Researchers have found well-preserved fossils of organisms that lived on the ocean bed about 550 million years ago.
The specimens are preserved in three-dimensional detail
The fossils, unearthed in Southern China, are of some of the earliest complex lifeforms known to science.
Frondose vendobionts died out before the Cambrian explosion about 540 million years ago, when animals with bones and shells appeared.
They are not closely related to any modern species but share similarities with fungi, lichen, and algae.
Evidence of complex life older than 540 million years is hard to study.
Many of the organisms that appeared in the Ediacaran Period (the time period between about 600 and 542 million years ago) had soft body parts that do not make an impression in rock.
Most Ediacara fossils found to date are buried in sandstone, and the coarse sand grains limit how much can be learned about their anatomy.
Now, a research team from Virginia Tech, US, and Nanjing Institute of Geology and Palaeontology, China, has discovered fossils preserved in limestone rock.
"We think that the fossils were preserved where they lived," Chuanming Zhou, of the Nanjing Instititue of Geology and Paleontology, said.
"In other words, they had not been transported a long way from their deathbed to their graveyard.
"And the way they occur in the rocks suggests that they were flat-lying organisms sprawling on the ocean floor 550 million years ago, much like some fungi, lichen, or algae do today."
The organisms consist of numerous tube-like structures that fan out from a central axis. Biologists believe they lived horizontally on the ocean floor, spreading out like vines.
The fossils reveal unprecedented information about how the bodies of the first multi-cellular organisms were constructed more than half a billion years ago.
"Present in rocks ranging from 575 to 540 million years in age, these fossils provide the key to understand the prelude to the Cambrian Explosion when animals with skeletons and familiar morphologies began to bloom about 540-520 million years ago," Shuhai Xiao, a geoscientist from Virginia Tech, said.
The research is published in the online edition of the Proceedings of the National Academy of Sciences (PNAS).