Amoeba tracks trump trace fossils?

Super-sized amoebas lumbering along the ocean floor at the bottom of the Caribbean Sea may shake up a long-standing debate on the timing of animal evolution, according to a paper published in today's issue of Current Biology.

"There is nothing paleontologists like more than a controversy," said Mikhail Matz, a University of Texas integrative biologist and the main author on the study. "I'm looking forward to this. It's going to be fun."

The evolutionary history of complex animals - those that are composed of multiple cells and have bilateral symmetry - is marked by a dramatic burst of speciation, called the Cambrian explosion, about 542 million years ago. The Cambrian explosion has always been somewhat troubling to evolutionary biologists - starting with Charles Darwin - because of the seeming rapidity with which complex animal groups appear to have evolved.

Some researchers point to rare Precambrian "trace fossils" - such as slither prints left in ancient sea bottoms - as evidence for complex animal life predating the Cambrian. The oldest of these trace fossils yet found are 1.8 billion years old, about three times older than any animal in the fossil record.

The discovery by Matz and his team casts doubt on the argument that Precambrian trace fossils were made by animals. The researchers identified giant protists at the end of raised, linear impressions in the seafloor. Though Matz did not directly observe the protists making the roll marks, their positioning at the end of furrows, some up to half a meter long, suggests that the amoebas left the marks as they rolled along the seafloor.

The discovery also suggests that Precambrian trace fossils weren't necessarily left by what biologists would consider animals and could have instead been made by unicellular organisms.

"We now have an actual example of an organism that can [leave such a trace] without being an animal," Stefan Bengtson, head of the paleozoology department at the Swedish Museum of Natural History, told The Scientist.



Bengtson, who was not an author on the paper, was one of the scientists who first described 1.8 billion-year-old trace fossils similar to the tracks Matz found. Bengtson said it was never fully clear that "animals" left those ancient traces - called the Stirling fossils after the Australian mountain range in which they were found - but that the fossils did suggest that the organism who left them was moving like a complex animal and was potentially multicellular or bilateral. "We ended up with only one plausible scenario," Bengtson says, "and that was that [the Stirling fossil] was from a motile organism." Others in the paleontological community took the fossils as evidence of a more slow and steady animal evolution than the Cambrian explosion model would suggest.

Now, Bengtson says, Matz has found an organism that leaves tracks very much like the Stirling trace fossils, but is not a complex animal. This weakens the argument that the Stirling traces are evidence for animals predating the Cambrian, and instead suggests that simpler organisms could have made the ancient tracks. "I think it's definitely a likely scenario, and a more likely scenario now that we know that there can be large protists that can make these traces," Bengtson said. "It certainly makes it a little more palatable to the paleontological community, or at least I hope it will be so."

Matz and his team sent a submersible research vessel about 750 meters below the waves near Little San Salvador Island, Bahamas and saw greenish globs covered in fine sediment. Though the team first believed the grape-shaped globs to be snails or even feces, closer inspection revealed them to be Gromia sphaerica, a testate (or shelled) amoeba that can grow up to 30 mm in diameter - massive for usually microscopic protists. Matz played down his discovery of a giant protist making tracks. "It's not resolving the controversy once and for all," he told The Scientist. "We're just trying to tip the balance," in favor of holding that the start of the Cambrian did coincide with the rise of animal groups.

Matz did say, however, that if a paleontologist happened upon fossil impressions of the "bilobed" Gromia sphaerica traces he found, it would be logical to conclude that they were left by a multicellular, bilateral animal. "If this particular track was fossilized and found by a paleontologist, it would undoubtedly be attributed to a bilaterian animal."

Soren Jensen, an ichnologist, or trace fossil scientist, at the University of Extremadura in Spain, agreed that Matz's paper aids evolutionary biologists in reconstructing animal evolution. "It shows that traces don't automatically mean that there were animals," he told The Scientist. "We don't always have to jump to that conclusion. We always have to keep an eye open for alternatives to animal originators [of trace fossils]."

Gromia sphaerica is not a new species - the amoebas also inhabit the Arabian Sea, though in a less motile form - but Matz and his colleagues were the first to observe them in close association with such tracks and said that little is known about the species or the biology of other deep sea protists. He added that hopes that his discovery fuels more research on such organisms.