Botanical affinities of Leiospheres

24 July, 2009

 

Leiospheres have generally been considered as resting cysts and their aperture (pylome) has been interpreted as an excystment opening. However, they may also represent vegetative stages of the algal life cycle, as in modern acid resistant microalgae apertures similar to pylomes may develop also in vegetative stages, e.g. as flagellar pores.

There may be no possibility of discriminating between vegetative and resting stages (cysts) in fossil material. The acid resistance and the possibility to be fossilized are widespread in the algae and occur variably in all their life stages, in vegetative cells and also in resting cysts.

 

Apertures

Apertures have a restricted taxonomic significance and they are not useful for the determination of life stages. Vegetative stages of algae and also encysted resting stages are likely to be preserved as fossils. Apertures on vegetative cells, however, cannot always be definitely discriminated from excystment openings. The pylome may be a flagellar pore similar to that of certain modern algae (e.g. Trachelomonas, see below).

If the fossils are of flagellate affinity, every one should have an aperture in its vegetative stage. Forms without apertures may be cysts of the same or different species, but also vegetative stages of unflagelled taxa. When the aperture is an excystment feature, it is present only during one life stage. Cells without apertures then may be closed cysts, but also vegetative stages of other than flagellate affinity. Cells with apertures may be mature cysts, but also vegetative stages of flagellate affinity.

In the original diagnosis of Leiosphaeridia (Eisenack 1958) pylome (aperture) is said to be present. In the following original description is remarked that pylomes are common in some species but rare or absent in others. This variation is proposed to be depending either on specific characters or on the stage of the life cycle of the organisms at the time when they were buried. Microfossils with pylomes were interpreted as obviously abandoned cells in their final life stage, and the function of the opening, was without doubts considered to be making it possible for the protoplast to extrude. Consequently pylomes are not necessarily present on every fossil specimen of a taxon, and their occurrence has no taxonomic value. Only their structure may be a feature important for the classification, possibly limited to the rank of species.

In some taxa a pylome is always present, in others pylomes are never or only occasionally observed with certainty. In “thousands of specimens” of the original material of Leiosphaeridia plicata and L. ralla there was no pylome (Felix 1965), and in 3324 specimens of L. wenlockia there were “one or two” with pylome (Downie 1959).

Generally, leiospheres have been considered as cysts having pylomes and ruptures as excystment features. However, since excystment is a genetically controlled characteristic process that is constant within a taxon, pylomes and ruptures cannot exist both as excystment features in the same species.

 

Environmental modifications

In the fossil record most often it is impossible to elucidate if variability is due to environmentally controlled factors or to different stages of the algal life cycle. Thus, different fossil genera from distant deposits may be only modifications of the same natural species.

Cells that do not differentiate hard parts do not always obtain identical morphological features when they are developed separately in different environments. Except this environmentally controlled morphogenesis the different life stages of the algal cell may generate morphologic variability. Further, in restricted environments disastrous events may occur which may often generate radical changes in the metabolism and morphological appearance of the cells.

A satisfactory classification of modern algae must pay sufficient attention to the morphological variability and life history of each taxon. Many species first described on the basis of specimens representing only one stage of their life cycles were later defined as life stages of other species.

 

Coenobia

I have identified fossil algal coenobia in a sample from the Upper Riphean Mirojedikha Beds in the Turukhansk region, eastern Siberia. The fossils I investigated derive from the Mirojedikha River and were extracted from a sample from the Upper Riphean Mirojedikha Beds.

The Mirojedikha Beds are about 200 m thick shallow water sediments consisting of dolostones, limestones, and shales. The present sample was collected in l967 by my friend, the Late Professor Boris V. Timofeev at the type locality at the Mirojedikha River, four kilometers from its confluence with the Yenisey River, 30 kilometers north of the town of Turukhansk. The sample is a dark-grey, clayey shale which derives from the lower part of the Mirojedikha Beds.

Vesicles of coenobia and solitary leiospheres do not display morphologic differences. The dimensions of vesicles of coenobia correspond to those of larger specimens in the bimodal size distribution of solitary leiospheres from the same sample.

Algal coenobia are sparsely reported as fossil findings. From the Cretaceous-Tertiary they are described as the genus Palambages. The first findings of coenobia in the Palaeozoic were reported from the Silurian of Libya in 1962. There were not any records from the Precambrian until my findings in 1982.

