07:58 GMT15 June 2021
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    Large and heavy rocks known as 'sailing stones' regularly cruise across the dry lake of Racetrack Playa in Death Valley, leaving long tracks behind them – and now scientists have found these tracks in a fossil dating back 200 million years, right across some dinosaur footprints.

    The fossil, which clearly shows the shape of a foot and even the skin texture of a prosauropod - an early sauropod - has been on display since 1896. No one had thought anything of the long, smeared track next to the prints until palaeontologist Paul Olsen of Columbia University examined it closely in 2017.

    Since then, he and his colleagues have been trying to determine the origins of these tracks, presenting their findings at the 2019 Fall Meeting of the American Geophysical Union. They argue that the track we see on this fossil was left by an ancient sailing stone.

    According to Olsen and his colleagues, their discovery could be evidence of briefly freezing temperatures in the tropics during the Early Jurassic period 200 million years ago, when the dinosaur world expanded and mammals started to evolve and diversify at a furious rate. The team came to the conclusion by studying how the stones might travel long distances now.

    Geologists have been fascinated with these skidding boulders for decades, and have come up with two mechanisms: one of which involves ice, and the other – microbial mats. However, the second option was ruled out due to the fossil tracks.

    "When the microbial mat gets thick, it actually shields the mud from the details of the foot," Olsen said, adding that skin textures wouldn't have survived a microbial mat.

    According to research done back in 2014, the movement of the rocks happens when the cooling temperature creates a layer of ice on the water filling the area during the winter. The temperature at night falls enough to create "windowpane" ice, which must be thick enough to maintain strength, but thin enough to be able to move freely. When the sun rises, the ice melts and cracks into pieces, floating on top of the liquid water. These floating panels are blown across the water by surprisingly light winds, pushing the rocks in front of them.

    Due to the ice, rocks weighing up to 320 kilograms can "sail" across the ground, gouging out tracks in the slippery wet mud. Since the wind blows in one direction, the tracks of several stones all end up oriented in the same way.

    The sailing stone-like track in the ancient fossil slab looks a lot like the sailing stone tracks on the Racetrack Playa. It even has striations seen in some modern sailing stone tracks, created by uneven textures on the stone as it goes through – and geologists regard it as a troublesome sign that the tropics had experienced sudden freezing at some point.  The fossil was excavated from Portland, Connecticut – a region that, 200 million years ago, would have been more equatorial, sitting at around 18 degrees latitude with low elevation. Most of the plants and animals in the region at the time were not suited to frosty conditions.

    There is one more possible explanation though. Around 201.3 million years ago, something happened, a mass extinction that marked the beginning of the Triassic period and the beginning of the Jurassic – the Triassic-Jurassic extinction event that killed off 76 percent of all life on Earth. It is believed that it was caused by volcanic activity, which spewed gases into the atmosphere leading to significant cooling – known as a volcanic winter. Such winters can have far-reaching effects – the eruption of Mount Pinatubo in the Philippines in 1991, for instance, cooled global temperatures by an average of 0.6 degrees Celsius for 15 months.

    The researchers believe that the effect of volcanic activity at the Triassic-Jurassic boundary could have tipped the conditions in the tropics to produce sailing stone tracks akin to those we see in the Racetrack Playa today.

    "This may be evidence of the cooling caused by the volcanic winter," Olsen said.

    The team continues their research to find additional evidence for their theory, for example, more sailing stone tracks, especially grouped together. If such tracks are all oriented in the same direction, that would be pretty compelling evidence in favour of the ice-propelled sailing stone interpretation.


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