Extinct for 50 million years, an enigmatic fossil species
may still live at the bottom of the seabut it defies capture.
By Peter A. Rona
o little is known about the deep ocean, that those of us who explore it should expect surprises. Yet even I and my research team from the National Oceanic and Atmospheric Administration were dumbfounded in 1976, when we studied photographs of the seafloor in the middle of the Atlantic Ocean. From our research ship Discoverer, we had slowly towed a deep-sea camera on a cable two and a half miles long. Pings of sound had guided the camera roughly ten feet above the seafloor, while strobe lights fired every twenty seconds to illuminate patches about the size of a bed sheet. In those days we had to wait until we were back on landFlorida, in this caseto process the film and view the images.
At first all we saw was the silt that coats the ocean floor. Then something a little bigger than a poker chip caught our attention. Under a magnifying glass, a distinctive pattern of black dots appeared in our photograph. The dots were evenly spaced and arranged in crisscrossing rows, forming a perfect six-sided figure that resembled the center of a board of Chinese checkers.
Once we knew what to look for, we recognized thousands of these hexagonal forms in our sequence of hundreds of photographs. Could such a uniform pattern be the sign of some unimagined life-form? Certain corals, for example, build structures with hexagonal symmetry, but not in seafloor sediment. Our imaginations ran wild. Was this a hoax perpetrated by the people who had processed our film? Was this some strange cargo spilled from a shipwreck? A message left by extraterrestrials? Surely my local marine biology colleagues at the University of Miami would quickly enlighten us. But they were just as puzzled as we were. They referred me to their counterparts at the National Museum of Natural History, Smithsonian Institution, in Washington.
The area we had surveyed was in the rift valley that lies along the center of the Mid-Atlantic Ridge, a mile-high undersea volcanic mountain range that traverses the Atlantic from north to south. The ridge links with similar ridges in the Arctic, Indian, and Pacific oceans. Along this global ridge system, continent-size tectonic plates, which form the outer shell of the planet, are moving apart, and new crust is constantly being created by the upwelling of magma from the Earth's hot interior. As it emerges, the molten rock cools, solidifies, and spreads apart at a rate of a few inches per year. Earthquakes accompany the slow widening of the seafloor.
Armed with a dozen black-and-white photographs, I made the rounds behind the scenes at the Smithsonian. My first consultation was with Frederick M. Bayer, an expert on corals. But Bayer concluded that the form was not a coral at all, and introduced me to an expert on another phylum of marine invertebrates. By the end of the day, specialists in every major group of marine invertebrates had examined the photographs and had drawn a blank. Their only advice was to prepare an article for publication in a scientific journal, with photographs showing the pattern as related to an invertebrate of uncertain identity.
No sooner had the suggested article appeared, than I received a letter and a reprint of a paper from Adolf "Dolf" Seilacher, a paleontologist at the University of Tübingen in Germany. Seilacher is an expert on classifying and interpreting traces of lifesuch as the trails left by wormsthat are preserved in ancient marine sediments. Your pictures were a real thrill to me, his letter began. Hoping that you have in the meantime received my reprint, the perfect identity with the trace fossil Paleodictyon nodosum of my paper is beyond any doubt.
Seilacher's paper described a fossil form preserved at least 50 million years ago in sediments of the deep-sea floor, which were now exposed on land at various sites in continental Europe. He was particularly excited that our discovery would enable us to find out what had left this enigmatic form in the fossil record. In other words, he proposed, we had stumbled onto evidence that a creature presumed to have been long extinct was still alive today. If we could confirm Seilacher's confident belief in the identity of the fossil and the pattern on the seafloor, it also seemed possible that we could discover what creature had produced it.
From the sketches in Seilacher's paper, we learned that the black dots visible in our photographs might be holes that led straight down a fraction of an inch to a horizontal network of tubes or tunnels just beneath the sediment surface. The tubes in the fossil forms interconnected in an orderly hexagonal network. Seilacher's interpretation was that the network was a tunnel system excavated by some kind of worm. This creature, he believed, augmented its sparse supply of food in the deep ocean by farming and harvesting bacteria in the tunnels. Furthermore, he proposed, the hexagonally arranged network was an evolutionary descendant of simpler traces preserved in 500-million-year-old sediments. As he envisioned it, the first organisms that excavated such tunnels inhabited shallow waters, but soon they retreated to the deep sea, perhaps a place where they could pursue their feeding strategy undisturbed. Over time the tunnels became more regular, and multiple exits were added to improve circulation, culminating in the strikingly regular Paleodictyon form.
In 1977, a discovery was made near the Galápagos Islands, in the eastern equatorial Pacific, that fundamentally changed the biological understanding of life on Earth. At a depth of about a mile and a halffar deeper than sunlight can penetrate to provide the energy needed for photosynthesisan oasis of life was unexpectedly found in the desert of sediment and lava flows that covers most of the deep-sea floor. In an area about the size of a football field lived foot-long clams and red-plumed tubeworms that stood taller than a person.
