The earliest macrofossils of plants occur in the Silurian Period of Earth history. However, palynology (the study of plant spores, pollen and particulate organic matter in rocks) has pushed the history of plants back to the Middle Ordovician. Newly reported work has documented spores in strata that are dated 8-12 Ma earlier that the previous record holder. Both the research paper (Rubinstein et al. 2010) and the commentary (Wellman 2010) present the findings within a framework of evolutionary transformation. Press coverage puts it this way:
"As land plants matured, they evolved from liverworts into mosses, and then into plants known as hornworts and lycopods. Then ferns appeared before seed plants, of which there are many species today, finally evolved."
Liverworts are ideally suited for colonising barren ground (source here)
The concept of evolution as a 'maturing' trend is itself loaded with cultural baggage. People read into the concept all sorts of ideas that are not explicit or implied by the theory of evolution they espouse. However, landscapes mature, as do ecosystems. By employing the word in an ecological way, we can, perhaps, escape from always viewing the fossil record through evolution-tinted glasses.
The Ordovician spores are referred to as cryptospores because they have some unusual features. Wellman lists seven reasons why the spores should be associated with bryophytes in general and liverworts in particular. Interestingly, the research team found fossilised spores from five different types of liverwort, which is evidence of diversification:
"Spores of liverworts are very simple and are called cryptospores," Dr Rubinstein told the BBC. "The cryptospores that we describe are the earliest to date." These spores, dating from between 473 and 471 million years ago, come from plants belonging to five different genera - groups of species. "That shows plants had already begun to diversify, meaning they must have colonised land earlier than our dated samples," said Dr Rubinstein.
To appreciate the ecological significance of the discovery, we need to remind ourselves of the inhospitable environments that existed in the Ordovician. What land plants could conceivably have survived, let alone prosper, when faced with such arid terrains?
"Colonization of the land by plants presumably occurred in a step-wise fashion starting during the Early Paleozoic with plants at a bryophyte, most likely liverwort, grade of organization. It resulted in acceleration of weathering processes and in the formation of modern terrestrial environments, including structured soils and complex microbial communities; it also profoundly affected carbon cycling, changed the atmosphere composition and irreversibly altered climates."
In such environments, plants lacking stems and roots have significant advantages. Liverworts were able to colonise the land. The lack of a good soil is no disadvantage to a plant without roots, although they do have structures to anchor them to the ground and to absorb water. Their ability to survive both droughts (desiccation up to 90% of water content) and water-logged periods is an asset. The complex process of photosynthesis allows these plants to gain whet they needed to survive and multiply. "Bryophytes assist in the stabilisation of soil crust by colonising bare ground and rocks, and are essential in nutrient recycling, biomass production, and carbon fixing." (source here)
On ecological grounds, the plants that grew in the Ordovician were ideally suited to initiating the colonisation process. They were not there because they were primitive (because there are plenty of complexities if we look for them) but because they were pioneers in the colonisation process. Furthermore, although there is evidence of diversification, the message we need to take home is one of stasis. Having established diversity, the authors of the research paper are constrained to comment that this same diversity is apparent at higher stratigraphical levels. It would appear that diversification was accompanied by stasis. This is not a story of macroevolutionary transformation, but of variations within a basic type. Today there are over 6,000 liverwort species: there is plenty of evidence for diversification, but all of it is within the Liverwort group.
"The assemblage described here includes five cryptospore genera. Both from morphological and systematic points of view, this Dapingian assemblage is not different from younger cryptospore occurrences, including Aeronian (Early Silurian, c. 439-436 Ma) assemblages. This seems to indicate that the evolutionary rate of the earliest embryophytes was extremely low, or that selective pressures did not act on the morphology of propagules."
Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana)
C. V. Rubinstein, P. Gerrienne, G. S. De La Puente, R. A. Astini, P. Steemans.
New Phytologist (October 2010) 188(2): 365-369 | doi: 10.1111/j.1469-8137.2010.03433.x
Summary: The advent of embryophytes (land plants) is among the most important evolutionary breakthroughs in Earth history. It irreversibly changed climates and biogeochemical processes on a global scale; it allowed all eukaryotic terrestrial life to evolve and to invade nearly all continental environments. Before this work, the earliest unequivocal embryophyte traces were late Darriwilian (late Middle Ordovician; c. 463-461 million yr ago (Ma)) cryptospores from Saudi Arabia and from the Czech Republic (western Gondwana). Here, we processed Dapingian (early Middle Ordovician, c. 473-471 Ma) palynological samples from Argentina (eastern Gondwana). We discovered a diverse cryptospore assemblage, including naked and envelope-enclosed monads and tetrads, representing five genera. [snip]
Wellman, C.H. The invasion of the land by plants: when and where? New Phytologist, (October 2010) 188(2): 306-309 | DOI: 10.1111/j.1469-8137.2010.03471.x
Walker, M. Fossils of earliest land plants discovered in Argentina, BBC News, 12 October 2010.
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