Paleodictyon

 A few years ago I wrote a couple articles for Cryptid Culture magazine. While the publication itself seems to have gone under, you can still order copies of it on their website. Definitely go check it out. There are lots of great articles and illustrations. Here I'd like to share one of the articles I wrote on a most unusual deep-sea cryptid which may have existed since before the dinosaurs.

PALEODICTYON

 It’s often said that the deep sea is as mysterious and unexplored as the distant gulfs of space. There is much truth in this, for new and strange species are constantly being discovered in the unlit abyss. Even those scant few creatures which have been seen by surface-dwellers may be little more than ghosts: brief flickers in a submersible’s lights, enigmatic shapes glimpsed for but a second on an ROV camera, strange carcasses damaged almost beyond recognition by the rough journey to the surface in a fisherman’s net. And in some cases, these phantoms may only be known indirectly from the marks of their passage left in the environment.

In 1976 oceanographer Peter A. Rona was using an underwater camera towed behind a research boat to explore the seafloor along the Galapagos Rift, a volcanic hotspot in the Pacific Ocean near the islands made famous by Charles Darwin. Though the Rift is dotted with numerous hydrothermal vents that are abundant with life, the area Dr. Rona was exploring was far away from the hot, black-smoking chimneys. It was little more than an undersea desert- a flat expanse of mud and silt almost devoid of life. So it was a great surprise when the camera came across a hill of sediment about the size of a silver dollar covered with an intricate pattern of holes. The placement of the holes was so geometrically perfect, and the find itself so out of place, that Dr. Rona at first assumed that the other researchers on the exploration team had somehow played a trick on him. But they were just as surprised as he was by the find.

Later excavations of the holes revealed that they led to shafts that connected to a network of honeycomb-like interconnected tunnels about an inch below the surface. A burrow of some sort.  A burrow so geometrically perfect as to almost seem made by intelligent beings- though there is no need to invoke an intelligent builder here since many animals can create startlingly precise structures.

The first question Dr. Rona and the others asked, of course, was just what animal had made these remarkable structures. No living organisms were discovered when the burrows were dug up. There weren’t even any telltale food scraps, bits of DNA, or other detritus to provide a clue to the identity of these deep-sea engineers.

The mystery only grew deeper as word of the discovery spread. A few years after Dr. Rona formally described the strange structures, he was contacted by paleontologist Adolf Seilacher, who showed him fossil burrows that were nearly identical to the Galapagos Ridge hills. Seilacher’s fossils, dubbed Paleodictyon (trace fossils such as burrows, footprints, and coprolites are given their own distinct scientific names), dated from the Eocene Epoch, approximately 55 million years ago. Other, simpler but still very similar fossil burrows dated all the way back to the early Cambrian Epoch, when large multicellular animals first appeared in the fossil record.  Whatever was making the mystery burrows had apparently existed on Earth with minimal evolutionary change since before the dinosaurs had evolved. Or, at least, with little change in the way it constructed its dwellings. Once this striking continuity was brought to light, the still-unknown maker of the modern tunnels was given the name Paleodictyon nodosum.

Since the initial discovery in 1976, Doctors Rona and Seilacher, along with other researchers, have found thousands of Paleodictyon burrows along volcanic rifts in both the Pacific and Atlantic oceans. Like those first specimens, the other structures were found on the barren seafloor far from the hydrothermal vents and their diverse ecosystems. Perhaps these unknown animals have evolved to survive in this rough environment, adapting to feast on what few nutrients are present in the form of “marine snow”- tiny bits of organic matter formed from bits of dead plankton and other organisms that constantly rain (or rather, snow) down from above.

A large xenophyophore in its shell.

But the question still remains: what sort of animals are making these Paleodictyon structures, exactly?  One hypothesis is that they are a species of giant amoebae known as xenophyophores. These single-celled organisms can be found in the deepest parts of every ocean. Most species cement sand and other debris together to build complex, rippled shells or “tests” which can resemble brains, sponges, heads of lettuce, or other corrugated objects. Some, however, are known to live buried in the sediment, admittedly in much simpler burrows.    

If Paleodictyon nodosum is indeed a xenophyophore, though, why does it excavate such geometrically complex burrows? Laboratory tests have shown that the convex lens-shaped mound over a Paleodictyon burrow draws water down through the vertical shafts, so perhaps the structure is used to pull suspended marine snow from the water column down to the buried amoebae. Alternatively, the tubes could be used for “farming” bacteria on their walls, much like how leafcutter ants in the Amazon will farm fungi on the bits of vegetation, they bring into their nests.  The farming hypothesis is suspect, though, since research has shown that the concentration of bacteria in the tubes is no higher than the concentration on the sea floor above, indicating that there is no deliberate cultivation occurring.

Dr. Rona’s favored possibility as to the identity of Paleodictyon is that the “burrows” may actually be the outline casts of sediment-dwelling sponges or other soft, filter-feeding organisms. This idea is not as strange as it may seem. Many sponges will burrow into mud, coral, or even the hard shells of oysters (the latter by using a weak acid secreted by the sponge’s cells) to protect their soft bodies from predators. As to why no remains of the animals themselves have been found, Dr. Rona suggests that they may have died and been completely devoured by bacteria and small deep-sea scavengers, leaving behind the geometric pit structure as the only evidence of their existence.

It is still curious, though, that in all these years of searching not a single burrow has been found that contains a live Paleodictyon nodosum, or even a few small scraps of their remains. Perhaps it is because the structures are actually much older than they appear. Though the burrows seem freshly dug, the seafloor deserts where they are found are still, quiet places lacking any current and only rarely disturbed by other organisms.  These unusual conditions may have allowed the burrows to persist intact for hundreds of years, long after their builders had completely rotted away.

The lack of a living builder may also simply be due to the fact that the area where Paleodictyon burrows are found has not been explored that extensively. Investigating the deep sea is an expensive endeavor that requires reserving highly competitive slots of time with a submersible or diving robot. Rona and Seilacher simply haven’t had enough time or money to search for these enigmatic builders. Though they have found thousands of Paleodictyon burrows, perhaps these are only the graveyards of a long-dead colony of the creatures. Perhaps the living animals lie just a few miles beyond the submersible’s light, waiting for someone to finally stumble across them.

Paleodictyon has received some media attention, though. Most notably in the form of “Volcanoes of the Deep Sea”, an IMAX documentary chronicling the researcher’s discovery and search for the animal.

On a final, interesting side note, the first recorded reference to Paleodictyon fossils may have come from Leonardo da Vinci. In the Leicester Codex, Leonardo records extensive notes on fossilized shells and other traces of prehistoric marine organisms. Among his drawings is a small, quick sketch of a honeycomb-like structure. Though the sketch is unlabeled, it is not a huge stretch to postulate that this may have been a representation of Paleodictyon, especially since these striking trace fossils are common around the inventor’s childhood home in the valley of the Arno River.