Imagine a mask made when you die that could preserve your face for millions of years. In a way, this is what happened to some of our planet's oldest known animals. Encased in “death masks” made of the mineral pyrite (“fool's gold”), these soft-bodied organisms avoided rot and decay long enough to make it into the fossil record, paleontologists say.

The creatures are known to have thrived around the world roughly 575 million to 541 million years ago, during the Ediacaran period. They looked like aliens: one, Kimberella, resembled an avocado wearing a garter; another, Dickinsonia, could pass for a cross between a pancake and an earthworm. Where this group fits on the evolutionary tree is a mystery—not all its members were animals, but some were, and those species most likely include ancestors or close relatives of all subsequent animal life. Another nagging mystery has been how Ediacaran organisms became fossils in the first place because most are thought to have been soft-bodied. Such squishy critters are prone to immediate consumption or decay, so they rarely fossilize when they die.

To investigate these questions, a team led by paleontologist Brandt Gibson of Vanderbilt University euthanized sea anemones and mollusks, the modern animals whose bodies are thought to be most similar to Ediacaran biota. They put the corpses in seawater tanks to mimic the ancient ocean's chemistry and watched as iron-rich pyrite was deposited around the bodies over the course of about a month. The study, published in May in PALAIOS, was the first to observe these death masks forming in the laboratory.

The shrouds did not completely impede decay, however. Sea anemone tentacles, for instance, “disappeared rapidly,” Gibson notes. This result suggests Ediacaran fossils may not be complete pictures of the original organisms. Filling in that information could be key to understanding how these strange creatures fit into the tree of life.

Alex Liu, a paleobiologist at the University of Cambridge, who was not involved in the research, says the study “adds to a growing realization that the Ediacaran period is not the ‘enigmatic’ interval it has been portrayed as for decades ... and the questions within it are tractable.”