Living fossils in a petrified forest
James O’Donoghue (UK)
It’s a Welwitschia! Nestling among 280 million year old fossil tree trunks, this is the rare plant I have been searching for. Two wonders of the natural world – the living, sprawling Welwitschia and the ancient, petrified trees – have fortuitously come together here in Namibia. Separated by more than an immense period of time, they also bear witness to completely different environments. Welwitschia is adapted to the extremes of dryness and heat of a desert while the archaic trees once lined a great floodplain and were felled by melting glacial waters. However, in spite of their many differences, these two plants share a hidden secret. Between them, they reveal the fascinating story of a gigantic landmass that broke apart over 100Ma.
The ancient southern supercontinent of Gondwana existed for hundreds of millions of years and embraced South America, Africa, Madagascar, Australia, Antarctica and parts of Asia. By 280mya, vast cool temperate forests of Cordaites trees covered parts of Gondwana. Cordaites is a primitive relative of the conifers that grew to 30m in height and is best known from the coal swamp forests of 325 to 295mya in Europe and North America.
As Gondwana emerged from a great ice age, Cordaites trees lining river floodplains were engulfed by melting glacial waters. This catastrophic flooding resulted in their burial under hundreds of metres of sediment. Over an immense period of time, silica dissolved in groundwater replaced the original woody tissue and turned to quartz. Traces of iron stained the quartz beautiful shades of red, brown and yellow. This process of petrification preserved, in exquisite detail, features such as growth rings, bark and even branch attachments.
Only with the break up of Gondwana were the logs finally re-exposed. Around 120mya, tectonic movements caused South America and Africa to separate and lifted western Namibia by as much as 1,000m. Erosion increased dramatically. Over many tens of millions of years, vast amounts of sedimentary rock were eroded away until the ancient trees once again saw the light of day.
Today, the idea of catastrophic flooding seems about as far removed as you can get from the site of the petrified forest near Khoraxis in north-western Namibia. The land is parched. Vegetation is sparse and stunted. Any plants that do survive have been shaped by millions of years to cope with the harsh conditions. Here and there, sprawling among the petrified logs, is one of the world’s most celebrated plants. Welwitschia mirabilis elicits an extreme reaction from pretty well everyone who sees it. The Daily Mail newspaper recently dedicated a full page feature to Welwitschia, describing it as ‘the most hideous plant on Earth’. It has even had an obscure Belgian film named after it: The Curse of Welwitschia.
Why should an obscure desert plant attract such hyperbole? For a start, it doesn’t look remotely like any other living plant. Nor does it behave like a typical desert plant. It lives only in the Namib Desert and neighbouring dry lands in a narrow strip along the southwest coast of Africa.
Welwitschia can live for 1,500 years and, for the past 150 years, has been baffling evolutionary biologists. Regarded as a ‘living fossil’, it is the sole surviving species in a unique taxonomic family. Darwin himself referred to it as the platypus of the plant world, so strange did it appear to him. Like the platypus, it bears both primitive and advanced features, as well as those found nowhere else. The fossil record of Welwitschia has been threadbare – until now. Thanks to finds made in the past few years, its living fossil tag has been justified as never before.
In 2005, an international team led by David Dilcher of the Florida Museum of Natural History and Mary Bernardes-De-Oliveira of the University of São Paulo in Brazil made an extraordinary discovery in the fossil-rich Cretaceous deposits of Brazil’s 110 million year old Crato Formation. Rocks that had given up pterosaurs, crocodiles, fish and insects now revealed perfectly preserved Welwitschia leaves, seedlings and cones. Both fossil and living plants appeared quite similar.
During the Cretaceous, Welwitschia was far more widely distributed and diverse than it is today. Fossils of Welwitschia-like pollen have now been found at sites all over the world. “The Namibian plants are remnants of a population that must have lived all the way from Brazil to China,” says Dilcher. “They are survivors that have adapted to a unique environment to which other plants have not been able to adapt.”
So how did Welwitschia come to be a relict in one of the driest places in the world?
The ancient Crato environment was nothing like as dry as the Namib Desert. With the fragmentation of Gondwana 120mya, Welwitschia found itself in geographically separated populations. Where South America and Africa had previously been joined together, the southern Atlantic Ocean started to form. As it gradually widened, oceanic circulation patterns caused moisture to blow far inland from the south-western coast of Africa. The narrow strip along that coast that is the Namib Desert was thus born over 80mya, making it the most ancient desert in the world.
