Some 635 million years ago, as the Earth shook off massive shrouds of glacial ice, an alien world blossomed on the ocean floors.
No complex animals prowled the seas of the Ediacaran Period. Instead, the depths held microbial mats and strange, frond-like creatures that resembled nothing alive today. Paleontologists have suggested that this was a sort of Garden of Eden, a simple ecosystem wiped away by the more vibrant fauna of the following Cambrian Period.
But recent research is complicating this view, suggesting that Ediacaran ecosystems were more complex than previously thought. Fossils also hint at the beginnings of a massive shift: scavenging that later evolved into predation.
“It’s the beginning of a major change in the ecosystem of the Earth, an irrevocable change,” said Mary Droser, a paleobiologist at the University of California, Riverside. “The ability to eat another animal is a big deal, and is a major ecological and biological innovation.”
The fossils of the Ediacaran, found in desolate locations like Namibia and South Australia, have been notoriously controversial and difficult to interpret. They include strange, quilted blobs; stationary filter-feeders; and organisms like enormous ferns, some up to eight feet long.
“At first, they were all thought to be jellyfish,” said Simon A.F. Darroch, a paleontologist at Vanderbilt University. “Later, others argued that they were stem-animals or lichens.” (Stem-animals were proto-animals: not quite plants, not yet creatures.)
Scientists also have debated how sophisticated these seafloor ecologies were, Dr. Darroch added. A complex ecosystem holds multiple species with a multitude of feeding strategies, like a modern forest or a reef. A simple ecosystem comprises only a few species with the same basic strategies.
Looking at the Ediacaran’s apparently uncomplicated organisms — without legs or obvious guts — most researchers assumed this ecosystem was on the simple side. But in a study recently published in Nature Ecology and Evolution, Dr. Darroch and his colleagues surveyed seafloor fossils from various parts of the Ediacaran and concluded that the early stages of the period featured surprisingly intricate communities.
Dive into the Ediacaran seas, and you’d see sediments covered in thick sheets of green or white microbes. There were no deep burrowers here; the ocean mud had the solidity and stickiness of a tile floor after a fraternity party.
The few multicellular organisms grazed slowly on the congealed mats or sat atop them, using different body shapes to pluck nutrients from the currents. Sea jellies, or something like them, may have floated in the open waters — nobody’s sure.
Many of these organisms had an architecture unlike any seen in modern organisms, said Martin Smith, a paleontologist at Durham University in England. While they may have appeared plantlike, they lived in waters too deep for sunlight to penetrate.
Even their precise shapes are unknown. Some may have been folded into complex patterns; others, inflated like a water bed.
“They probably grew very slowly,” Dr. Smith said, “arriving as spores on a virgin seafloor after a mudslide or land slip, then reproducing asexually to form interconnected colonies that gradually populated the seafloor.”
Dr. Darroch and his colleagues suggest that different feeding strategies were at the root of this unexpected diversity, but Dr. Smith finds it hard to imagine such organisms feeding in radically different ways.
Instead, the organisms may have competed with one another with varying reproductive strategies, he suggested. Some were able to alter their shapes according to their surroundings and then to spread their spores over a greater distance.
Even with such unexpected diversity, the Ediacaran was a still world, without much in the way of predation. But dead organisms were an easy resource, Dr. Droser said, and some animals began to take advantage.
According to a recent paper Dr. Droser co-wrote in the journal Emerging Topics in Life Sciences, a new set of fossils shows the oldest known traces of such scavenging. Recovered from the Flinders Ranges of South Australia, the rocks hold beautifully preserved fossils of Ediacaran organisms, with tiny passages carved through them.
Dr. Droser and James Gehling, a paleontologist at the South Australian Museum, suggest that these burrows were made by small animals nibbling through microbe mats decaying under thin drifts of sand, and devouring the dead organisms they found in the mats.
In a world where nothing dug very deep, burrowing would have been a biological and ecological innovation.
“The scavenging occurred after the organisms were buried, so we know that they were dead,” Dr. Droser said. “Eating a dead animal requires more oxygen and also a metabolism that can digest it.”
But Dr. Smith disagrees, pointing out that signs of scavenging after the Ediacaran Period are not like these. Scientists tend to find elaborate patterns in fossils, left by animals exploiting every inch of decaying organic matter.
“These Ediacaran burrowers seem not to care that they’ve just passed through a potential meal,” Dr. Smith said.
“This doesn’t sound like a great strategy for a scavenger, less still for a predator,” he added. “But perhaps these early organisms were still learning the ropes.”
Either way, the advent of simple scavenging and burrowing heralded larger changes to come. Scavenging may have been a steppingstone to active predation, and the evolution of the first predators kicked off a massive arms race. Once prey began attempting to escape, defending themselves or fighting back, competition and natural selection began to escalate.
There are clues that something was changing toward the end of the Ediacaran, Dr. Darroch said: signs of movement in the mud fall dramatically, as do the number of species recorded — symptoms of an ecosystem under stress.
As the microbial mats disappeared in a world of burrowers, the fragile species that lived off them went, too. The appearance of more recognizable animals — mobile and powerful, able to punch through the mats and devour other living organisms — might have caused a biologically driven mass extinction.
The Cambrian explosion gave rise to many recognizable animal groups, and for a time these overshadowed the strange, silent gardens of the Ediacaran. But the key to how multicellular ecosystems developed seems to lie in sandstones left by those chilly seas, in an ecosystem much more diverse than previously suspected.
“Life has impacted our world today from the production of oxygen to churning up soil,” Dr. Droser said. “We can see in the Ediacaran the firsts of a number of these biological and ecological processes that later became essential to our planet.”