Tympanicysta Balme, 1980

Afonin, Sergey A., Barinova, Sophia S. & Krassilov, Valentin A., 2001, A bloom of Tympanicysta Balme (green algae of zygnematalean affinities) at the Permian-Triassic boundary, Geodiversitas 23 (4), pp. 481-487 : 484-486

Genus Tympanicysta Balme Tympanicysta stoschiana Balme ( Figs 1 ; 2 A-C, E, F)

EMENDED DIAGNOSIS. — Unbranched, rarely branched filaments of cylindrical to barrel-shaped cells with elliptical to ovate terminal cell. Cell walls smooth. Septa smooth or folded, forming lens-shaped joints. Chloroplasts cylindrical, microreticulate-perforate, lining cell walls. Akinetes thick with constricted chloroplast.

DESCRIPTION

Fragments of filaments from four to five cells long are preserved alongside with dispersed cells. The cell shapes vary from cylindrical thinwalled in vegetative filaments to swollen barrelshaped and thick-walled, supposedly representing an initial dormant stage, or akinete, 70-130 µm long, 43-97 µm wide, lacking median constrictions or segmentation of cell walls. The cell walls are unilayered, smooth, lacking pores. The transverse septa are smooth or, in the thick-walled filaments, folded forming lenticular joints. The terminal cells, when preserved, are swollen, elliptical or ovate, bluntly pointed, about 120 µm long, 56 µm wide. Chloroplasts are preserved as a granular dark matter in the central part of the cells. The better preserved chloroplasts appear as cylindrical membranes with microreticulate-perforate structure lining the cell walls ( Fig. 1A ). A few cells representing the fully formed akinetes are cylindrical to slightly bell-shaped owing to a median constriction, fitting the initial cell, with thick walls, 178 µm long, 65 µm wide.

COMPARISON

In the character of cell joints, these microfossils resemble zygnematalean green algae ( Zygnematophyceae). Classification of extant zygnemataleans is based primarily on the conjugation and zygotic characters that are often lacking even in living material, while their vegetative characters are widely convergent in different genera. In the general morphology, Tympanicysta is most similar to the extant zygnemataceous green algal genera Spirogyra Link and Mougeotia Ag. which have typically unbranched filaments of cylindrical cells that can be swollen at forming thick-walled dormant cells, or akinetes. In the same way, Tympanicysta is represented by two kinds of filaments-slender with cylindrical cells and relatively robust with barrel-shaped thick-walled cells, the latter perhaps overrepresented in microfossil assemblages. Moreover, a typical akinete is found in our material.

Other morphological features of Tympanicysta also match the typical zygnematalean characters. In Tympanicysta , the transverse cell walls, or septa are either smooth or, in the thick-walled forms, folded by invagination of the cell wall. In the extant Mougeotia , the septa are typically smooth, forming a lenticular joint, whereas folded septa more commonly occur in Spirogyra . A granular dark matter in the central part of the cells corresponds to chloroplasts of zygnematalean algae in which it can be either axial or parietal, laminar or star-shaped, but in compressed cells appearing exactly as in the fossil. The better preserved chloroplasts resemble those of the Cladophorales ( Vasser et al. 1989).

Occasional cells in the fossil material are similar to the typical akinetes of zygnematalean algae that are thick-walled dormant cells formed of vegetative cells in adverse environments ( Transeau 1951). A terminal cell shown in ( Fig. 2C ) corresponds in shape to that of filamentous colonies developing from zygospores ( De Bary 1858).

Although the fossil form is close to the Zygnematales in a number of characters, it might also show characters typical of other groups of green algae. Thus a frequent branching, as well as the parietal chloroplasts, occur in Cladophorales rather than Zygnematales . Such unususal combinations of characters are to be expected in Palaeozoic forms.

DISCUSSION

The Zygnematophyceae include widespread extant forms of which Spirogyra is the largest and the most common genus including about 350 species. They typically inhabit relatively clear still or running, sometimes swamped fresh waters forming dense mats but occur also in brackish waters and mineral springs. Conjugation takes place in warm season preferably in warm shoals. Fossil zygnemataleans are represented primarily by their zygospores that appear in the Carboniferous and are common later on ( Van Geel & Grenfell 1996).

If the ecology of extinct zygnemataleans has been comparable to their living representatives, then an increase of these algae in the terminal Permian to early Triassic might have been owing to the rise of ground water level and a widespread ponding of rivers at the early stage of global transgression that started in the late Changhsingian (latest Permian) time ( Yang Zunyi et al. 1995). Incidentally, the fossil plant bed of Nedubrovo locality is an anoxic estuarine deposit with well-preserved terrestrial plant compression material and planktonic unicells. This kind of aquatic environment might have been favourable for zygnemataleans. At the same time, a rapid change of lithofacies indicates an unstable sedimentary environments, with lithologies fluctuating from coaly graystones to redbeds within a few meters of the stratigraphic sequence. Such fluctuations, apparently characteristic of transboundary sequences, might impose environmental hazards on zygnematalean populations enhancing the encystment and formation of thick-walled cells owing to which these algae became conspicuous as fossils.

Figures

FIG. 1. — Tympanicysta stoschiana Balme from the basal Vetlugian of Nedubrovo, Vologda region; A, two vegetative cells with chloroplasts; B, vegetative cell with well-preserved chloroplast. Scale bars: 10 µm.

FIG. 2. — A-C, E, F, Tympanicysta stoschiana Balme from the basal Vetlugian of Nedubrovo and Sholga, Vologda region; A, vegetative cell showing chloroplast and infolded septa at the top; B, vegetative cells with infolded septae; C, swollen terminal cell; E, akinete-like cell; F, vegetative filament; D, Spirogyra sp., extant, from a floating mat of a floodplain pond, Bitsa River, the Moscow River Basin, collected in June 1999, vegetative filament at transition to akinete. Scale bars: A, B, D, E, 10 µm; C, 20 µm; F, 30 µm.