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Sentusidinium varispinosum
Sentusidinium "?varispinosum" (Sarjeant, 1959, p.338–340, pl.13, fig.7; text-fig.6) Sarjeant and Stover, 1978, p.50.
NOW Impletosphaeridium. Originally Baltisphaeridium (acritarch), subsequently Tenua Eisenack, thirdly Sentusidinium, fourthly Sentusidinium?, fifthly Cleistosphaeridium, sixthly (and now) Impletosphaeridium. Questionable assignment: Stover and Evitt (1978, p.81).
Holotype: Sarjeant, 1959, pl.13, fig.7; text-fig.6; Fauconnier and Masure, 2004, pl.49, fig.8.
Age: early Callovian.
Original description as Baltisphaeridium varispinosum: [Sarjeant 1959, p. 338-340]:
Diagnosis:
A species of Baltisphaeridium of spherical shape and having a dense cover of slender spines, sometimes simple, sometimes knobbed, sometimes bifurcate with very short branches. Length of spines equal to about half the shell radius. Spines densely set but separately visible throughout their length under high magnification.
Description:
Shell yellowish-brown in colour, with walls of moderate thickness. The type specimen is somewhat damaged and does not possess its full number of spines: it is estimated that if the spine cover were complete, between 150 and 200 spines would be present around the periphery. The spines are sometimes simple, but more frequently they are knobbed, hammer-headed, or bifurcate (see Text-fig. 6).
Dimensions:
holotype: diameter of shell 48 µm; overall diameter 70 µm; length of spines around 10 µm. Other specimens observed, usually more severely damaged, differed little in dimensions from the type.
Remarks/Affinities:
Only four existing species show sufficient common characters with B. varispinosum to merit detailed consideration. Baltisphaeridium fimbriatum (White, 1844), from the Cretaceous of England and New South Wales, has numerous spines with capitate or furcate apices, but these spines are much shorter, in length equal to about one-fifth of the radius of the shell, and are of very constant length, giving an appearance of an outer shell quite lacking in the outline of B. varispinosum. Baltisphaeridium pilosum (Ehrenberg, 1838), from the Corallian of Poland, and its form nanum (O. Wetzel, 1933), from the Cretaceous of Germany, have entirely simple spines proportionately shorter in relation to body radius than those of B. varispinosum. Baltisphaeridium whitei (Deflandre and Courteville, 1939) [= Xanthidium hisutum White, 1844], from the Cretaceous of England and France, has a much denser cover of simple, hair-like spines, producing what White termed a " furred " appearance. Baltisphaeridium polytrichum (Valensi, 1953), from the Bathonian of France and the Senonian of Western Australia, has spines in comparable density to those of B. varispinosum but entirely simple and of a length equal to or greater than the radius of the shell. The closest comparison is found in the forms from the Kimmeridgian of Dorset attributed to B. polytriehum by Downie, 1957. These are described as having smooth, spherical shells with numerous processes, generally simple but sometimes branching. Examination of Downie's specimens has shown that they differ from B. varispinosum in having a less dense cover of somewhat longer processes, but they appear nonetheless closer to it than to Valensi's species. Relatively few hystrichospheres in proportion to the number of dinoflagellates were present in the Cornbrash fauna. B. varispinosum is the most prominent species, forming up to 30 per cent of the hystrichosphere assemblage: fourteen specimens were seen, plus five obscured specimens probably also of this species.
NOW Impletosphaeridium. Originally Baltisphaeridium (acritarch), subsequently Tenua Eisenack, thirdly Sentusidinium, fourthly Sentusidinium?, fifthly Cleistosphaeridium, sixthly (and now) Impletosphaeridium. Questionable assignment: Stover and Evitt (1978, p.81).
Holotype: Sarjeant, 1959, pl.13, fig.7; text-fig.6; Fauconnier and Masure, 2004, pl.49, fig.8.
Age: early Callovian.
Original description as Baltisphaeridium varispinosum: [Sarjeant 1959, p. 338-340]:
Diagnosis:
A species of Baltisphaeridium of spherical shape and having a dense cover of slender spines, sometimes simple, sometimes knobbed, sometimes bifurcate with very short branches. Length of spines equal to about half the shell radius. Spines densely set but separately visible throughout their length under high magnification.
Description:
Shell yellowish-brown in colour, with walls of moderate thickness. The type specimen is somewhat damaged and does not possess its full number of spines: it is estimated that if the spine cover were complete, between 150 and 200 spines would be present around the periphery. The spines are sometimes simple, but more frequently they are knobbed, hammer-headed, or bifurcate (see Text-fig. 6).
Dimensions:
holotype: diameter of shell 48 µm; overall diameter 70 µm; length of spines around 10 µm. Other specimens observed, usually more severely damaged, differed little in dimensions from the type.
Remarks/Affinities:
Only four existing species show sufficient common characters with B. varispinosum to merit detailed consideration. Baltisphaeridium fimbriatum (White, 1844), from the Cretaceous of England and New South Wales, has numerous spines with capitate or furcate apices, but these spines are much shorter, in length equal to about one-fifth of the radius of the shell, and are of very constant length, giving an appearance of an outer shell quite lacking in the outline of B. varispinosum. Baltisphaeridium pilosum (Ehrenberg, 1838), from the Corallian of Poland, and its form nanum (O. Wetzel, 1933), from the Cretaceous of Germany, have entirely simple spines proportionately shorter in relation to body radius than those of B. varispinosum. Baltisphaeridium whitei (Deflandre and Courteville, 1939) [= Xanthidium hisutum White, 1844], from the Cretaceous of England and France, has a much denser cover of simple, hair-like spines, producing what White termed a " furred " appearance. Baltisphaeridium polytrichum (Valensi, 1953), from the Bathonian of France and the Senonian of Western Australia, has spines in comparable density to those of B. varispinosum but entirely simple and of a length equal to or greater than the radius of the shell. The closest comparison is found in the forms from the Kimmeridgian of Dorset attributed to B. polytriehum by Downie, 1957. These are described as having smooth, spherical shells with numerous processes, generally simple but sometimes branching. Examination of Downie's specimens has shown that they differ from B. varispinosum in having a less dense cover of somewhat longer processes, but they appear nonetheless closer to it than to Valensi's species. Relatively few hystrichospheres in proportion to the number of dinoflagellates were present in the Cornbrash fauna. B. varispinosum is the most prominent species, forming up to 30 per cent of the hystrichosphere assemblage: fourteen specimens were seen, plus five obscured specimens probably also of this species.