Published by the American Geological Institute
and Trends in the Geosciences
Question marks and pretzel twists are puzzling paleontologists rethinking formations in a rock slab found in 1974. The markings, which date back 620 million years, were originally heralded as the world’s oldest animal imprints. Now some scientists are claiming the marks were made not from animals, but from scratches created during folding of the rock layers.
Worms and other soft-bodied animals leave bookmarks of themselves in the geologic record, known as trace fossils. Instead of bones, their tunnels, worm tubes, fecal mounds and feeding patterns are fossilized as impressions in the sedimentary rock.
For years, Precambrian trace fossils were known from around the world — Namibia, Australia, Russia, England and Newfoundland — but not from the United States. Then, in 1974, geologist Lynn Glover of Virginia Polytechnic Institute found fossil imprints while mapping rocks in North Carolina. The late paleontologist Preston Cloud of the University of California, Santa Barbara (1912-1991) confirmed the impressions were soft-bodied polychaete annelids, segmented worms common on seacoasts.
Quarrymen from Duke University removed pieces of a slab, which, when reassembled, measured more than two square meters by 30 centimeters thick and weighed more than five tons. For years the slab, Vermiforma antique Cloud, was exhibited at the U.S. Geological Survey in Reston, Va., before it was recently moved to the Smithsonian Institution in Washington.
Now, paleontologist Adolf Seilacher of Yale University and colleagues from the Universität Tübingen, Germany, claim the similarities between impressions are too remarkable to be made by animals.
|“A meat hook, kink and pretzel reoccurs in all the forms in that bedding
plane and all have the same orientation that would be impossible in trace
fossils,” Seilacher says. “Spaghetti-like worms in their tubes washed by
currents locally, never get the same wave.”
The markings are scattered throughout the slab with the same type of twists and turns occurring in each one. The only differences among them are in the size of the loops and the length of the tails, like a writer repeatedly practicing the cursive letter “e.” Although living polychaete worms are known to orient in the same direction and build characteristic burrows Seilacher and colleagues proposed a tectonic origin for Vermiforma.
Heralded as one of the oldest animal imprints, the
worm fossil Vermiforma antique Cloud is being cast as a
Duncan Heron, Duke University
Instead, says Seilacher, the markings were the “signature of the mountain builder,” created with particles caught between the shifting beds and scratched out over millions of years. At first, the scientists thought rocks caught in the upper slab made the traces. But, the marks are different enough to warrant a second hypothesis. Perhaps the rocks were not fixed in the upper slab but free to move between slabs, says Yale colleague Edward Bolton.
To test the hypotheses the scientists propose removing parts of the turbidite bed still overlying the Vermiforma surface. If rocks were trapped in the upper bed no traces would exist, only the remaining spikes. But, if broken bits of rock were trapped between the beds, then, as each moved in opposite directions, an inverse trace would remain in the upper bed. Still, if indeed these are animal fossils, and many maintain they are, the mirror image of the animal’s burrow would be pressed in the upper rock.
So far, Seilacher’s interpretation of the celebrated fossil is not being well received. “The tectonic structures (not the fossils) that do exist in this rock mass were formed by flattening (pure shear) and not by rotational simple shear as Professor Seilacher has hypothesized,” says structural geologist Robert D. Hatcher Jr. of the University of Tennessee, Knoxville.
“The rock mass here is not sufficiently deformed for simple shear to have affected these rocks, nor is the style of deformation in these rocks suitable to produce such features by tectonic processes,” says Hatcher, who originally visited the field with Preston Cloud, Jim Wright and Lynn Glover. “The article in Geology appeared as a surprise to me.”