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Wilsonidium imperfectum
Etymology. From Latin imperfectus (imperfect).
Holotype. IGG, no. 7/23-02; Russia, Lower Volga region, Ravninnaya R-43 borehole, interval of 129.0–128.1 m, sample R-43/58; Middle Eocene.
Description. Peridinioid cysts are rhomboidal in shape, with a narrow pericoel, compressed in the dorsoventral direction. The lateral sides of the epicyst and hypocyst are straight. The lateral horns and left antapical horn are strongly reduced. The endocyst is large and oval in shape; sometimes, it reaches the lateral sides of the pericyst, which gives the cornucavate shape of the cyst. The pericoel is usually very narrow. The periphragm and endophragm are dense and granular. Tabulation is imperfect and marked on the periphragm by sparse short spines and denticles located along the sutural sutures; rows of suturally oriented low-relief structures (tubercles and grains) are occasionally observed. The archaeopyle is rounded– square in shape and formed by the prolapse of intercalary plate (2a).
Dimensions. Holotype (Pl. 12, fig. 17): total length 109.0 μm, total width 82.6 μm, central body length 86.4 μm, central body width 82.8 μm. Paratype (Pl. 12, fig. 19): total length 104.6 μm, total width 91 μm, central body length 73.0 μm, central body width 64.0 μm. Other specimens: total length 100– 109 μm, total width 75–91 μm, central body length 73–87 μm, central body width 64–83 μm. Seven specimens were measured.
Comparison. The species Wilsonidium imperfectum sp. nov. differs from the other Wilsonidium species in a large oval endocyst, a narrow pericoel, and the reduced, weakly defined tabulation of the periphragm. With respect to the rhomboidal shape of the pericyst and the pattern of periphragm ornamentation, the new taxon resembles Wilsonidium michouxi Vasilyeva, sp. nov., which also has a reduced ornamentation of the periphragm; however, the latter has a wide pericoel. The new species also resembles Wilsonidium ornatum (Wilson, 1967) with respect to the imperfect ornamentation of the periphragm; however, it differs in its smaller size, shortened lateral horns, and the elongated rhomboidal shape of the pericyst. In addition, unlike W. imperfectum sp. nov., the species W. ornatum has no ornamentation on the mid-dorsal and ventral surface of the pericyst.
Remarks.The cysts of the dinoflagellate Wilsonidium imperfectum sp. nov. are strongly compressed in the dorsoventral direction, so that the sutural ornamentation is fused on the dorsal (Fig. 3a) and ventral surfaces of the pericyst and hardly distinguishable.
Material. Seven specimens from the type locality.
Holotype. IGG, no. 7/23-02; Russia, Lower Volga region, Ravninnaya R-43 borehole, interval of 129.0–128.1 m, sample R-43/58; Middle Eocene.
Description. Peridinioid cysts are rhomboidal in shape, with a narrow pericoel, compressed in the dorsoventral direction. The lateral sides of the epicyst and hypocyst are straight. The lateral horns and left antapical horn are strongly reduced. The endocyst is large and oval in shape; sometimes, it reaches the lateral sides of the pericyst, which gives the cornucavate shape of the cyst. The pericoel is usually very narrow. The periphragm and endophragm are dense and granular. Tabulation is imperfect and marked on the periphragm by sparse short spines and denticles located along the sutural sutures; rows of suturally oriented low-relief structures (tubercles and grains) are occasionally observed. The archaeopyle is rounded– square in shape and formed by the prolapse of intercalary plate (2a).
Dimensions. Holotype (Pl. 12, fig. 17): total length 109.0 μm, total width 82.6 μm, central body length 86.4 μm, central body width 82.8 μm. Paratype (Pl. 12, fig. 19): total length 104.6 μm, total width 91 μm, central body length 73.0 μm, central body width 64.0 μm. Other specimens: total length 100– 109 μm, total width 75–91 μm, central body length 73–87 μm, central body width 64–83 μm. Seven specimens were measured.
Comparison. The species Wilsonidium imperfectum sp. nov. differs from the other Wilsonidium species in a large oval endocyst, a narrow pericoel, and the reduced, weakly defined tabulation of the periphragm. With respect to the rhomboidal shape of the pericyst and the pattern of periphragm ornamentation, the new taxon resembles Wilsonidium michouxi Vasilyeva, sp. nov., which also has a reduced ornamentation of the periphragm; however, the latter has a wide pericoel. The new species also resembles Wilsonidium ornatum (Wilson, 1967) with respect to the imperfect ornamentation of the periphragm; however, it differs in its smaller size, shortened lateral horns, and the elongated rhomboidal shape of the pericyst. In addition, unlike W. imperfectum sp. nov., the species W. ornatum has no ornamentation on the mid-dorsal and ventral surface of the pericyst.
Remarks.The cysts of the dinoflagellate Wilsonidium imperfectum sp. nov. are strongly compressed in the dorsoventral direction, so that the sutural ornamentation is fused on the dorsal (Fig. 3a) and ventral surfaces of the pericyst and hardly distinguishable.
Material. Seven specimens from the type locality.