Faulty or rough processing may be the reason for the scantiness of reports of fossil colonies. Maybe most samples yield colonies rather than single specimens if they are treated with gentle care. Clusters, flakes, and different kinds of agglomerations of uniform fossil microalgae have been reported as colonies though they do not strictly represent colonies.

A coenobium is a definitely integrated colony composed of a certain number of cells which are determined during its early development; after the embryonic phase cell-division does not occur until the following reproductive phase. In coenobia the joints and sheaths are usually resistant and likely to be preserved as fossils. However, unicell production and colony formation are influenced by environmentaly controlled factors.

In the algae there is a parallel development from simple to more elaborate forms so that practically all types of cellular and colonial organizations have their counterparts in different classes. The characters necessary for discrimination between classes are not preserved as fossils. Consequently, relating fossil coenobia to modern algal taxa is impossible.

I referred both coenobial and solitary forms to a species with the new combination name Leiosphaeridia asperata (Naumova) Lindgren. Kildinella hyperboreica is not the correct name of this taxon. Protoleiosphaeridium, Kildinella, and Polyedrosphaeridium are congeneric with Leiosphaeridia, as well as the not validly published genera Leiopsophosphaera, Macroptycha, and Scaphita.

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Morphology and taxonomy
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Diagnostic features in modern algae
Morphometry of modern and fossil algae
Genus Leiosphaeridia
Distribution of Leiosphaeridia

References

Downie, C., 1959.
Hystrichospheresfrom the Silurian Wenlock Shale of England. Paleontology, 2: 26—71. London.
Eisenack, A., 1958.
Tasmanites Newton 1875 und Leiosphaeridia n.g. als Gattungen der Hystrichosphaeridea. Palaeontographica Abt.A, 110: 1—19. Stuttgart.
Felix, C.J., 1965.
Neogene Tasmanites and leiosphere from southern Louisiana, U.S.A Palaeontology, 8: 16—26. London.
Lindgren, S., 1981.
Remarks on the taxonomy, botanical affinities, and distribution of leiospheres. (Summary in Russian) Stockholm Contrib. Geol., 38(1): 1—20. Stockholm. ISBN 91-22-00500-5. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.
Lindgren, S., 1982.
Taxonomic review of Leiosphaeridia from the Mesozoic and Tertiary. Stockholm Contrib. Geol., 38 (2): 21—33. Stockholm. ISBN 91-22-00502-1. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.
Lindgren, S., 1982.
Algal coenobia and leiospheres from the Upper Riphean of the Turukhansk region, eastern Siberia. Stockholm Contrib. Geol., 38 (3): 35—45. Stockholm. ISBN 91-22-00504-8. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.
Lindgren, S., 1982.
A new taxon of Leiosphaeridia (algae) from Upper Cretaceous clays, southern Sweden. Stockholm Contrib. Geol., 37 (11): 139—143. Stockholm. ISBN 91-22-00487-4. ISSN 0585-3532.
Lindgren, S., 1984.
A new taxon of Leiosphaeridia (algae) from Upper Cretaceous clays, southern Sweden. Stockholm Contrib. Geol., 39 (5): 139—144. Stockholm. ISBN 91-22-00517-x. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.

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Diagnostic features in modern algae comparable to Leiosphers

24 July, 2009

 

Chlorococcales

The features considered to be of greatest taxonomic importance at the rank of genus in the modern Chlorococcales are chromatophores, pyrenoids, and zoospores. None of these are preserved in fossils. Modifications due to environmental influences cannot always be discriminated from hereditary variability by microscopic analyses alone. In preserved modern material it is impossible to extract the various stages necessary for distinguishing genera and species. That is also a major difficulty in mixed cultures of living material—and of course in fossil material. Thus, cultivation is the essential method for taxonomic classification of many unicellular modern algae.

The fossil leiospheres lack all the characters mentioned above, which are necessary for their classification in modern algal taxa—even at the rank of class. It is evidently impossible also to discriminate between different morphological appearances due to ontogenetic variability and environmental modifications. Consequently the leiospheres must be arranged in an artificial system without biological implications, but as far as possible according to biological principles.

Cysts are metabolically dormant cells with a considerably higher resistance than vegetative cells to detrimental physical conditions, other than heat. Desiccation resistance may be a survival attribute in nature, and this property seems to be related to the outer cell wall.

