Desert Fossils Reveal 540-Million-Year-Old Jellyfish 'Graveyard'

By Mindy Weisberger, Senior Writer | August 7, 2017 06:49am ET

Walk across a beach's wet sand, and you'll leave behind imprints that briefly hold the shape of your feet before blurring and falling apart.

But 540 million years ago, seaside sand contained densely packed microbial communities, which created a gooey glue that was excellent for preserving impressions of ocean creatures left high and dry by retreating waves.

And on a long-gone seashore in what is now arid Death Valley, sticky sand retained impressions of the oldest known example of a jellyfish stranding, saving a fossilized snapshot of Cambrian period marine life that researchers excavated and described in a new study. [Cambrian Creatures Gallery: Photos of Primitive Sea Life]

The ancient jellyfish were preserved in a slab of sandstone found in southeastern California. Scientists identified 13 of these oval specimens on the rocky surface, ranging from 1.2 to 8.3 inches (3 to 21 centimeters) in diameter. The fossils were lighter than the rock surrounding them, and they varied not only in size but also their style of preservation. Some included convex, circular ridges; others held concave rings around a convex interior; and several were fossilized as more pronounced, rounded mounds, the scientists wrote in the study.

In one jellyfish specimen, shapes of some of the animal's body parts were still faintly visible. Additional marks in the rock around the fossilized jellyfish hinted at the movements of ancient currents, which may have pushed and distorted the bodies of the stranded jellyfish prior to fossilization. Other marks might have been made by a stranded jellyfish's attempts to move back into the water, according to the study authors.

Unique conditions for fossil formation

Ancient, soft-bodied animals are exceedingly rare in the fossil record, compared with animals with robust skeletons or shells, the study authors wrote. But a unique combination of environmental conditions can preserve even jellyfish in surprising detail, study lead author and geologist Aaron Sappenfield told Live Science.

Illustrations set the scene for a jellyfish stranding during the Cambrian period. A group of jellyfish were swept toward the shore and were beached by the receding tide.

Credit: Aaron Sappenfield/University of California, Riverside

Jellyfish that wash up on beaches today are frequently eaten by scavenging birds and crustaceans, Sappenfield said. But during the Cambrian period, when marine life was bountiful and diverse, there were no large terrestrial scavengers to pick at the jellies' carcasses. If they became stranded, chances were good that their remains would stay in one place long enough to fossilize, he said.
However, the jellies' preservation was equally dependent on the gummy, microbe-rich sand that they stranded themselves on, which was also a characteristic of the Cambrian period, Sappenfield said.
"A jellyfish lands on the beach — that big, wet sack settles in the sand — and you get this nice impression with really high resolution because of that binding agent," he said.

This jellyfish was likely buried in sand after it became stranded; its body collapsed, and the carcass was preserved in the microbe-rich sediment.

Credit: Aaron Sappenfield/University of California, Riverside

Most of the known fossils of mass jellyfish strandings date to the Cambrian, likely because that period presented these unique conditions — few scavengers, and sticky sand — that enabled fossilization in an organism that was very difficult to preserve, Sappenfield told Live Science.

Solving an ancient puzzle

Early Cambrian fossils such as these are also helping paleontologists investigate a long-standing mystery about a group of bizarre marine organisms known collectively as the Ediacaran biota, which appeared around 575 million years ago and abruptly disappeared from the fossil record around the beginning of the Cambrian period, about 540 million years ago, Sappenfield said.

"Trying to compare types of fossils preserved on either side of the boundary is a very important step, to say if they vanished because preservation conditions didn't favor them, or because of another reason, such as a mass extinction," he explained.

This oval impression is all that remains of a jellyfish that washed up on a beach 540 million years ago, part of a mass stranding in what is now Death Valley.

Credit: Aaron Sappenfield/University of California, Riverside

These "boundary" fossils could offer clues about what factors may have led to dramatic shifts like those that occurred for the Ediacaran biota. And with that information, scientists could better understand how ecosystems today may be affected by changing conditions, such as those driven by human activity, Sappenfield said.

"Minor perturbances to ecology and how global ecosystems behave can manifest in very significant changes in the way the biosphere [places on Earth that harbor life] is structured," he said.
The findings were published online in the July 2017 issue of the journal Geological Magazine.