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Estiastra
From Williams et al., 2017:
[Estiastra, Eisenack, 1959, p. 201; Emendation: Sarjeant and Stancliffe, 1994, p.50.
Acritarch genus.
Type species: Estiastra magna, Eisenack, 1959 (pl.16, fig.17)]
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Emended description: [Sarjeant and Stancliffe, 1994, p. 50]:
Emended diagnosis:
Acritarchs of stellate aspect, composed of 4-10 processes arising in more than one plane. Processes very broad-based, conical to phalloid in outline; distally they may be acuminate, sometimes with a nipple-like prominence, or (rarely) briefly bifurcate, but they are never blunt or rounded and lack distinct branches. Central portion of vesicle formed by the confluence of process bases. Eilyma composed of one layer or of two layers in continuous contact; process tips may be solid or plugged. Surface psilate, punctate, granulate or pustulose, with or without striae on the processes, but not echinate and without systems of ridges connecting the process bases. Opening, where observed, by cryptosuture; when fully open, a section of the eilyma including one or two processes may be lost.
Remarks: The genera Pulvinosphaeridium Eisenack 1954, Estiastra Eisenack 1959, Palacanthus Wicander 1974 and Rhiptosocherma Loeblich Jr. and Tappan 1978 share the characters of having broad-based processes of large size, whose confluence forms the central body of the vesicle-the central body shape being thus entirely determined by the number, size and shape of the processes. Palacanthus is readily differentiated, as its processes all arise in a single plane; the acceptation of the other three genera, however, has been confused. Loeblich Jr. and Tappan (1978, p. 1284) considered that Rhiptosocherma differed from Pulvinosphaeridium in that the former had a clearer differentiation of the processes from the vesicle and had elongate processes with more varied surface ornament. Using the criteria outlined by Eisenack, Cramer and Díez (1979b, p. XIII-XXX), these differences were not sufficient to justify separate generic status. Consequently, Cramer and Díez (1979, p. 102) treated the former genus as a taxonomic junior synonym of the latter; consequently also, Fensome et al. (1990, p. 434) transferred the type, and only constituent, species of Rhiptosocherma, R. improcera, to Pulvinosphaeridium. However, R. improcera differs from Pulvinosphaeridium, as herein conceived, in having acuminate processes in larger number and with longitudinal striae: the genus Rhiptosocherma is therefore instead considered to be a taxonomic junior synonym of Estiastra.
The emendation here proposed accords with the concepts of Cramer (1970, p. 115) in that the feature utilized to differentiate Estiastra from Pulvinosphaeridium is the acuminate character of its processes. Estiastra differs from Polyplanifer Cramer 1964 in that its processes are distally unbranched and from Stellinium Jardiné et al. 1972 in the lack of ridges connecting process bases. The very large type species of Estiastra, E. magna, is the only one exhibiting an escape structure; among Eisenack's illustrations of the type material is one that certainly shows, and one that may possibly show, opening by loss of one pole (1959, pl. 16, figs. 18 and 19 respectively).
[Estiastra, Eisenack, 1959, p. 201; Emendation: Sarjeant and Stancliffe, 1994, p.50.
Acritarch genus.
Type species: Estiastra magna, Eisenack, 1959 (pl.16, fig.17)]
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Emended description: [Sarjeant and Stancliffe, 1994, p. 50]:
Emended diagnosis:
Acritarchs of stellate aspect, composed of 4-10 processes arising in more than one plane. Processes very broad-based, conical to phalloid in outline; distally they may be acuminate, sometimes with a nipple-like prominence, or (rarely) briefly bifurcate, but they are never blunt or rounded and lack distinct branches. Central portion of vesicle formed by the confluence of process bases. Eilyma composed of one layer or of two layers in continuous contact; process tips may be solid or plugged. Surface psilate, punctate, granulate or pustulose, with or without striae on the processes, but not echinate and without systems of ridges connecting the process bases. Opening, where observed, by cryptosuture; when fully open, a section of the eilyma including one or two processes may be lost.
Remarks: The genera Pulvinosphaeridium Eisenack 1954, Estiastra Eisenack 1959, Palacanthus Wicander 1974 and Rhiptosocherma Loeblich Jr. and Tappan 1978 share the characters of having broad-based processes of large size, whose confluence forms the central body of the vesicle-the central body shape being thus entirely determined by the number, size and shape of the processes. Palacanthus is readily differentiated, as its processes all arise in a single plane; the acceptation of the other three genera, however, has been confused. Loeblich Jr. and Tappan (1978, p. 1284) considered that Rhiptosocherma differed from Pulvinosphaeridium in that the former had a clearer differentiation of the processes from the vesicle and had elongate processes with more varied surface ornament. Using the criteria outlined by Eisenack, Cramer and Díez (1979b, p. XIII-XXX), these differences were not sufficient to justify separate generic status. Consequently, Cramer and Díez (1979, p. 102) treated the former genus as a taxonomic junior synonym of the latter; consequently also, Fensome et al. (1990, p. 434) transferred the type, and only constituent, species of Rhiptosocherma, R. improcera, to Pulvinosphaeridium. However, R. improcera differs from Pulvinosphaeridium, as herein conceived, in having acuminate processes in larger number and with longitudinal striae: the genus Rhiptosocherma is therefore instead considered to be a taxonomic junior synonym of Estiastra.
The emendation here proposed accords with the concepts of Cramer (1970, p. 115) in that the feature utilized to differentiate Estiastra from Pulvinosphaeridium is the acuminate character of its processes. Estiastra differs from Polyplanifer Cramer 1964 in that its processes are distally unbranched and from Stellinium Jardiné et al. 1972 in the lack of ridges connecting process bases. The very large type species of Estiastra, E. magna, is the only one exhibiting an escape structure; among Eisenack's illustrations of the type material is one that certainly shows, and one that may possibly show, opening by loss of one pole (1959, pl. 16, figs. 18 and 19 respectively).