The dried-up Lake Racetrack Playa in California's Death Valley is famous for its unique geological phenomenon. Here, large stones slowly move along the clay bottom of the lake, as evidenced by the long footprints that remain behind them.
And now the same stone paths were found in a completely unexpected place - in a fossil of 200 million years old, on which the footprints of dinosaurs are also well preserved.
The fossil, which clearly shows the shape of the foot and even the texture of the skin of the proauropod, has been on public display since 1896. No one ever gave much importance to the long, smeared footprint next to the prints until, in 2017, paleontologist Paul Olsen of Columbia University took notice.
He and his colleagues have now presented their findings at a meeting of the American Geophysical Union. They claim that the footprint that is visible on this fossil was made by an ancient moving stone.
According to Olsen and his colleagues, their discovery may indicate a brief drop in temperature in the tropics during the early Jurassic period 200 million years ago.
This conclusion is due to the fact that the mechanism of movement of stones is associated with wind and ice. A similar mechanism can be seen at the Playa Racecourse, where the secret of the moving stones was revealed only a few years ago, in 2014.
The area is filled with enough water to form a floating layer of ice on the surface, but not so much that the stones are completely submerged. A thin crust of ice forms on a frosty night. When the sun rises, the ice melts and breaks into pieces that float on the surface of the liquid water. This floating ice, carried away by the wind and under-ice currents, is capable of moving massive rocks. Thus, stones weighing up to 320 kg can "float" on the ground, leaving footprints in the slippery wet mud.
Then, as the water evaporates, the dirt solidifies, leaving tracks similar to those seen in the fossilized print.
The footprint on the ancient fossil slab is very similar to the footprint from the bottom of Lake Racetrack Playa.
So we're back to the team's hypothesis of sudden freezing in the tropics, which actually poses a little problem.
The fossil plate was excavated in Portland, Connecticut, a region that would have been more equatorial 200 million years ago. Most of the plants and animals in the region at that time were not adapted to frosty conditions.
There is a possible explanation for this. About 201.3 million years ago, there was a mass extinction that marked the end of the Triassic and early Jurassic periods and killed 76 percent of all life on Earth.
It is believed that this was caused by volcanic activity that released gases into the atmosphere. In the short term, such volcanic events can lead to a significant cooling - the so-called volcanic winter.
Such winters can actually have quite far-reaching consequences - for example, the eruption of Mount Pinatubo in the Philippines in 1991 caused global temperatures to drop by an average of 0.6 degrees Celsius over 15 months.
The researchers believe that the influence of volcanic activity at the border of the Triassic and Jurassic periods could have influenced the climatic conditions in the tropics and created conditions for the movement of stones, similar to those we see today in Racetrack Playa.
“This could be evidence of a cold snap caused by a volcanic winter,” Olsen said.
To find out more and confidently assert about the causes of the appearance of moving stones 200 million years ago, you need to find more such tracks in fossil rocks, preferably grouped together, and if the tracks go in the same direction, then this will be convincing evidence in favor of the ice theory of moving stones. …