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Areosphaeridium arcuatum
Areosphaeridium arcuatum, Eaton, 1971
Tax. jr synonym of Areosphaeridium dictyostilum (Menendez, 1965) Sarjeant, 1981, according to Sarjeant, 1981. Lentin and Williams, 1985, retained Areosphaeridium arcuatum it as a separate species.
Holotype: Eaton, 1971, pl.3, fig.1; Bujak et al., 1980, pl.2, fig.6
Locus typicus: Whitecliff Bay, Isle of Wight, England
Stratum typicum: Middle Eocene
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Original description: [Eaton, 1971, p. 360-363]:
Diagnosis:
The central body has a sub-circular to sub-quadrate outline and a finely granular surface.
The processes are intratabular and they have solid fibrous stems of variable length and breadth. Each process is distally expanded and bifurcate; the attitude of the bifurcations varies from patulate to recurved. The bifurcations are of variable length, and they vary in breadth from slender with a denticulate distal margin, to broad frequently net-like, with an irregular distal margin. Fenestration may be developed in the process stems producing stem branches which are sometimes distally free.
The tabulation of 4", 6", 6""", 1p, 1"""" is not always fully reflected by the processes. The processes reflecting plates 6", 6""" and 1p are not always present. The cingular zone is typically devoid of processes although occasionally a process may occur on this zone. The antapex is typically offset to the right of the mid-ventral line and bears one process (1""""). Between the postcingular processes and the antapical process there may be one or occasionally two additional processes. The archaeopyle is apical, tetratabular.
Dimensions:
Holotype: central body (without operculum) length 40 µm, breadth 53 µm; processes, length 22-30 µm, breadth 2-5 µm.
Observed range: central body length 29 (without operculum) - 50 µm (complete), breadth, 31-60 µm; processes, length 6-39 µm, breadth 1-8 µm; number of specimens measured, 22.
Discussion:
The central body is frequently dorso-ventrally flattened, with the antapical process (1"""") offset to the right of the mid-ventral line (fig. 4). When present, the additional processes are situated between the antapical process and the postcingular processes. The additional processes are probably antapical or postcingular rather than reflecting distinct plate areas. Two variants of A. arcuatum are recognized based on the arrangement of the processes.
Var. A is distinguished by not having the tabulation of the species fully reflected by the processes (fig. 4). On the apical zone there are four processes (1"-4"). On the precingular zone there are five processes (1"-5"), and plate 6" is devoid of processes. The cingular zone is devoid of processes. On the postcingular zone there are five processes (1"""-5"""), and plate 6""" is devoid of processes. There is always one process reflecting the posterior intercalary plate (1p). The antapex bears one process (1""""). The additional process or processes are situated between the antapical process and the postcingular processes in the vicinity of plates 3""" and 6""". The processes show considerable variation in length and breadth. In some specimens the majority of the processes are slender, and one or two are broad. The broader processes frequently reflect plates 2""" and 4""" (pl. 3 figs. 1, 2). Stem fenestration when present, is frequently developed in only one or two processes (pl. 3 figs. 1, 3). When large fenestrations are present the process stem is branched, although the branches are usually distally united. Var. A is the more common of the two variants.
Var. B (fig. 5) is distinguished by having at least six processes on the precingular zone (1"-6") and on the postcingular zone (1"""-6"""). Fenestration may be developed in the process stems producing stem branches which may be distally free. When seven precingular processes are present, one of the precingular plates bears two processes. The cingular zone is typically devoid of processes although some specimens have a process on this zone. There is one process at the antapex (1""""), but the process reflecting the posterior intercalary plate (1p) is not always present. The central body usually has a sub-circular outline. Var. B is the rarer of the two variants.
Affinities:
A. arcuatum differs from A. diktyoplokus in the overall form of the processes. The distribution of the processes in var. B, the rarer variant of A. arcuatum is sometimes identical to that of A. diktyoplokus, while var. A, the more common variant is distinguished from A. diktyoplokus by not having processes on plates 6" and 6""", and by the presence of at least one additional process on the hypotract. For further comparisons see discussion of Areosphaeridium multicornutum.
