Back
Exiguisphaera trichota
Plate 29, Figures 3, 4, 7, 8, 13, 14, 17, 18
Exiguisphaera phragma Duxbury 1979a in HARDING 1990, pl. 12,
figs. 1, 2, 4, 5, 6, 8 only
Holotype: Plate 29, Figures 4, 8.
Type Locality: Speeton Clay Formation, early Barremian, rarocinctum ammonite Zone. Holotype: Slide LB5B(8), E.F. U51.0.
Derivation of Name: From the Greek trichotos, hairy - in reference to the fine sutural and intratabular ornament of this species.
Diagnosis: A small to intermediate, thin-walled, spheroidaldinoflagellate cyst with a sexiform gonyaulacacean tabulation pattern typical of the genus. Sutural crests are formed by solid elements which may be free-standing. These may furcate or anastamose and may intermittently be connected distally by a trabeculum, although distally-free spinose elements are the norm. Intratabular ornament is invariably present, in the form of slender spines, equal in height to those marking sutures. Ornament distribution is patchy, with frequent “bald” areas. The archeopyle is precingular, type 2P, formed by the loss of plates 3" and 4", which are often separately retained within the cyst cavity.
Dimensions: Holotype: 56 × 56 μm.
Overall: 61 (53) 46 μm × 63 (55) 48 μm
Specimens measured: 11
Remarks: Exiguisphaera trichota n. sp. is most similar to Exiguisphaera phragma Duxbury 1979a but differs in its irregular, short, spinose ornament which is unevenly distributed over the cyst surface and along the sutures (see remarks under Exiguisphaera phragma, above). The regular “fence-like” sutural structures observed in E, phragma are not seen in E. trichota, and whatever sutural spine connection occurs distally is sporadic.
The thin-walled nature of E. trichota leads to common cyst distortion, and retention of free (i.e. unconnected) opercular plates within the cyst cavity is a common feature.
Exiguisphaera trichota n. sp. appears to encompass most of the forms assigned to E. phragma in Harding (1990) (see remarks under E. phragma, above). It was recorded between Speeton Beds LB6 and LB4D (early Barremian, variabilis to rarocinctum ammonite Zones), being common at the higher level. This is a range very similar to that quoted by Harding for E. phragma.
Exiguisphaera phragma Duxbury 1979a in HARDING 1990, pl. 12,
figs. 1, 2, 4, 5, 6, 8 only
Holotype: Plate 29, Figures 4, 8.
Type Locality: Speeton Clay Formation, early Barremian, rarocinctum ammonite Zone. Holotype: Slide LB5B(8), E.F. U51.0.
Derivation of Name: From the Greek trichotos, hairy - in reference to the fine sutural and intratabular ornament of this species.
Diagnosis: A small to intermediate, thin-walled, spheroidaldinoflagellate cyst with a sexiform gonyaulacacean tabulation pattern typical of the genus. Sutural crests are formed by solid elements which may be free-standing. These may furcate or anastamose and may intermittently be connected distally by a trabeculum, although distally-free spinose elements are the norm. Intratabular ornament is invariably present, in the form of slender spines, equal in height to those marking sutures. Ornament distribution is patchy, with frequent “bald” areas. The archeopyle is precingular, type 2P, formed by the loss of plates 3" and 4", which are often separately retained within the cyst cavity.
Dimensions: Holotype: 56 × 56 μm.
Overall: 61 (53) 46 μm × 63 (55) 48 μm
Specimens measured: 11
Remarks: Exiguisphaera trichota n. sp. is most similar to Exiguisphaera phragma Duxbury 1979a but differs in its irregular, short, spinose ornament which is unevenly distributed over the cyst surface and along the sutures (see remarks under Exiguisphaera phragma, above). The regular “fence-like” sutural structures observed in E, phragma are not seen in E. trichota, and whatever sutural spine connection occurs distally is sporadic.
The thin-walled nature of E. trichota leads to common cyst distortion, and retention of free (i.e. unconnected) opercular plates within the cyst cavity is a common feature.
Exiguisphaera trichota n. sp. appears to encompass most of the forms assigned to E. phragma in Harding (1990) (see remarks under E. phragma, above). It was recorded between Speeton Beds LB6 and LB4D (early Barremian, variabilis to rarocinctum ammonite Zones), being common at the higher level. This is a range very similar to that quoted by Harding for E. phragma.