross Mountain , Poland, Ludlow Series, Silurian .

Measurements (in mm).—Size range: L = 2, H = 1.05 ( ZPAL O.63/16, Fig. 8A) to L = 4.25, H = 2.2 ( ZPAL O.63/31). Holotype: L = 2.07, H = 1.67 (NM-L 23322; CE 1222, lnv. No. 1648). L/H ratio and adductor muscle position for 19 specimens from two localities ( Fig. 4 ).

Emended diagnosis.— Entomozoidae with a triangular lateral valve outline, and a large, posterolaterally projecting ventral ala. Dorsal margin straight or slightly concave, ventral margin gently curved. Anterior and posterior cardinal corners pointed. Adductorial sulcus narrow, well developed, extending up to 1/2 of valve height, ending ventrally at a simple muscle spot. Faint notch in anterior valve margin. Valves smooth or weakly ribbed (modified after Gooday 1983).

Description.—Valve large, having an inverted triangular lateral outline. Maximum length at the dorsal margin; maximum height at the level of the lateroventral ala. Valve lateral outline is straight to weakly concave dorsally, gently curved posteriorly and anteriorly and more strongly curved ventrally. Valve surface is gently inflated overall. Adductorial sulcus narrow, concave forward, occurs just in front of mid-length and extends from the dorsal margin to just below mid-height ending with a simple adductorial muscle spot. The anterodorsal frontal projection resembles the straight rostrum of Recent halocyprid ostracods, and bears a well-developed notch on its valve margin and very weakly developed rostral incisure. Immediately adjacent to the posterodorsal cardinal corner a short posterodorsally projecting spine extends just above valve margin. Lateroventral ala is large, projects posteroventrally from the ventral part of the valve; in lateral view its anterior margin is convex anteriorly, its posterior margin is concave. During ontogeny, the ala becomes larger and projects more posteroventrally, and the rostrum and posterodorsal spine become more distinct.

Remarks.—The type specimen from Bohemia (Fig. 8G) was originally described as a phyllopod by Barrande (1872: 464, pl. 31: 14, 15). The Polish material is very similar to this type specimen, differing only by its seemingly flatter ala. The unique ribbed specimen figured by Bouček (1936: pl. 6: 3) and redrawn by Přibyl (1951: pl. 1: 2) is one of a kind: it has ribs that are absent in both the type specimen and the Polish material. More material would be needed to determine if it belongs to a separate species.

Stratigraphic and geographic range.—Ludlow Series, Holy Cross Mountains, Poland. Also known from the Ludlow

Series, Silurian of the Czech Republic: rarely in the lower part of the Kopanina Formation, “ Cromus ” beaumonti horizon, together with Richteria migrans , Vltavina perneri and other ostracodes and trilobites. Praha-Podolí (old quarry Dvorce, now the swimming pool) and Kozel (below Listice) near Beroun ( Barrande 1872).

Figures

Fig. 7. Idealized reconstructions and terminology of Nudator elegantulus sp. nov. (A) and Rhomboentomozoe rhomboidea (B) from the Holy Cross Mountains, Poland; Ludlow Series, Silurian. In left lateral (A1, B1) and dorsal (A2, B2) views.

Fig. 4. Valve length/height plot (A) and position of the adductor muscle scar (A/B, where A and B are the distance between the muscle scar and the posterior end of the valve and the ventral margin, respectively) against length/height (B) of complete specimens of entomozoid species from the Holy Cross Mountains, Poland; Ludlow Series, Silurian.