Excystment structures

If some acritarchs were resting cysts, comparable to those produced by the dinoflagellates, then the contents must have escaped through an opening, the excystment structure. Excystment structures are not found in all acritarchs but sufficient are known to

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Fig. 9.1 Acritarchs. (a) Leiosphaeridium, x400. (b) Baltisphaeridium, x250. (c) Acanthodiacrodium, x400. (d) Visbysphaera, x700. (e) Diexallophasis, x250. (f) Tunisphaeridium, x345. (g) Micrhystridium, x1200. (h) Ammonidium, x390. (i) Cymatiosphaera, x400. (j) Cymatiogalea, x600. (k) Pterospermella, x330. (l) Leiofusa, x400. (m) Deunffia, x400. (n) Domasia, x400. (o) Ooidium, x450. (p) Veryhachium, x300. (q) Pulvinosphaeridium, x300. (r) Estiastra, x300. (s) Octoedryxium, x300. (t) Polyodryxium, x350. (u) Neoveryhachium, x600. (v) Melanocyrillium, x300. ((a), (i), (j), (l) Redrawn from Mendelson in Lipps 1993; (g) redrawn from Tappan 1990; (c) redrawn from Evitt in Tschundy & Scott 1969; (u) redrawn from Molyneux et al. in Jansonius & McGregor 1996.)

Fig. 9.1 Acritarchs. (a) Leiosphaeridium, x400. (b) Baltisphaeridium, x250. (c) Acanthodiacrodium, x400. (d) Visbysphaera, x700. (e) Diexallophasis, x250. (f) Tunisphaeridium, x345. (g) Micrhystridium, x1200. (h) Ammonidium, x390. (i) Cymatiosphaera, x400. (j) Cymatiogalea, x600. (k) Pterospermella, x330. (l) Leiofusa, x400. (m) Deunffia, x400. (n) Domasia, x400. (o) Ooidium, x450. (p) Veryhachium, x300. (q) Pulvinosphaeridium, x300. (r) Estiastra, x300. (s) Octoedryxium, x300. (t) Polyodryxium, x350. (u) Neoveryhachium, x600. (v) Melanocyrillium, x300. ((a), (i), (j), (l) Redrawn from Mendelson in Lipps 1993; (g) redrawn from Tappan 1990; (c) redrawn from Evitt in Tschundy & Scott 1969; (u) redrawn from Molyneux et al. in Jansonius & McGregor 1996.)

suggest that the style of opening is taxon specific. Excystment openings form six major types. A simple lateral rupture (or cryptosuture) is the most common and comprises a simple, more or less straight suture that does not divide the vesicle completely (e.g. Micrhystridium, Fig. 9.1g). The lateral rupture is similar but has an ornamented border or thickening (e.g. Diexallophasis, Figs 9.1e, 9.3a). A median split divides the vesicle into two roughly equal halves (e.g. Ammonidium, Fig. 9.1h). The trochospiral suture traces a lateral split found in the spindle-shaped acritarchs (e.g. Leiofusa, Figs 9.1l, 9.3c). The epityche opening forms a hemispherical flap of wall and is characteristic of Veryhachium (Fig. 9.1p). A pylome is a circular opening situated above the equator (e.g. Cymatiogalea, Fig. 9.1j). A circinate suture is only found in Circinatisphaera and defines a circular suture coiling in a levorotary direction, often with an attached lid-like operculum. Munium and munitium are apical apertures with denticulate margins commonly found in vesicles fossilized prior to excycstment.

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