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Parvocysta triquetra

Parvocysta? triquetra Riding and Shaw in Riding et al., 1991

Riding and Shaw in Riding et al., 1991 questionably included this species in Parvocysta.
Holotype: Riding et al., 1991, pl.1, fig.9
Locus typicus: 62 m from the base of the section at Beinn na Leac, Raasay (NG 598 379)
Stratum typicum: Late Toarcian-Aalenian

Original diagnosis: Riding and Shaw, 1991, p.152
Proximate, acavate dinoflagellate cysts, rounded subtriangular to vaguely subpentagonal in dorso-ventral outline. Shape modified by a deep paracingular constriction. Squat in overall shape, the maximum breadth may exceed the length. Hypocyst broader than epicyst. Epicyst cone shaped, often narrows into a short conical/subconical apical horn. Antapical area flat to slightly concave. Cyst wall thin, smooth. Paratabulation and archeopyle not indicated.

Original description: Riding and Shaw, 1991, p.152
Small dinoflagellate cysts with a rounded subtriangular to rounded subpolygonal ambitus. Epicyst cone-shaped, which may narrow anteriorly to a short, hollow apical horn. The paracingular region is wide and deeply incised. The hypocyst is relatively broad (this dimension may exceed the cysts total height), with a flat to slightly concave antapical region. The autophragm is thin, single-layered and smooth. There are no unequivocal indications of paratabulation and an archeopyle has not been observed.

Dimensions (35 specimens measured):
Minimum Mean Maximum
Overall length: 24.2 - 29.9 - 36.8
Maximum breadth of epicyst: 13.1 - 15.4 - 17.9
Maximum breadth of hypocyst: 23.2 - 27.6 - 31.7


Affinities:
Riding and Shaw, 1991, p.152: Parvocysta? triquetra is the only representative of this genus with a subtriangular dorso-ventral outline (i.e., the epicyst is markedly and consistently narrower than the hypocyst). The species P. barbata Bjaerke 1980, P. bjaerkei Smelror 1987 and P. nasuta are similar in shape but the epicyst and hypocyst are generally equal in breadth; also P. harhata and P. hjaerkei are spinose.
An archeopyle was not discerned in this species thus, until the precise nature of the excystment aperture is found, triquetra is questionably placed in Parvocysta.
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