Worms in the Mist
|Australia’s Stirling Range Formation.|
Credit: Albany GateWAy Coooperative Limited
To see what sort of wildlife gathers at the shoreline, look for prints pressed into the wet sand. Hermit crab, sandpiper, five-toed Speedo-wearer… Look quickly, for the incoming tide will soon wipe the sands clean.
Some animals can leave a more permanent footprint, however. Preserved in Cambrian sandstone are trails made by worm-like creatures wriggling in what was once the wet sand of tidal areas. Such "trace" fossils are fairly common in Cambrian rocks.
Most of these trace fossils were made by creatures burrowing down into the sand. The tracks were preserved partly because they were made below the surface, away from the erasing action of water. But some tracks made on the surface also have been preserved, especially if the organism who made them used mucus for locomotion. As it slid along, ploughing a ditch in the sand, the mucus coating the body caused the sand grains to clump together. The clumped sand withstood being broken apart by water long enough to be buried by more sand.
Last year, Birger Rasmussen, Ian Fletcher, and Neal McNaughton of the University of Western Australia, along with Stefan Bengtson of the Swedish Museum of Natural History, reported finding such surface trace fossils in rocks from Australia’s Stirling Range Formation. However, the Stirling rocks pre-dated the Cambrian period by several hundred million years, forming long before multi-celled mobile animals were supposed to have evolved. In the journal Science, the scientists reported that the age of the Stirling rock was somewhere between 1.2 to 2 billion years old.
|Click for animated gif of worm tracksCredit: Javier Herbozo|
Life 1.2 to 2 billion years ago was mainly composed of microscopic, single-celled bacteria and archaea. Single-celled algae – members of the eukaryotic branch that now includes plants, fungi, and animals – made their first tentative appearance 1.8 billion years ago.
According to the fossil record, multi-cellularity took a long time to get going. There are well-preserved fossils of multi-cellular algae dating back 1.2 billion years. The oldest multi-celled animal fossils are less than 600 million years old, dating back to right before the Cambrian explosion.
Why did multi-cellular life take so long to get going? The origin of multi-cellular life may be related to the rise of oxygen on Earth, because a larger body composed of multiple cells needs the extra energy that oxidation provides. Based on clues in rocks, the first major boost of oxygen probably occurred between 2.5 and 2 billion years ago.
Presumably, multi-cellular life could have appeared soon after the rise in oxygen. Molecular clock dating is not inconsistent with this idea, placing the emergence of multi-cellular life to sometime between 700 million and 1.5 billion years ago. [Molecular clocks are based on the assumption that evolutionary changes in the genetic code are roughly correlated with the passage of time.]
The tracks in the Stirling rocks, if they were made by multi-cellular organisms, would provide another line of evidence for the early emergence of multi-cellular life.
The Stirling tracks are about two millimeters wide and several centimeters long. Some of the tracks are wider at one end, suggesting that whatever made them stretched out and became slimmer when it moved. Some of the tracks terminate in a U-shape, suggesting a point where the creature may have stopped in its wanderings.
According to Bengtson, although the Stirling tracks look like they were made by a worm-like animal, the creature that made the tracks was most likely not a worm, or even the primitive ancestor of a worm. Instead, it may have been an organism that died out long before multi-cellular animals emerged.
|A sea sponge’s ancestors date back to approximately 600 million years ago.|
The rock itself tells us that this creature – whatever it was – lived in a shallow-water tidal environment. Ripple marks and mud cracks indicate the sand was deposited in a shallow sea. The rocks can’t yield any biological information, such as DNA, because during burial the rocks became heated and oxidized. Any biological remains have long since been degraded.
|Stirling fossils. Click for larger image.|
Image Credit: Rasmussen, et al. Science
"DNA, in particular, is a fragile molecule and is not known to have survived for more than some tens of thousands of years," says Bengtson. "The Stirling fossils are about a hundred thousand times older than that."
The Stirling rocks also display disc-shaped structures in several of the rock layers. Similar discoidal fossils have cropped up in other ancient rocks, and one interpretation is that they are the holdfasts of sponge-like animals that embedded themselves in the ocean bottom.
