These ancient plantlike organisms were animals — and we don't know why they disappeared

Half a billion years ago, Earth hit its stride. After millions of years of slow going, evolutionarily speaking, the "Cambrian explosion" saw a rapid acceleration in the number of species evolving from existing life. Fossilized evidence from the era suggests that the Cambrian explosion is when the ancestors of most of the animals we know now came into being.

Still, scientists have long theorized about what might have predated the Cambrian explosion. New research is providing some answers.

To solve this mystery, researchers turned to Ediacaran organisms, named after the Ediacaran Period that lasted from 635 million to 542 million years ago, The Conversation explained. Fossils of these ancient creatures have been turning up for decades, but scientists never quite knew what they were — animals, plants, fungi, or perhaps something else entirely. Their "frond-like" appearance led many to believe they were more closely associated with plant species than our own Kingdom Animalia.

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But in a study published this week in the journal Paleontology, scientists took a hard look at these Ediacaran organisms — and determined that despite being unlike any others on Earth today, they were in fact animals, Science Magazine reported. By analyzing over 200 fossils of a known animal species, Stromatoveris psygmoglena, which existed after the Cambrian explosion, researchers concluded that their biology was close enough to other Ediacaran organisms to count as one of them.

The sheer volume of Ediacaran fossils that have been discovered suggests that they "dominated Earth's seas" during their time, Science Magazine explained. Before this study, scientists more or less agreed that the development of animals was what killed them off. But with these new fossils linking the Ediacarans to a species that lived 200 million years after the Cambrian explosion, "it's not quite so neat anymore," said study author Jennifer Hoyal Cuthill. Read more at Science Magazine.