Half a billion years before humans came up with the term "baby wearing," a shrimp-like animal was already swimming through ancient seas, carrying its eggs protectively under its carapace as it looked for food and dodged predators.

Waptia fieldensis was a crustacean that lived in tropical seas 508 million years ago, during the Cambrian period, in tropical seas that are now Canada's famous Burgess Shale fossil site in Yoho National Park in southeastern B.C.

The eggs, as seen in this diagram, are each about two millimetres in diameter and were attached under the shell on either side of Waptia's thorax, but outside its body. (Danielle Dufault)

Waptia fossils found with eggs tucked under their shells represent the oldest direct evidence of animals caring for their young, says Jean-Bernard Caron, lead author of a report describing the find in the journal Current Biology.

Caron is the curator of invertebrate paleontology at the Royal Ontario Museum in Toronto and an associate professor of earth sciences and ecology and evolution at the University of Toronto.

Waptia lived 508 million years ago, during the Cambrian period, in tropical seas that are now Canada's famous Burgess Shale fossil site in Yoho National Park in southeastern B.C. (John Niddrie/Parks Canada)

Normally, he said, paleontologists can't describe much more than the physical appearance of fossil specimens, as information about their way of life is rarely preserved.

"That it gives us this information is really amazing," he said. "This is a window into the ecology of the animal that we didn't expect to find."

He suggested that the eggs likely identified the animals carrying them as females, as eggs are usually carried by the female in modern arthropods, a group that includes insects, spiders and crustaceans such as shrimp.

The eggs, each about two millimetres in diameter, are highlighted in this scanning electron microscope image of Waptia, which grew to about seven centimetres long. (Royal Ontario Museum)

Many crustaceans, such as lobsters, shrimp and ostracods, still carry their eggs around today, but Caron said it was surprising to see that the strategy evolved so long ago.

The discovery suggests Waptia lived in an environment with lots of stresses, such as predators or poor water quality, that required it to take special measures to ensure its young survived.

Waptia, which grew up to seven centimetres long, was one of the largest animals living in the ancient seas that became the Burgess Shale. It looked a bit like a shrimp with a shell on either side of its thorax and a hinge joining them. It had legs built for swimming, but likely lived close to the sea bottom, which was teeming with organisms such as sponges and worms.

Caron said Waptia was probably a scavenger or filter feeder, as it doesn't have any big claws that would mark it as a predator.

Caron first noticed little balls that looked like eggs on Waptia fossils when he was a graduate student. The balls, each about two millimetres in diameter, were attached under the shell on either side of Waptia's thorax, but outside its body. There were up to 12 eggs per side. Years later, Caron decided they were worth a closer look.

Rare find

Among hundreds of Waptia fossils, he found half a dozen with egg-like structures. He brought them to France to study with ancient arthropod expert Jean Vannier, who works at France's centre national de la recherche scientifique.

Vannier said the find was "amazing" because eggs are typically very fragile.

"They decay very rapidly. It's very exceptional that there are such things fossilized."

When the researchers looked at the chemical composition of the egg, they found that minerals varied in different areas, suggesting that there were embryos inside that had a different composition from the egg itself. (Royal Ontario Museum)

The scientists examined the egg-like structures with a microscope that analyzed their chemical composition. They found that the minerals varied in different parts of the egg, suggesting that there were embryos inside that had a different composition from the egg itself.

There is one other fossil crustacean from around the same time that has been found carrying what might be eggs. That animal, also an arthropod with a hinged, two-sided shell, called Kunmingella douvillei, lived in China about seven million years earlier than Waptia.

It had tiny little balls attached to its legs, but they were not as well preserved and did not contain evidence of embryos, making it difficult to confirm that those were eggs. But if they were, that would show that animals were already evolving different ways to care for their young.