All creatures strive to leave their mark on earth
by Jill Neimark
Can time ever leave a fossil? Yes, if you consider the six-and-a-half-foot trace etched on a Jurassic outcrop of limestone near the parish of São Bento in Portugal—the actual path taken in the final hours of life by a single sea lily 170 million years ago. Sea lilies, also known as crinoids, are marine animals graced with multiple flowerlike arms. One ordinary afternoon eons in the past, a powerful wave or high tide swept at least one crinoid onto the beach. The water retreated and the little invertebrate vainly spent the last hours of its life crawling nearly 80 interminable inches toward the receding sea, using its feathery arms to hoist and push itself. Its central stalk left a groove, its arms a wake of shallow impressions. It died before reaching the water.
In May of 2016, paleontologist Anthony Martin of Emory University in Atlanta, Georgia, stood with Portuguese geologist Carlos Neto de Carvalho and a few dozen colleagues, and peered down at the record of this ancient death march—formally known as a mortichnia, or death trace. “It was incredible,” says Martin, the author of the 2017 book The Evolution Underground that looks at tracks, burrows, and other fossils of movement, labor, and motion on the Georgia coast and barrier islands—fossils of, in essence, time itself. “You could follow this little trail all the way to the actual fossil of the dead body,” he recalls. “The last hour of this creature’s life was all recorded right there.”
Genuine mortichnia are rare—an equally impressive one was discovered in 2002 in the Upper Jurassic Solnhofen Lithographic Limestone near Wintershof, Germany, a region already famous for dinosaur fossils. 150 million years ago a horseshoe crab was likely swept into a deep lagoon during a storm and crawled nearly 32 feet before dying of oxygen deprivation. Those final moments are captured for us today by the creature’s footprints as well as impressions where its head and abdomen dragged. In the 1990s, a different kind of death trace—that of a “zombie,” or already-dead marine mollusk—was discovered in the same region of Bavaria. The mollusk’s shell slowly sank to a lagoon bed, fell on its side, and was pulled steadily along by calm currents for at least 28 feet before finally settling into the sea floor. This deep-time snapshot reveals ocean currents one afternoon back in the Jurassic age.
Mortichnia fascinate because they offer a glimpse into the behavior of creatures in a world long ago. They also give us a visible record of the last moments before a transition that all of us must make and few of us can fathom.
Last year more than three million tourists stood before the plaster casts of Pompeii, sculptures of the death agonies of ancient Romans who suffocated when Mount Vesuvius erupted two thousand years ago. And beginning in the 19th century, scientists hoped they might tease out the last image that living creatures saw before they died—imprinted on the retina. The principle, called optography, became a favorite of science fiction and lurid crime thrillers. In 1914, newspapers reported that a 19-year-old murder victim named Theresa Hollander had been found by her father. Soon after her eyeball was photographed in the hopes that her retina would reveal the last image (the murderer) in her vision before she died. Unfortunately, the photographs of retinal images are always, at best, impressionistic swirls and lines.
As for death traces, the ‘zombie’ drag marks of the marine mollusk have now been crafted into a virtual 3-D model by paleontologist Dean Lomax of the University of Manchester and his colleagues. Using a method called photogrammetry, they compiled 645 digital photos of the tracks into an intact virtual mollusk, with all the original ridges and furrows of its long-dissolved shell. (See image above.) After the results were published in May 2017, in the online journal PLOS ONE, paleontologists around the world can study the mollusk. But that raises the question of the longevity of digital traces: How long can a virtual mollusk live?
We might also wonder about the persistence of our own tracks, which are increasingly digital. In the summer of 2016, a Minnesota man named Philando Castile was shot by a police officer as Castile reached for his wallet. Diamond Reynolds, Castile’s girlfriend, switched on her phone’s video camera, and livestreamed the aftermath of the shooting to the world. “Please don’t tell me my boyfriend just went like that,” she cried on the video that would be viewed millions of times over the next 24 hours. Someday, will a post-anthropocene human dig up a smart phone and toss it aside, unaware that so many life—and death—traces were buried in the tiny circuitry under the glass?