520-Million-Year-Old Fossil of Fuxianhuia Protensa and Four Babies Is Our Oldest Evidence of Parenting

The fossil showing Fuxianhuia protensa and young. Dong Jing Fu et al.

Parenting isn't just for mammals, nor is it a new strategy, according to the fossil record. A recently-discovered fossil shows that the behavior of raising young is at least 520 million years old.

A tiny, trilobite-like creature called Fuxianhuia protensa swam the ocean floors of the Cambrian, laying eggs and eating smaller animals. Scientists have several fossilized imprints of these animals, but it's usually hard to tell much about an animal's behavior just by looking at its anatomy. However, one fossil of F. protensa told a story of motherhood.

A pristine fossil shows an adult F. protensa and four babies of the same size and age together. Paleontologists could tell that the four were younger because they were smaller and thinner than the adult.

The researchers who analyzed the fossil argue that these young animals were likely siblings from the same clutch of eggs and died alongside their mother. This indicates that they lived with her, too, and perhaps she protected them. Researchers at the University of Cambridge who studied this fossil, found in the Chengjiang fossil site in Yunnan province, China, published their analysis in the journal bioRxiv.

Fuxianhuia protensa once swam the ocean floor and likely cared for its young. Jing Shan Fu

"Extended parental care," or care for offspring past birth, is a strategy that some animals use to maximize the chance that their offspring will survive. It's especially useful for animals that have vulnerable young or very few young, helping to ensure that those offspring live long enough to pass on their genes to the next generation.

But it's rare for arthropods like spiders and shrimp to parent their young. They usually prefer to hedge their bets by laying large numbers of eggs. They'll protect the eggs from predators, but once the offspring are born, they're usually on their own.

This fossil is most likely the oldest evidence of parenting juveniles in the fossil record, and demonstrates that a wider variety of creatures are capable of caring for their young than previously known.