Fossil evidence of jellyfish
dates back to the Cambrian Period, 500 million years ago. This fossil
jellyfish shows similarity to the modern jellyfish, Cunina (right). It
was one of four different types of jellyfish dated back to the Cambrian
by researchers in 2007. These ancient jellyfish showed the same
complexity as modern jellyfish, meaning they either developed rapidly
500 million years ago, or today’s varieties are much older.
Credit: Fossil photo by B. Lieberman. Cunina photo by K. Raskoff, copyright.
The oldest known fossils of jellyfish have been found in rocks in Utah
that are more than 500 million years old, a new study reports.
The fossils are an unusual discovery because soft-bodied creatures, such
as jellyfish, rarely survive in the fossil record, unlike animals with
hard shells or bones.
"The fossil record is biased against soft-bodied life forms such as
jellyfish, because they leave little behind when they die," said study
member Bruce Lieberman of the University of Kansas.
These jellyfish left their lasting imprint because they were deposited
in fine sediment, rather than coarse sand. The film that the jellyfish
left behind shows a clear picture, or "fossil snapshot," of the animals.
"You can see a distinct bell-shape, tentacles, muscle scars and possibly
even the gonads," said study team member Paulyn Cartwright, also of KU.
The rich detail of the fossils allowed the team to compare the cnidarian
(the phylum to which jellyfish, coral and sea anemones belong) fossils
to modern jellyfish.
The comparison confirmed that the fossils were, in fact, jellyfish and
pushed the earliest known occurrence of definitive jellyfish back from
300 million to 505 million years ago.
The fossils also offer insights into the rapid species diversification
that occurred during the Cambrian radiation, which began around 540
million years ago and when most animal groups start to show up in the
fossil record, Lieberman said.
The complexity of these early jellyfish seems to suggest that either the
complexity of modern jellyfish developed rapidly about 500 million
years ago, or that jellyfish are even older and developed long before that time.
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.
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.