Sponges branched away very early in the history of animals, and therefore are quite different from what we normally think of as an "animal." They are rooted in place like a plant, and composed of various cells that work to draw in nutrients from the surrounding seawater. The earliest sponge fossils date back to about 600 million years ago.
There is no evidence that the disc-shaped structures in the Stirling rocks are the remains of sponges. And although the discoidal fossils appear in the same rock as the trace fossils, Bengtson says the different structures don’t have anything to do with each other.
Bengtson doesn’t think the tracings in the rock could have been made by algae, with grass-like strands that traced loops into the wet sand. The reason, he says, is that such random tracings are inconsistent with the patterns in the Stirling rocks.
"The Stirling fossils form a very conspicuous double-stranded structure, where the two strands join in a loop at one end and form a flaring opening at the other," says Bengtson. "Interpreting them as folded single threads likewise meets with difficulties, as there is no reason why these would always be folded at the middle and remain at the same distance from each other."
|Erwin suggests that the trace fossils could have been created by such non-biological means as cracks, various sedimentary processes, or torn microbial mats, like the one shown above.|
Image Credit: Pennsylvania State University
Doug Erwin of the Smithsonian Museum of Natural History thinks looking for biological causes for the Stirling markings is the wrong approach. Instead, he says we must presume the markings are made by non-biological sources, unless proven otherwise.
"Extraordinary claims require extraordinary evidence, as the saying goes, and I don’t think the Science article rises to that level," says Erwin. "The variety of apparent traces is striking, and many do not appear to be biologic. I guess my concern is that the authors, unintentionally, have selected the most biologic looking objects from a large population."
Bengtson counters this concern by saying that the illustrations published with their Science article showed the total variation of markings associated with the traces, without selection for biological-looking objects.
But the age of the rocks remains a sticking point with Erwin. Because the rocks are so old, he says, more evidence is needed to prove biology. Just because the tracings look similar to Cambrian tracings is not sufficient evidence for a biologic origin.
Erwin suggests that the trace fossils could have been created by such non-biological means as cracks, various sedimentary processes, or torn microbial mats (which are non-biological in the sense that they are accumulations of trapped matter).
Bengtson and his colleagues considered these possibilities in their Science paper, but discounted them. The markings were not made by geological processes, they say, because the rocks lack the features you would expect to see from such an origin. The tracings don’t resemble swash marks, rill marks, or shrinkage cracks.
Bengtson and his colleagues don’t believe that microbial mats could have made the tracings, either. They say that torn mats are dominated by irregular, contorted, and twisted structures, which are not found in the Stirling rocks.
|A trace fossil at Lyme Regis (Dorset, UK) – burrows seen in plan and section, on the top and side of a limestone block.|
Credit: Lois Wakeman, 1999-2003
Interpreting the structures as microbial mats, says Bengtson, "now seems to be the popular excuse to dismiss the Stirling fossils, even though nobody has even tried to explain how these structures could have been formed by disruption of mats."
The Stirling fossils are not the only trace fossils under contention. Other wiggly grooves have been found in 1.6-billion-year-old sandstone from central India. The scientists who reported these tracks concluded they were made by a worm-like animal that propelled itself with rhythmic muscle contractions. They also concluded that the animal grazed on decayed microbial mats on the sea floor, because the tracings follow the base of a thin veneer of darker sandstone that may be the remains of a mat.
According to Erwin, there is a very long history of such claims of ancient trace fossils.
"Most of them simply die off and don’t get cited," says Erwin. "I think it is fair to say that they haven’t achieved much acceptance within the paleontological community."
The reluctance of the paleontological community to accept the Stirling markings as trace fossils dismays Bengtson, but he seems even more pained when scientists don’t even try to suggest plausible alternative hypotheses for the structures.
"So far I’m disappointed by the general reception – I had hoped for more challenging and constructive criticism than we have received so far," he says.
Still, Bengtson and Birger Rasmussen are continuing to study the Stirling fossils, as well as other trace-like fossils in ancient rocks.
"In a field full of potential pitfalls, we need to analyze all new finds with the utmost care," says Bengtson, "An unconditional search for such traces is necessary, if we are ever to get a representative picture of the emergence of multi-cellular life on Earth."