How the earliest plants made our world muddy

Originally published at Lost Worlds Revisited for the Guardian Science Blog Network on 23 March 2018:

https://www.theguardian.com/science/2018/mar/23/how-the-earliest-plants-made-our-world-muddy

The first plants to make it on to land altered mud production and where it formed rocks, changing our planet forever

How and when the earliest plants made the first move on to land is always a hot topic for palaeobotanists. We know that early land plants likely evolved from freshwater algae, gaining a bunch of necessary adaptations in the process. Plants needed to support themselves, protect themselves from drying out and from the harmful effects of UV light, and gain water and nutrients from a finite supply on land. A study published last week by Mariusz Salamon and colleagues described fossils that push back the earliest evidence of land plants to around 445 million years ago.

The new fossils come from mudstones in central Poland, in beds that have been dated using other, much more common and cosmopolitan, fossils. The plant remains are tiny, branched fragments, up to about 3mm long. Some specimens appear to have spore-cases at the top of their branches, similar to those seen in younger, better-known early land plants such as Cooksonia. The preservation of the plants means details are hard to discern, but Salamon and colleagues present a single, tantalising stoma, or air pore, on one of the fragments as a key piece of evidence. If this plant had stomata for gas exchange, it was likely to have been living in land, a good 15 million years earlier than previously known plant fragments.

A reconstruction of Cooksonia, one of the earliest known land plants. Photograph: Matteo De Stefano/MUSE via Wikimedia Commons

Of course, you can come at the problem from the other direction. What global changes would you see in the rock record that could be explained by the activities of plants? This is the same kind of approach taken by geologists looking for evidence of life on other planets. What changes would you see preserved in rocks that cannot be explained away by inorganic processes, and by land plants in particular?

William McMahon and Neil Davies took this approach in a recent study, looking at the relationship between the buildup of muddy sediments on land and colonisation by the earliest plants. It has long been assumed that until plants colonised the land, most fine-grained sediments eroding from the continents were washed by river systems into the sea, where they formed marine mudrock. Once land plants became established, changes to both how rocks weathered and how fine, muddy particles were trapped by plants meant that muddy sediments could accumulate on land in much greater quantities. This long-held assumption was crying out for some testing with hard data.

McMahon and Davies assembled data from more than a thousand published reports and made more than a hundred field investigations, looking at rocks formed by flowing water on land, known as alluvial formations. Their database comprised all 704 known alluvial formations, ranging from the Archean eon 3.5 billion years ago to the Carboniferous period, a mere 300 million years ago.

Their analysis of the proportion of mudrock (made of grains 0.063mm or smaller) in alluvial deposits agreed with long-held geological hunches. For the first 3 billion years of the rock record, mudrocks form an average 1% of sedimentary deposits on land. By around 300 million years ago, mudrocks had risen to an average of 26% of alluvial formations. They pinpoint an upsurge between the Late Ordovician period (458 Ma) and the Devonian period (359 Ma) – which agrees remarkably well with the fossil record for earliest colonisation by land plants.

No tectonic or other purely geological events seem to explain the upsurge in alluvial mudstone, but three mechanisms associated with the appearance of early land plants would have boosted muddy deposits on land. First, plants would promote the weathering of rocks to fine clay minerals, and as rooting systems evolved, and symbiotic relationships with microbes increased, the depth of the uppermost layer of rocky soil in which chemical weathering took place would increase. In these ways, plants were adding to the production of fine-grained sediments. Second, roots would have had a binding effect, helping to retain fine-grained sediments and preventing erosion. The root systems of the earliest land plants were limited, though, so this explanation cannot be the whole story for the Late Ordovician to Devonian muddy upsurge. The final mechanism that may well have been pivotal in creating a muddy planet is the baffling effect that even tiny early land plants would have produced above ground, trapping fine sediments between their stems, leaves and other organs. This effect can be seen today: in environments where liverworts and mosses are the only plants that can make a living by forming a low ground cover, fine-grained mud and silt are incorporated in their clumps.

Once they were established more than 400 million years ago, rates of mud production and retention were permanently altered and mud became a permanent fixture of the rock record on land, thanks to the work of tiny plants over deep time.

Exquisitely preserved fossil deposit a window on early life on land

Originally published at Lost Worlds Revisited for the Guardian Science Blog Network on 15 March 2017:

https://www.theguardian.com/science/2017/mar/15/exquisitely-preserved-fossil-deposit-is-a-window-on-early-life-on-land-rhynie-chert

The Rhynie Chert fossil deposit in Scotland is just over 400 million years old and reveals secrets of life’s conquest of the land

How life made the move onto land is one of the big questions for palaeobiologists. The physiological challenges were immense, and affected most facets of life. Organisms needed to adapt how they gained water (and prevented themselves drying out), how to obtain nutrients, how to exchange gases with the atmosphere, how to support and move a body without water buoyancy, not to mention reproducing out of water.

Life doesn’t have a grand plan, and terrestrialisation (as the process is known) was a messy, incremental process, with different organisms adapting to life on land at different paces. It’s a bit like going camping with a gang of friends. There will be the keen, pioneering types who like to get there early, set up camp and resolve any problems. Then there’s the late-comers, who roll up with the beer when most of the hard work has been done. The popular image of a lobe-finned fish lolloping up a beach to conquer the land is attractive, but there’s plenty that happened on land before the tetrapods joined the party.

