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Parvocysta tricornuta
Parvocysta? tricornuta 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.11
Locus typicus: 62 m from the base of the section at Beinn na Leac, Raasay (NG 598 379), Scotland
Stratum typicum: Late Toarcian-Aalenian
Original diagnosis: Riding and Shaw, 1991, p.149-150
Proximate, acavate dinoflagellate cysts, longitudinally elongate with a rounded subpentangular to subtriangular dorso-ventral outline. Epicyst an elongate cone, surmounted by a prominent conical to subconical apical horn. The hypocyst has two antapical, laterally-positioned bulbous horns and two hollow, hemispherical protuberances inserted between the horns in the antapical area. Autophragm thin, smooth except the antapical protuberances, which may be scabrate or sparsely denticulate. Paratabulation comprises possible discontinuous low, smooth paracingular ridges. Archeopyle not developed.
Original description: Riding and Shaw, 1991, p.150
Small, elongate, subtriangular to subpentangular dinoflagellate cysts. The epicyst is cone-shaped with a prominent, hollow apical horn. Two antapical, laterally-positioned, distally blunt horns are present on the hypocyst. Between these horns, in the antapical area, two hollow, hemispherical protuberances with low-relief ornamentation are inserted. The lateral sides are markedly convex in dorso-ventral view.
Wall: The autophragm is single-layered, thin and smooth.
Paratabulation: often indicated by low, discontinuous paracingular ridges.
Archeopyle: not observed.
Dimensions (28 specimens measured):
Minimum - Mean - Maximum in µm
Overall length: 48.3 - 62.0 - 76.1
Overall breadth: 27.8 - 43.2 - 51.7
Length of apical horn: 9.1 - 13.0 - 19.2
Length of lateral hypocystal horns: 5.3 - 7.2 - 9.0
Breadth of antapical protuberances: 5.4 - 7.3 - 9.0
Length of antapical protuberances: 4.1 - 5.2 - 6.1
Height of paracingulum: 1.9 - 2.5 - 3.8
Affinities:
Riding and Shaw, 1991, p.150, 152: Parvocysta? tricornuta is questionably placed in the genus due to the apparent lack of an archeopyle. It is hoped that the examination of further material will elucidate the archeopyle style. Prauss (1989) figured a specimen of P. ? tricornuta (as "Reutlingia" cracens) and stated (p. 22) that this taxon had a type I (2a) archeopyle with an attached operculum on the basis of this single specimen. A photomicrograph of this specimen (Prauss, 1989, pl.1, fig.16) failed to demonstrate this adequately, and therefore is not used as sufficient proof to place tricornuta within Parvocysta herein.
Parvocysta? tricornuta is most similar to P? cracens (see Bjaerke, 1980) as the latter has a prominent apical horn and two large, slender antapical lateral hypocystal horns. These horns are, however, normally at least twice as long as those on P. ? tricornuta; Parvocysta ? cracens also lacks the characteristic two antapical protuberances of the former taxon. The archeopyle in P ? cracens is also not precisely determined, see Bjaerke (1980, p.66); type I was reported, but the posterior detail of the operculum is unclear.
Riding and Shaw in Riding et al., 1991 questionably included this species in Parvocysta.
Holotype: Riding et al., 1991, pl.1, fig.11
Locus typicus: 62 m from the base of the section at Beinn na Leac, Raasay (NG 598 379), Scotland
Stratum typicum: Late Toarcian-Aalenian
Original diagnosis: Riding and Shaw, 1991, p.149-150
Proximate, acavate dinoflagellate cysts, longitudinally elongate with a rounded subpentangular to subtriangular dorso-ventral outline. Epicyst an elongate cone, surmounted by a prominent conical to subconical apical horn. The hypocyst has two antapical, laterally-positioned bulbous horns and two hollow, hemispherical protuberances inserted between the horns in the antapical area. Autophragm thin, smooth except the antapical protuberances, which may be scabrate or sparsely denticulate. Paratabulation comprises possible discontinuous low, smooth paracingular ridges. Archeopyle not developed.
Original description: Riding and Shaw, 1991, p.150
Small, elongate, subtriangular to subpentangular dinoflagellate cysts. The epicyst is cone-shaped with a prominent, hollow apical horn. Two antapical, laterally-positioned, distally blunt horns are present on the hypocyst. Between these horns, in the antapical area, two hollow, hemispherical protuberances with low-relief ornamentation are inserted. The lateral sides are markedly convex in dorso-ventral view.
Wall: The autophragm is single-layered, thin and smooth.
Paratabulation: often indicated by low, discontinuous paracingular ridges.
Archeopyle: not observed.
Dimensions (28 specimens measured):
Minimum - Mean - Maximum in µm
Overall length: 48.3 - 62.0 - 76.1
Overall breadth: 27.8 - 43.2 - 51.7
Length of apical horn: 9.1 - 13.0 - 19.2
Length of lateral hypocystal horns: 5.3 - 7.2 - 9.0
Breadth of antapical protuberances: 5.4 - 7.3 - 9.0
Length of antapical protuberances: 4.1 - 5.2 - 6.1
Height of paracingulum: 1.9 - 2.5 - 3.8
Affinities:
Riding and Shaw, 1991, p.150, 152: Parvocysta? tricornuta is questionably placed in the genus due to the apparent lack of an archeopyle. It is hoped that the examination of further material will elucidate the archeopyle style. Prauss (1989) figured a specimen of P. ? tricornuta (as "Reutlingia" cracens) and stated (p. 22) that this taxon had a type I (2a) archeopyle with an attached operculum on the basis of this single specimen. A photomicrograph of this specimen (Prauss, 1989, pl.1, fig.16) failed to demonstrate this adequately, and therefore is not used as sufficient proof to place tricornuta within Parvocysta herein.
Parvocysta? tricornuta is most similar to P? cracens (see Bjaerke, 1980) as the latter has a prominent apical horn and two large, slender antapical lateral hypocystal horns. These horns are, however, normally at least twice as long as those on P. ? tricornuta; Parvocysta ? cracens also lacks the characteristic two antapical protuberances of the former taxon. The archeopyle in P ? cracens is also not precisely determined, see Bjaerke (1980, p.66); type I was reported, but the posterior detail of the operculum is unclear.