Tax. jr synonym of Areosphaeridium dictyostilum (Menendez, 1965) Sarjeant, 1981, according to Sarjeant, 1981. Lentin and Williams, 1985, retained Areosphaeridium arcuatum it as a separate species.
Holotype: Eaton, 1971, pl.3, fig.1; Bujak et al., 1980, pl.2, fig.6
Locus typicus: Whitecliff Bay, Isle of Wight, England
Stratum typicum: Middle Eocene
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Eaton, 1971, p. 360-363]:
Diagnosis:
The central body has a sub-circular to sub-quadrate outline and a finely granular surface.
The processes are intratabular and they have solid fibrous stems of variable length and breadth. Each process is distally expanded and bifurcate; the attitude of the bifurcations varies from patulate to recurved. The bifurcations are of variable length, and they vary in breadth from slender with a denticulate distal margin, to broad frequently net-like, with an irregular distal margin. Fenestration may be developed in the process stems producing stem branches which are sometimes distally free.
The tabulation of 4", 6", 6""", 1p, 1"""" is not always fully reflected by the processes. The processes reflecting plates 6", 6""" and 1p are not always present. The cingular zone is typically devoid of processes although occasionally a process may occur on this zone. The antapex is typically offset to the right of the mid-ventral line and bears one process (1""""). Between the postcingular processes and the antapical process there may be one or occasionally two additional processes. The archaeopyle is apical, tetratabular.
Dimensions:
Holotype: central body (without operculum) length 40 µm, breadth 53 µm; processes, length 22-30 µm, breadth 2-5 µm.
Observed range: central body length 29 (without operculum) - 50 µm (complete), breadth, 31-60 µm; processes, length 6-39 µm, breadth 1-8 µm; number of specimens measured, 22.
Discussion:
The central body is frequently dorso-ventrally flattened, with the antapical process (1"""") offset to the right of the mid-ventral line (fig. 4). When present, the additional processes are situated between the antapical process and the postcingular processes. The additional processes are probably antapical or postcingular rather than reflecting distinct plate areas. Two variants of A. arcuatum are recognized based on the arrangement of the processes.
Var. A is distinguished by not having the tabulation of the species fully reflected by the processes (fig. 4). On the apical zone there are four processes (1"-4"). On the precingular zone there are five processes (1"-5"), and plate 6" is devoid of processes. The cingular zone is devoid of processes. On the postcingular zone there are five processes (1"""-5"""), and plate 6""" is devoid of processes. There is always one process reflecting the posterior intercalary plate (1p). The antapex bears one process (1""""). The additional process or processes are situated between the antapical process and the postcingular processes in the vicinity of plates 3""" and 6""". The processes show considerable variation in length and breadth. In some specimens the majority of the processes are slender, and one or two are broad. The broader processes frequently reflect plates 2""" and 4""" (pl. 3 figs. 1, 2). Stem fenestration when present, is frequently developed in only one or two processes (pl. 3 figs. 1, 3). When large fenestrations are present the process stem is branched, although the branches are usually distally united. Var. A is the more common of the two variants.
Var. B (fig. 5) is distinguished by having at least six processes on the precingular zone (1"-6") and on the postcingular zone (1"""-6"""). Fenestration may be developed in the process stems producing stem branches which may be distally free. When seven precingular processes are present, one of the precingular plates bears two processes. The cingular zone is typically devoid of processes although some specimens have a process on this zone. There is one process at the antapex (1""""), but the process reflecting the posterior intercalary plate (1p) is not always present. The central body usually has a sub-circular outline. Var. B is the rarer of the two variants.
Affinities:
A. arcuatum differs from A. diktyoplokus in the overall form of the processes. The distribution of the processes in var. B, the rarer variant of A. arcuatum is sometimes identical to that of A. diktyoplokus, while var. A, the more common variant is distinguished from A. diktyoplokus by not having processes on plates 6" and 6""", and by the presence of at least one additional process on the hypotract. For further comparisons see discussion of Areosphaeridium multicornutum.