At about the same time as the early break up of Gondwana, flowering plants evolved and rapidly diversified. With their ability to grow quickly, they were an immediate evolutionary success story. Over time, much of the slower growing vegetative old guard such as Welwitschia found itself displaced from its old haunts by the fast-growing flowering upstarts. Welwitschia became confined to marginal habitats where flowering plants were less able to thrive, such as the proto-Namib Desert.
Dieter von Willert of the University of Münster is the world’s leading authority on Welwitschia, having spent 30 years studying the plant. He argues that it is full of paradoxes but is supremely well adapted for its environment. Take its large leaves. On the face of it, they don’t make any sense. Most desert plants make do with tiny leaves to minimise water loss. “From an energy point of view it is suicide what Welwitschia is doing because the heat load is so tremendous,” says Von Willert.
He has calculated that, with desert ground temperatures regularly reaching 65 to 70oC, the average sized Welwitschia will lose a litre of water through its leaves every day. This is a tremendous amount to lose in a desert where many months or even years may go by without rain. Yet, it is often the only large plant you will see in the Namib Desert. There must be a good reason why evolution has preserved its large leaves for 100 million years, even though they constantly grow and constantly lose large amounts of water.
It turns out that the leaves provide a finely tuned survival strategy for dealing with unpredictable desert rainfall. To understand why, compare Welwitschia’s leaves with those of deciduous flowering plants, which shed their leaves when conditions are unfavourable. This generally serves them well, permitting their continued survival in seasonally hot or cold conditions where leaves could be easily damaged.
But how is a plant to survive where rainfall is unpredictable and rare, as in the Namib Desert? Routinely shedding leaves in these conditions is not a viable option as the water supply may well have dried up by the time the leaf has had a chance to re-grow. Von Willert explains that Welwitschia balances the growth of its leaf from its trunk against the death of the leaf at it outermost end. This part of the leaf appears frayed and shredded, revealing that it is no longer live tissue and cannot further lose water. When there is a lot of water around, the live green portion of the leaf will grow quickly and expand, permitting more photosynthesis to take place. In severe drought conditions the dead frayed end of the leaf will take over and the live green portion will be reduced to a stump. In this way Welwitschia can always respond immediately when the rare rainy periods occur.
Welwitschia also tops up its constantly diminishing water supply by having tough deep roots that can grow to a depth of 4.5 metres and more. There, it can reach deep aquifers of trapped groundwater. Welwitschia needs water, and lots of it, to thrive. Its seedlings will only survive during very rare wet years in the desert. Von Willert has grown many seedlings and cautions: “If you think Welwitschia doesn’t need to be watered regularly then you will not succeed”.
So did Welwitschia evolve as a desert specialist back in its Gondwanan heyday or was it forced to adapt to desert environments as it became displaced by faster growing flowering plants? Ernst van Jaarsveld of the South African National Biodiversity Institute argues that it probably started out as a plant of the grassy open savannah. Von Willert agrees. The rapid growth of its thick, corky trunk may be evidence for its ancient savannah origins as this kind of trunk would protect against the frequent fires that characterise life there. However, when push came to shove that same trunk was also well adapted for conserving water in the desert.
The immensity of deep time is all around you at Petrified Forest in Namibia. Ancient trees swallowed by a catastrophic flood and freed through a hundred million years of erosion. A living fossil trapped in an island of aridity. Nowhere captures the spirit of Gondwana more evocatively than this desolate spot.
All photographs were taken by the author in August 2006.
Acknowledgements
I am grateful to Professor David Dilcher, Helke Mocke of the Geological Survey of Namibia and Professor Dieter von Willert for their generous cooperation in the production of this feature.
References
Darwin, C. 1861. Letter 609 to J.D. Hooker. In Darwin, F. & A. C. Seward, eds. 1903. More letters of Charles Darwin. A record of his work in a series of hitherto unpublished letters. London: John Murray. Volume 2.
Dilcher, D.L. et al. 2005. Welwitschiaceae from the Lower Cretaceous of northeastern Brazil. American Journal of Botany, 92(8): 1294-1310.
Thomas, D. 29 May 2007. Bloomin’ Ugly. Daily Mail, London.