Some very old rocks and a nifty imaging technique have yielded what could be the oldest known animal fossils—spongelike organisms that lived on ocean reefs on what is now South Australia.
Princeton University geologist Adam Maloof wasn't looking for fossils. He was wandering the mountains of South Australia with graduate student Catherine Rose, looking at rocks from just before a major glaciation, about 635 million years ago, which may have covered much of the planet in ice. While poking around, they kept seeing rocks with the same shapes—anvils, rings, wishbones, and others—tucked among the fossils of stromatolite reefs, which are formed by bacteria. "They looked like fossils, but we weren't expecting fossils, so we ignored them," says Maloof. "Eventually, they became too common and repetitive to ignore."
Maloof and Rose thought they might have found specimens of Namacalathus, a hard-bodied organism with a long stalk that lived about 550 million years ago, but they needed a better look. Back in the lab, the researchers ground down the first couple of centimeters of the strange-shaped rocks 50 microns at a time, taking a picture of each newly revealed surface. The team then used computers to put together a three-dimensional image of the animal. "We learned pretty quickly that they looked nothing like Namacalathus," he says. Instead of a stalk, the researchers found elliptical, asymmetric blobs. They were able to model two complete fossils and fragments of several others.
Each fossil was about a centimeter across and shot through with 1-millimeter-diameter tunnels. After eliminating several possibilities, the duo concluded that the organism most closely resembled sponges, which have internal canal systems. That would make these fossils far older than the oldest animal fossil currently known, a 555-million-year-old snail-like creature known as Kimberella.
There are reasons to think that sponges were around this early. Scientists have used evidence on the rate of evolution—known as molecular clocks—to date the origin of sponges back to 600 million or 700 million years ago. Others have also found lipids that are thought to be made by sponges in rocks of about that age. Maloof says the finding is exciting because it means that animals may have been around before a planetwide glaciation and probably survived it. The team reports its findings online today in Nature Geoscience.
Still, experts are skeptical about whether the fossils represent ancient animals. "They're just, at present, really tantalizing fossils for which a really cool argument has been made," says geologist Whitey Hagadorn of the Denver Museum of Nature and Science in Colorado. "I wish we had some DNA in these suckers to figure it out for sure." The good news, he says, is that lots of other scientists will see this article and start poking around for animal fossils in similar-aged rock. "I can't wait to see if someone finds a better-preserved deposit from somewhere else."