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Dead Sea fossils hint at earlier start for some ancient plants

Surprising finds unearthed in Jordan suggest that fern and conifer species survived the 'Great Dying' 250 million years ago.

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A well-preserved twig of a podocarp conifer fossil

A fossilized twig from an ancient conifer that was found along the shore of the Dead Sea.Credit: Palaeobotany Research Group Münster

On the banks of the Dead Sea, preserved in 260 million-year-old rock, lie memorials to the resilience of plants in the face of environmental catastrophe.

Unusually detailed plant fossils show that several major lineages survived the world’s greatest extinction event — a cataclysm of climate change and environmental upheaval that took place around 250 million years ago, sometimes called ‘the Great Dying’. Some studies suggest that up to 90% of all land animals went extinct. It is the closest that life on Earth has come to being extinguished.

But there are signs that plants fared better. The fossils, described on 20 December in Science1, suggest that certain plant groups — those that lived in harsh environments and endured seasonal droughts — were particularly well primed to survive environmental cataclysm.

“We believe that’s because they had already been adapted to cope with stress and disturbance,” says lead author Benjamin Bomfleur, a palaeobotanist at the University of Münster in Germany.

Postcards from the Permian

Tracking plant evolution is particularly difficult: it is rare to find fossils that preserve the intricate details of a plant’s key anatomical features, such as its reproductive organs or characteristics of its epidermis. Without those traits, it is impossible for researchers to unambiguously identify the species of a fossil.

But armed with sledge-hammers and pickaxes, Bomfleur and his colleagues unearthed an unusual patch of mummified plants in a rocky outcrop along the eastern shore of the Dead Sea in Jordan. The waxy covering, or cuticle, of the plants was exquisitely preserved, enabling the team to identify the plants on the basis of key epidermal features.

The collection of fossils features the oldest example yet of a member of the plant family that includes modern conifers. The finds also include the remains of an ancient seed fern called Dicroidium, which could help to settle a debate over the species' history.

Dicroidium was long thought to live only in more-southern regions during the Mesozoic era, after the Great Dying. But in 2006, researchers reported Dicroidium fossils near the Dead Sea in Jordan that predated the massive extinction event2 — suggesting that the ancient fern had survived the Permian extinction.

Some scientists disputed those results because the fossils did not contain examples of reproductive organs that some consider crucial for identifying the plant. The latest specimens, however, include pollen- and seed-bearing structures, confirming the 2006 findings.

An isolated frond fragment of an extinct seed-fern

This fragment of a fossilized seed fern was freed from the rock that preserved it using strong acid.Credit: Palaeobotany Research Group Münster

Ancient ancestry

One reason the collection is so important is because it comes from a relatively dry environment, says palaeobotanist Cindy Looy of the University of California, Berkeley. There are few fossils from such environments, which were hotspots of seed-plant evolution.

“It has far-reaching consequences for our understanding of plant evolution and the origins of several of the major plant lineages,” she says. “The major innovations in the seed plants were taking place in drier environments.”

Researchers use such fossils to calibrate models of plant evolution, Looy notes, so the latest finds could affect estimates of when different groups diverged.

The fossils could also help scientists to understand why some plants survived the extinction event, yet others perished. Looy is hopeful that more clues could be unearthed as researchers share data and expand their digs into unexplored regions. “There are not that many palaeobotanists,” she says, “and there’s a lot of planet to cover.”

doi: 10.1038/d41586-018-07826-8

References

  1. 1.

    Blomenkemper, P., Kerp, H., Hamad, A. A., DiMichele, W. A. & Bomfleur, B. Science 362, 1414–1415 (2018).

  2. 2.

    Kerp, H., Hamad, A. A., Vörding, B. & Bandel, K. Geology 34, 265-268 (2006).

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