The size is not a sole diagnostic character. Anyhow related modern taxa (genus, species) seem to have a limited size range so measurements may be important features for determination of modern algae. I have compared the size distribution in Leiosphaeridia to vegetative stages of modern unicellular Chlorococcales.

Cyanophyceae

In the Cyanophyceae the normal reproductive cell, the akinete, is a spherical or cylindrical cell with a smooth or granulated surface. It is characterized by a considerable increase in size compared with the vegetative cell, and before maturation it will expand still more. The ultrastructure is similar to that of the vegetative cell, but with a new outer wall added. Thus the akinete can be regarded as an enlarged vegetative cell enclosed by a thick outer envelope.

Akinete-like cells with thickened walls develop also in the Chlorophyceae, but conditions associated with the production of this stage were not sufficiently investigated.

Akinete germination occurs in different ways. Frequently it generates the rupturing of the outer wall. The excystment mechanism is a true taxonomical feature. According to genetical control, some taxa will dehisce by an aperture (pylome), others by a rupture. However, this is recognizable only in certain stages of their life cycles.

 

Modern algae Trachelomonas

My laboratory investigations of the modern euglenophycean algae Trachelomonas volvocina showed that the lorica of these specimens is acid resistant and that after treatment in acids their flagellar pore is similar to the pylome of leiospheres. The living cells are spherical but they collapse during the acetolyzis process and form folds similar to those said to characterize the fossil genus Kildinella (=Leiosphaeridia).

Trachelomonas is characterized by a lorica that is often iron impregnated, with an apically located flagellar pore, and a very long flagellum. The species are discriminated by shape, ornamentation, processes, and flagellar pore features. A series of 447 illustrations (Conrad & van Meel 1952) of Trachelomonas species shows that shape, ornamentation, processes, and aperture characteristics are not significant taxonomic features at the rank of genus.

About 250 species of Trachelomonas are known. The cells show a considerable variability in size and shape, which makes the taxonomy of the group complex. This variability is also due to different stages of the life history of the organisms and to environmentally controlled morphogenesis. In acidic water some species may have a hyaline, smooth lorica, whereas the same species in alkaline water have thickened lorica impregnated with iron- and manganese-compounds. Exposed to unfavourable environmental conditions some species of Trachelomonas will develop into round, thick-walled resting cells that may be difficult to discriminate from specimens of other genera, even of different classes, e.g. the chlorophycean genus Chlamydomonas.

Specimens of Trachelomonas shown on the internet.

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Morphology and taxonomy
Botanical affinities
  FORWARD TO


Morphometry of modern and fossil algae
Genus Leiosphaeridia
Distribution of Leiosphaeridia

References

Conrad, W. & van Meel, L., 1952.
Matériaux pour une monographie de Trachelomonas Ehrenberg, C., 1834, Strombomonas Deflandre, G., 1930 et Euglena Ehrenberg, C., 1832, Genres d’Euglénacées. Mém. Inst. R. Sci. Nat. Belg., 124: 1—176, pls. 1—13.
Lindgren, S., 1981.
Remarks on the taxonomy, botanical affinities, and distribution of leiospheres. (Summary in Russian) Stockholm Contrib. Geol., 38(1): 1—20. Stockholm. ISBN 91-22-00500-5. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.
Lindgren, S., 1982.
Taxonomic review of Leiosphaeridia from the Mesozoic and Tertiary. Stockholm Contrib. Geol., 38 (2): 21—33. Stockholm. ISBN 91-22-00502-1. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.
Lindgren, S., 1982.
Algal coenobia and leiospheres from the Upper Riphean of the Turukhansk region, eastern Siberia. Stockholm Contrib. Geol., 38 (3): 35—45. Stockholm. ISBN 91-22-00504-8. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.
Lindgren, S., 1982.
A new taxon of Leiosphaeridia (algae) from Upper Cretaceous clays, southern Sweden. Stockholm Contrib. Geol., 37 (11): 139—143. Stockholm. ISBN 91-22-00487-4. ISSN 0585-3532.
Lindgren, S., 1984.
A new taxon of Leiosphaeridia (algae) from Upper Cretaceous clays, southern Sweden. Stockholm Contrib. Geol., 39 (5): 139—144. Stockholm. ISBN 91-22-00517-x. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.