There are clues to the earliest stages in the timeline of the move onto land. Microbial mats had probably been sliming the margins of water bodies since the Precambrian, more than 500 million years ago. Fossil soils (known as palaeosols) from about 460 million years ago suggest that organic matter was being deposited and then recycled by microbes, arthropods were burrowing, and clumps of plants were even stabilising mature soils (but with no direct evidence of the plants themselves). Microfossils preserved in near-shore sediments also tell us that plants were already producing spores (an adaptation to life on land), and dispersed plant tissues show that early land plants had already evolved by that point.

 The Rhynie Chert: a snapshot of the earliest ecosystems on land from just over four hundred million years ago. Photograph: James St. John

We are very lucky to have a snapshot of the earliest ecosystems on land from just over four hundred million years ago, in an exquisitely preserved deposit known as the Rhynie Chert, from Rhynie, a village in Aberdeenshire. A scientific meeting last week at the Royal Society in London brought together a global community of Rhynie Chert researchers to share their collective knowledge, on the centenary of the first scientific publication on the Chert by palaeobotanists Kidston and Lang.

Four hundred million years ago, Scotland was sitting just south of the Equator, and the upland area that became Rhynie would have looked similar to today’s Yellowstone Park. Hot springs rich in minerals would have dominated the landscape. We know from modern equivalents that the vent pools themselves would have been too hot and alkaline to harbour and preserve life, but as you move away from the vents, out towards cooler, less alkaline (but still inhospitable) geothermal wetlands, you see a sequence of different organisms making the best of an extreme habitat. The wonderful thing about the geothermal setting at Rhynie is that the precipitation of silica from the hot spring water rapidly preserved organisms in life position, and with cellular detail.

Artistic reconstruction of the Lower Devonian terrestrial plants from the Rhynie Chert. A.Rhynia gwynne-vaughanii B. Aglaophyton major C. Ventarura lyonii D. Asteroxylon mackiei E.Horneophyton lignieri F. Nothia aphylla Illustration: Falconaumanni

Preserved in those precious lumps of chert are everything from bacteria, fungi and algae, to early land plants and their spores, and animals including nematode worms, arachnids, centipedes, springtails, silverfish and harvestmen. The oldest known lungs belong to Palaeocharinus, a spider-like trigonotarbid. Another arachnid, the harvestman Eophalangium, briefly held the record for the world’s oldest known penis. The Rhynie Chert also preserved aquatic arthropods in exquisite detail, allowing the detailed structure of crustacean mouthparts to be studied to below a thousandth of a millimetre. Painstaking work using stacks of images to reconstruct larval crustaceans from the nearby (and contemporaneous) Windyfield Chert has resulted in a remarkable amount of knowledge about life within the hydrothermal waters, as well as above them.

The information that we have been able to glean about early land plants from the Rhynie Chert is the really astonishing thing. The early land plants at Rhynie formed the equivalent of modern-day ‘cryptogamic groundcover’: algae, lichens and mosses living in intimate association in the very top-most layer of soil.

A reconstruction of a nauplius crustacean larva from the Windyfield chert. Photograph: Joachim T. Haug et al/Palaeontologica Electronica

Modern spore-producing plants such as mosses, liverworts and even ferns all have life cycles which alternate between a gametophyte form of the plant, with a single copy of the genome and which produces gametes, and, following fertilization, a sporophyte phase with two copies of the genome, which produces and releases spores, from which the gametophyte emerges… and so on. Sporophyte-phase plants from Rhynie such as Rhynia, Aglaophyton and Horneophyton, with root-like rhizoids and naked branching axes (leaves had yet to evolve) were first described a hundred years ago. Remarkably, their equivalent gametophyte forms have all also been identified. In modern spore-producers the emphasis is either on the gametophyte phase (in mosses and liverworts) or on the sporophyte phase (in ferns) but in the Rhynie ecosystem, both phases were independent, free-living plants of similar complexity. This could mean that the Rhynie plants are the sister group to modern vascular plants, the group which includes the ferns and all other ‘higher’ plants with water-conducting tissues.

The release of sperm cells from the male gamete (antheridium) from the gametophyte form of Aglaophyton. Photograph: Hans Kerp

Rapid preservation in mineral-rich fluid has even captured near-impossible moments in a plant life cycle: the release of sperm cells from the male gamete (antheridium) from the gametophyte form of Aglaophyton, known as Lyonophyton. The details of intimate interactions between early plants and other organisms are also captured: mycorrhizal fungi living in symbiosis inside the plant tissues, 400 million years ago, can be discerned.

The biggest and most complex plant from the Rhynie Chert, Asteroxylon mackiei, almost has leaves. The plant resembles a modern clubmoss, but its leaf-like structures do not have any of the plumbing expected of true leaves and neither do its root-like rhizoids. Asteroxylon does, however, have water-conducting tissues in its stems, and the beautiful cross-sections found in thin sections of Rhynie Chert look uncannily like those of modern clubmosses.

Transverse section of a stem of Rhynia. Photograph: Paul Kenrick

One question which remains after a hundred years of Rhynie Chert research is how typical the ecosystem is for the Early Devonian, 400 million years ago. Hydrothermal wetlands are highly specialised ecosystems today, and a comparison with plant spores found in other rocks of equivalent age suggests that the Rhynie plants may represent the hardcore subset of pioneering early land plants which could survive in such a challenging environment. Exciting experimental molecular work with the modern equivalents to the Rhynie plants (such as the liverwort Marchantia and the moss Physcomitrella) is approaching the same question from the opposite direction: which genes which have been conserved in these plants are key to understanding how they adapted to life on land.