Ogmoopsis achaemenid, Pour & Williams & Vannier & Meidla & Popov, 2006

Pour, Mansoureh Ghobadi, Williams, Mark, Vannier, Jean, Meidla, Tõnu & Popov, Leonid E., 2006, Ordovician ostracods from east central Iran, Acta Palaeontologica Polonica 51 (3), pp. 551-560 : 554

Ogmoopsis achaemenid sp. nov. Williams, Vannier, and Meidla

Fig. 3C.

Derivation of the name: After the Achaemenid dynasty of ancient Persia, which ruled in the first millennium B.C.

Holotype: NMW2004.22 G.301, a complete tecnomorphic right valve.

Type locality: Dahaneh Kolut, east central Iran.

Type horizon: Shirgesht Formation, Section B, lithological Unit B5, sample B−D/4, probably late Middle Ordovician.

Material.— More than 20 specimens, NMW2004.22 G.301, NMW2004.22 G.333–353, from sample B−D/4. Specimens are 1.1 to 1.8 mm long, representing instars and adults .

Diagnosis.— Ogmoopsis species with narrow lobes (about 200 µm wide for specimens over 1 mm length), all of which are of similar width including the posterior lobe (L4). Anterior lobe (L1) inflated dorsally and overreaches the dorsal margin. L3 and L4 may also overreach the dorsal margin, but are not inflated dorsally. Ogmoopsis achaemenid is similar to Ogmoopsis bocki ( Öpik, 1935) , in having dimorphic juveniles with the velar ridge much broader in the heteromorph. Both Og. bocki and Og. achaemenid possess a dorsally inflated anterior lobe that overreaches the dorsal margin, but Og. achaemenid differs from Og. bocki by having all of its lobes of even width and by being longer (up to 1.8 mm long, compared to ca. 1.1 mm in Og. bocki ). In the type species Og. nodulifera , the lobes are long and relatively narrow, and the anterior lobe (L1) is weakly bulbous, and may overreach the dorsal margin (see Tinn and Meidla 2003: fig. 7.18). Lobe L2 and occasionally also L1 are narrow, but L3 and L4 are wider and flattened compared to the lobes of Og. achaemenid . Og. vesperi Sarv, 1959 differs from Og. achaemenid by having both L1 and often L3 inflated dorsally, overreaching the dorsal margin (see Sarv 1959: pl. 18: 12, 13). Relative to Og. achaemenid , Og. variabilis Sarv, 1959 , has stout lobes and the posterior lobe, in particular, is flattened (see Sarv 1959: pl. 19: 1, 5, 6). Og. estonica Sarv, 1959 has lobes of even width ( Sarv 1959: pl. 18: 14–21), but none of these overreach the dorsal margin and they are broader, relative to the size of the valve, than for Og. achaemenid . In Og. terpylae Sarv, 1959 , L2 is isolated from the connecting ridge, giving this species a distinctive morphology by which it can be differentiated from Og. achaemenid and other Ogmoopsis ( Sarv 1959: pl. 19: 7, 10, 11). Og. alata Sarv, 1959 has a strongly inflated L 3 in its ventral portion, a feature which is particularly obvious from a dorsal view (see Sarv 1959: pl. 19: 13; see also Schallreuter 1985: pl. 4: 1, 2), and which is absent from Og. achaemenid .

Description.—Valves weakly preplete and quadrilobate: all lobes narrow, including the posterior−most lobe (L4), and separated by well−developed sulci (S1–S3). S1 is narrow, S2 and S3 are broad. All lobes connected ventrally by a connecting ridge. L1 inflated and markedly overreaching the dorsal margin; L2 sigmoidal, not overreaching the dorsal margin; L3 curved (crescent−shaped), overreaches the dorsal margin; L4 curved, its outline following that of the posterior lateral outline, sometimes weakly overreaching the dorsal margin. In both dimorphs the velar ridge extends from the mid−posterior to the anterocardinal corner, but is much broader in heteromorphs. In ventral view the velar ridge overreaches the concave ventral surface. There are no other ventral ridges in either dimorph. Velar dimorphism is also manifested in preadult specimens.

Discussion.— Schallreuter (1993: 80–83) provides more modern description of several Ogmoopsis species based on material from erratic boulders (Geschieben in German), but questions the affinity of Og. variabilis and Og. vesperi to Ogmoopsis . He also refers Og. estonica to his new genus Ahla Schallreuter, 1993 , in which he also includes Og. terpylae , though somewhat tentatively, and Ceratopsis bocki Öpik, 1935 . As Tinn and Meidla (2003: 70) have shown, Ahla is a synonym of Ogmoopsis , and Og. bocki is very similar to the type species Og. nodulifera .