These and what turned out to be hundreds of other animal species new to science were prospering in warm springs issuing from cracks in pillow-shaped lava flows. Biologists immediately wondered how these animals could make a living, apparently without depending on nutrients generated through photosynthesis. It turned out that at the base of the food chain were bacteria that nourished themselves through a process of chemosynthesis. Drawing their energy from gases dissolved in the warm springs, mainly hydrogen sulfide, they were able to manufacture sugars and starches from carbon dioxide and water.
The Galápagos discovery was soon followed by the revelation of similar ecosystems at hot springs discharging from spectacular black smoker vents along the ridge system in the Pacific. The temperatures in these hydrothermal vents ranged as high as a scalding 750 degrees Fahrenheit (400 degrees Celsius). With these discoveries geologists realized that the ocean basins are really leaky places. The cold, heavy seawater can sink downward for miles, through cracks in the underlying volcanic rock. There it is heated as it flows near reservoirs of magma at sites beneath the ridge system, expanding and rising until it discharges from the seafloor. Along the way it dissolves metals and picks up gases from the rocks and the magma. The metals precipitate out of solution as iron-rich sulfides, coalescing into chimney-like structures and pouring into the surrounding cold seawater as a black cloud of particles (hence the name black smoker).
For marine biologists, discovering these new ecosystems was like being a member of a Star Trek crew and finding a previously unknown basis for life on another planet. More astonishing still, certain heat-loving, chemosynthetic microorganisms living in those ecosystems turned out to have genetic characteristics that place them near the base of the tree of life. That raises the tantalizing possibility that life on Earth began at such hydrothermal vents in the early ocean, rather than in shallow waters at Earth's surface.
The initial consensus of the scientific community was that the hot springs and their ecosystems were confined to the Pacific Ocean, where the seafloor is spreading as much as ten times faster than it is in other ocean basins.
For biologists, the most surprising aspect of our discovery was that the animals at the Atlantic vents differed from those in the Pacific. The red-plumed tubeworms and giant clams were nowhere to be seen. In their place, the dominant vent animal is a new variety of shrimp. But for me the greatest surprise was that the vent field lay in the same region of the Mid-Atlantic Ridgenear the latitude of Miami, Floridawhere we had photographed the hexagonal forms nearly a decade earlier.
Our Atlantic site now became the cutting edge of seafloor hydrothermal research. Almost overnight, we gained ready access to collaborators and support for undersea expeditions in human-occupied deep-sea-diving submersibles. Although we were focusing our efforts on an actively venting mound the size and shape of the Houston Astrodome, in 1990 I managed to piggyback some dive time with the submersible Alvin to visit the east wall of the rift valley, about a mile east of our mound. That was where our photographs of Paleodictyonor whatever the creature washad been taken years earlier.
Accompanied by Dudley Foster, Alvins pilot, I was able to view our mysterious hexagonal patterns up close. Then, with one of the sub's manipulator arms, Foster and I pushed clear plastic tubes, about two-and-a-half inches in diameter, a foot down into the seafloor sediments.
But to my dismay, when we sieved some samples to look for the worm or other organism that might have made the pattern, the sediment passed through the sieve and left nothing behind. Other samples that we preserved in formalin to take back for study by biologists were also a disappointment: the pattern of holes collapsed and disappeared, and the expected underlying hexagonal network of tunnels or tubes was nowhere to be seen.
My next opportunity to pursue this elusive phenomenon came in 1991, when the Canadian director Stephen Low began work on his IMAX film about the Titanic. Because the wreck lies in 12,500 feet of water several hundred miles off Newfoundland, Low had contracted with the P. P. Shirshov Institute of Oceanology in Moscow to use its two state-of-the-art submersibles Mir 1 and Mir 2. The underwater photographer Emory Kristof of the National Geographic Society in Washington, D.C., and I arranged for a series of dives en route to the filming off Newfoundland, in which we would test the IMAX cameras while exploring the TAG Hydrothermal Field. Our tests of the cameras contributed little to the film, revealing only the inadequacy of the lighting system then available. In research terms, however, the dives paid rich rewards.
The Mirs passenger compartmenta steel sphere no wider than one person's outstretched armscan accommodate only three people at a time. I made the first of the research dives with Yury Bogdanov, a senior research scientist with the Shirshov Institute, and Mir chief pilot Evgeny Chernjaev. Our aim was to explore the region from our Astrodome mound eastward to the hexagonal forms.
The dive was breathtaking. After a two-and-a-half-mile descent, at a rate of about one mile an hour, we landed on top of the Astrodome mound next to chimneys that poured forth turbulent clouds of black smoke and swarmed with shrimp. From there we set our course eastward, slowly gliding away from the rusted bright red and yellow mineral deposits of the mound and over the monotonous light tan sediments and pillow-shaped lava flows of the surrounding seafloor. Then we began to see the hexagonal patterns on the surface of the sediments.
As we continued eastward, the sediments gradually changed from light tan to reddish brown. Inactive chimneys, several feet high, began to appear, while the hexagonal patterns disappeared. Farther on, the chimneys became much taller, and we found ourselves traveling about a hundred feet above the seafloor near the level of the dead chimney tops, weaving our way as if we were flying through a forest of redwoods. Finally we were able to descend near to the seafloor, which was littered with fallen chimneys, each several feet in diameter and fluted like a column of a Greek temple.
