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Plants, as is true of the animals, did not appear on the land from one day to the next. At the origins of land based vegetation is a green alga which, much in the way of the amphibian which little by little freed itself from an aquatic lifestyle, played an essential bridging role which allowed the plant kingdom to emerge from the water. This emergence of the plants was, make no mistake about it, a major event in the development of life on the planet’s surface, notably through the independence granted the animal kingdom as regards the aquatic environment. But during which period in the history of the Earth did this crucial evolution take place? Up until recently scientists dated the transition to 460 million years ago (Read the article Plant life on earth earlier than originally thought). Nevertheless the analysis of Argentinean shale carried out by Philippe Steemans, a palynologist at the University of Liege, and his colleagues has once again brought to life the controversy over the appearance of the first embryophytes.
Embryophytes is the term used to designate land plants, in other words plants which have succeeded in successfully adapting to life on land. An important characteristic of embryophytes is their capacity to synthesise sporopollenin, an extremely resistant polymer which is found in the walls of the dispersion cells such as spores and pollen grains, and thanks to which, once the plants emerged from the water, the spores were able to disperse, protected from ultraviolet radiation, drying out and from attack by micro-organisms, triggering the first colonisation of the land masses. A colonisation which dates back to earlier than foreseen!
In effect, based on the study of fossilised cryptospores (the spores of basement membrane land plants) produced by hepatica (small land plants), and discovered in Argentinean shale, the first embryophytes are now thought to have appeared 471 to 473 million years ago. In other words some ten million years or so earlier than the previously estimated date, based on fossilised cryptospores found in Saudi Arabia and the Czech Republic. That at least is what is explained in the article (1) published in October, 2010, in the journal New Phytologist and written by palaeontology researchers, including Dr C. Rubinstein, a paleo-palynologist (a geologist who specialises in the study of organic walled micro-fossils such as spores, pollen, dinoflagelletes, etc) at the University of Mendoza in Argentina, and Doctors P. Gerrienne and P. Steemans, respectively a paleo-botanist (a scientist who studies plant fossils) and a paleo-palynologist at the University of Liege.
(1) C. V. Rubinstein, P. Gerrienne, G. S. de la Puente, R. A. Astini and P. Steemans. Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana). New Phytologist (2010) 188: 365–369
The terrestrial colonisation by plants is thus thought to have occurred during the Palaezoic era, and more precisely the Middle Ordovician (461-471 million years ago), with the hepatica. These small sized plants, some centimetres in length, generally live in damp and shaded places. They belong to the bryophytes group, which are characterised by the absence of a vascular system, in other words conductor tissues which in more developed plants allow the transport of sap and thus the ‘feeding’ of the plant. The bryophytes group contains mosses, hepatica and anthocerotes.
The discovery of an early arrival of plants on the land is an important evolutionary breakthrough in the study of the geobiological history of our Earth. In effect land plants contributed to important climate changes, through an impact on the carbon cycle and on the composition of the atmosphere, but also to changes and modifications in soil structures, and finally enabled other species, notably animals, to evolve and conquer the continental spaces. The dating of all these phenomena, closely linked to the appearance of plants, must thus also be reviewed.
The discovery so flabbergasted Dr. C. Rubenstein, behind this study of the cryptospores of fossilised hepatica, that the scientist subjected his samples to a second expert appraisal, carried out by Dr. P. Steemans. The first results obtained in the Argentinean laboratories were confirmed in Belgium.
But how do scientists arrive at such results? By what method do they manage to date the genesis of land plants to within a few million years?
Fragments of the shale showing on the surface of the Rio Capillas canyon, Sierras Subandinas, in North West Argentina, were first of all gathered. For this to be done the samples are carefully chosen, notably on the basis of their colour (the darkest compacted clays are more likely to contain organic fossils), then dissolved in acids such as hydrofluoric and hydrochloric acid. The residues are then dried, filtered and placed on a slide to be studied through a microscope. In this way ‘cryptospores’ can be identified in the samples. What are we dealing with here? Spores, as was explained earlier, are plant cells which allow the plant to be dispersed. In the case of basement membrane land plants one more simply speaks of cryptosopres, similar to the spores of modern plants, with the slight difference that they do not possess a dehiscence structure, a structure which allows the cell to be opened.
How can the age of the cryptospores be subsequently determined? This is done in an indirect manner, thanks to the presence of other micro-fossils, such as chitinozoans, organisms which are still poorly understood and doubtless belonging to the animal kingdom, or acritarchs, supposedly belonging to the plant kingdom. The known age of these micro-fossils allow the layer sampled to be dated with precision. The chitinozoans are in this case the subtlest indicators.
There was an extra surprise for the researchers. The samples analysed contained cryptospores with origins in several ‘types’ of hepatica. This would seem to indicate that the plants had already started to diversify at this time, and thus the colonisation of the land masses would have occurred even before the estimated date.
It remains to be seen if the spores analysed indeed come from land plants, and not from aquatic plants. To this question Dr. Gerrienne and Dr. P. Steemans reply that the ‘spores’ of algae are not preserved, as contrary to the spores of land plants they are lacking the resistant polymer which is sporopollenin. It thus does indeed seem that we can conclude that plants appeared on the Earth earlier than had been believed up until now.
© Université de Liège - http://www.reflexions.uliege.be/cms/c_31026/en/land-plants-have-suddenly-aged?printView=true - September 22, 2020