More than half a billion years ago, peculiar beings called rangeomorphs ruled the seas. They looked like modern ferns, with leaflike protrusions that branched off in a fractal pattern. Some stood over three feet tall, while others lay flat.

Scientists don’t know why rangeomorphs emerged 571 million years ago — before the Cambrian explosion filled Earth’s oceans with more familiar creatures, and when nearly all of their fellow organisms were microscopic — or how they ate or reproduced. We also don’t know how they fit into evolutionary history. There are “many strange questions about them,” said Alex Liu, a paleobiologist at the University of Cambridge who has been studying the frondy oddballs for 13 years.

New research by Dr. Liu and Frankie Dunn of the Oxford University Museum of Natural History adds another surprise: Although rangeomorphs were thought to have lived on their own, many were actually connected by long, thin filaments, forming large networks.

The authors hope that this discovery, published Thursday in Current Biology, will help shed light on some of those other mysteries.