After fifteen hours on the seafloor, my companions apologetically asked me whether I was ready to ascend. By comparison, the Alvin typically spends only about four hours on the deep-sea floor. When Bogdanov, Chernjaev, and I returned to the surface, we were chilled to the bone. The temperature of the deep water is near freezing, and deep-diving submersibles lose heat quickly through their metal hulls. But back up on the support vessel, we warmed up quickly in a mercifully hot sauna.
We collected no hexagonal forms during the Mir dive, but I did secure several a couple of years later, using the Alvin. One core was dried and impregnated with liquid epoxy resin, finally preserving the curious surface pattern of holes. But nothing more definitive was learned, and the sample was stored away.
My next close encounter of the hexagonal kind came in 2001, when I joined a team making a new IMAX film, again directed by Stephen Low. The star of the enterprise was the Alvin, now equipped with IMAX and high-definition TV cameras and a powerful underwater lighting array capable of illuminating an area half the size of a football field. The team had already made spectacular images of a vent site in the Pacific. I was there to help with filming the contrasting Mid-Atlantic Ridge.
The new equipment performed beautifully. Only one thing seemed to be missinga story that could tie together all the spectacular images. When I recounted how Seilacher and I had converged on the fossil Paleodictyon and its apparently living counterpart, and how we were trying to solve its mysteries, Low became intrigued. He invited me to make a dive with Emory Kristof to point out the form and its setting. We used the high-definition TV camera and replayed the video to a packed house in the ship's laboratory that evening. The next day, Low sent his director of photography, William Reeve, down with the IMAX camera to make some more images. Our detective story ultimately became the narrative thread for the film, Volcanoes of the Deep Sea.
With Paleodictyon in the limelight, some nagging questions resurfaced: Did the form on the seafloor really correspond to the fossil? If it did, where was its hexagonal network of tubes or tunnels? I remembered the sample I'd preserved in epoxy and gave it to Seilacher, who set out to dissect it. I received a photo from Seilacher with a handwritten note: Epoxy did not evenly penetrate. Still hexagonal network of tunnels can clearly be seen. Drawing on his mental picture of what it should look like, Seilacher could see a network, but I was skeptical. By now the film was a wrap, but to unequivocally prove the existence of the network, and to try to discover what was making the patterns on the seafloor, Low supported an Alvin dive for Seilacher and me.
The dive took place in July 2003. Twenty-five years had passed since I had received Seilacher's excited letter proposing a link between his ancient fossils and my seafloor photographs. We climbed into the Alvin together and, after our long descent, landed knee deep in hexagon country. Crouched by the window on his side of the sub, Seilacher exclaimed at the abundance of the patterns. We immediately began collecting cores, targeting the freshest-looking hexagons with the sharpest margins.
With time growing short, our pilot, Pat Hickey, dexterously aimed a hose he had rigged at a fresh pattern and began blowing away the surface sediment with a gentle stream of water. Within seconds, as we watched on the video monitor, the hexagonal pattern of tiny holes on the sediment surface disappeared and a hexagonal network of tubes or tunnels emerged, exactly like those in the fossil form.
For me, it was a eureka moment! Unable to jump up and down in the confined space, I reached across and shook hands with my companions. The living form on the seafloor and the fossil form that lived on the seafloor more than 50 million years ago were indeed one and the same. The deep ocean had served as a sanctuary, a place where Paleodictyon had lived on for an unimaginably long time, protected even from the global environmental changes that caused the extinction of many of the animals living in shallow water and on land.
We carefully preserved the sediment cores we had collected, and I felt confident that we finally had the answer in hand. Experts are now examining the structure of the forms and the microorganisms they contain, and chemically analyzing the sediment and sampling it for organic matter that will be genetically sequenced.
But Paleodictyon remains elusive. Two hypotheses for its origin are being tested. One is Seilacher's original explanation, that the form is constructed as a burrow by an as-yet-unknown worm. The alternative hypothesis is that the form itself reflects the shape of the organism, perhaps a large single-celled organism whose living tissue fills the horizontal network. In that event, the organism might take up the sediment to make a kind of hexagonal exoskeleton, leaving holes in the sediment open to catch food from above. These studies are in progress. After nearly thirty years, to my surprise, the mystery of Paleodictyon still seems as deep as the waters where it lives.
A leader in the exploration of the deep-sea floor, Peter A. Rona was thrilled to work with director Stephen Low and others on the IMAX film Volcanoes of the Deep Sea (see www.volcanoesofthedeepsea.com). The film highlights Rona's discovery of the enigmatic living fossil Paleodictyon, the subject of this article. He is also the author of an earlier article for Natural History, Metal Factories of the Deep Sea (January 1988). Rona is a professor of marine geology and geophysics at Rutgers University in New Brunswick, New Jersey, and a consultant to the United Nations on seafloor resources. He continues to make dives in deep-sea submersibles, an activity that he considers safer than driving to work.
Copyright © Natural History Magazine, Inc., 2004