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Bizarre sea creatures from 500 million years ago are resurfacing as fossils

By Elizabeth Rayne
Hemirhodon amplipyge trilobite fossil

Prehistoric seas were swarming with strange creatures. After hundreds of millions of years, rare fossils have revealed more than we could ever imagine about them.

Some 518 million years ago, during the Cambrian period, most life was found in the ocean — but we hardly know anything about how the bizarre lifeforms in these waters lived and evolved. Now, scientists who recently discovered the fossil deposit known as the Haiyan Lagerstätte in China have unearthed strange and amazingly preserved sea creature fossils, many of them soft-bodied invertebrates and even juveniles, which may help with understanding both the organisms themselves and how their evolution is related to marine and terrestrial life today.

The excavation was led by paleobiologist Xianfeng Yang of Yunnan University, China, who carefully gathered the fossils of 118 species (including 17 that had previously been unknown) at the site, near the Kunming Gulf. He then analyzed them with Julien Kimmig, who is the collections manager at Penn State’s Mineral Sciences Museum & Art Gallery. They recently published a study in Nature Ecology and Evolution.

These fossils are freaky. Not only were weird forms of life frozen in time, but soft-bodied invertebrates that often decomposed before ever getting a chance to fossilize were a rare find. Some of them were even looking back with 3D eyes.

Bizarre Sea Creatures

“The organisms were likely surprised by whatever event killed them, possibly a change in water oxygen levels and a quick burial by sediment, a catastrophic submarine mudslide caused by a storm, or something similar,” Kimmig tells SYFY WIRE. “To preserve the organisms as well as they are at the Haiyan locality they must have been buried very quickly and cut off from oxygen supply.”

So many juveniles surfaced because the area is believed to have once been a paleonursery that many types of organisms found to be a breeding haven safe from predators. That, or it could have been newly invaded by those creatures not long before the event that trapped them forever. If the abundance of organisms was the result of an invasion, they thrived and kept breeding in that environment, which explains why so many juveniles were found, until something disastrous took down any that could not crawl or swim away quickly enough.

There are so many layers from different time periods in the Lagerstätte that it supports the hypothesis that there were different “boom and bust” periods during which generations of organisms flourished, then died off, with the cycle repeating itself multiple times. What is so unique about this fossil deposit is that every single layer had numerous juveniles. So why did they all perish?

Bizarre Sea Creatures

Next to a mudslide brought on by a tsunami or other storm, other possibilities for the sudden mass death include drastic changes in salt or oxygen levels in the water. Right before the Cambrian explosion burst with new forms of life, the End-Permian Mass Extinction killed off nearly everything that lived in the oceans as the acidification of water became deadly. You don’t usually think of creatures like jellyfish or worms as having much of a brain, but they were obviously capable of creating a paleonursery. According to Kimmig, it might have not required much thinking or planning to begin with.

“Vertebrate and invertebrate organisms have nurseries nowadays, and this deposit shows there is a good possibility that the behavior to keep juveniles safe from predators might have been a very early trade that developed,” he says.

Knowing what could possibly eat you and staying away from that is something just about every living thing on the planet has in common. So is propagating the species. For a species to be successful, it can’t fall prey to anything with an appetite for it, and these animals must have figured out where they couldn't be found for so many juveniles to have been preserved in the same area. Half of all fossilized organisms found there were still going through a juvenile phase. Fossils of juvenile soft-bodied invertebrates are even rarer than those of adults.

Bizarre Sea Creatures

Though fish, worms, insects, proto-crustaceans like trilobites (top photo), and other things that lived in what is now the Haiyan Lagerstätte are now extinct, they could give us a portal into how plant and animal life evolved during the Cambrian period. Some of them are ancestors of extant species we recognize. Others could even be related to us. This has Kimmig excited for continuing his research in the future.

“Before, we were mostly familiar with adult specimens of these early animals,” he says. “Now we can see how they developed from larval stages to fully grown adults, possibly giving us information on how the different animals are related and as such giving us information on how modern animals are related to each other.”