The Paleodictyon nodosum is a creature thought to produce a certain form of Paleodictyon burrow found around mid-ocean ridge systems in the Pacific and Atlantic Oceans. Although scientists have collected many of the burrows of the Paleodictyon nodosum, they have never seen a live one. What a live specimen would look like is widely debated, with the debate being split into two main sides. Adolf Seilacher who discovered the original fossils of the Paleodictyon nodosum hypothesizes that the creature is a worm like species that burrows into the sediment around hydrothermal vents and deflects water flow through the burrows to catch food or farm its own food.[1] Peter A. Rona, discoverer of the modern burrows, suggests that the Paleodictyon nodosum may actually be a large protist.[1] There are other known examples of protists reaching the sizes that the Paleodictyon nodosum reaches, and they are known to be infaunal. Scientists ran various tests on the burrows of the Paleodictyon nodosum and were unable to reach a single conclusion as to the form of the Paleodictyon nodosum. The one thing that they can agree upon is that there are many markers that suggest that these forms are caused by a creature, and not by geological forces.[1]
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They were originally photographed in 1976 on the Galapagos Rift between 2400-3700m depth.[2] Later, Seilacher and Rona used the deep-water submersible DSV Alvin to recover samples of the same form near the Mid-Atlantic Ridge. These samples were collected between 3430m and 3575m depth, around 26°N and 45°W.[1] These burrows were found in very similar conditions as the ones found along the Galapagos Rift. The biggest similarity between the habitats of all Paleodictyon nodosum is that they are all found along convergent plate boundaries at both active and extinct hydrothermal vents.[1]
The burrows of the Paleodictyon nodosum are one of the few things about them that scientists are actually able to study, and so this is possibly the area of which we know the most about the Paleodictyon nodosum. The Top of the form is shaped like a shield, with the center raised, and a lip around the outside. The center is raised approximately 5mm above the low points. Each horizontal section consists of 3 equidistant rows of tiny holes (approximately 1mm in diameter) that connect at 120° angles. Each of these horizontal sections are connected by vertical shafts (approximately 2-3mm in diameter).[1] When actively being inhabited, the surface of the burrow is made of red metalliferous sediment. When it becomes inactive, this becomes covered with a light gray Lutite and the top flattens out. The red sediment is only found under the surface sediment in this environment, so its presence at the surface hints at a biotic factor which brings up the sediment.[1] The number of rows and the spacing of these rows increases in correlation with the size of the overall form. This indicates that these burrows are a result of organic growth.[1] The raised parts of the burrow force water to flow through the burrow. As a result of this, scientists found large numbers of foraminifera tests within the burrows. These were trapped in certain areas suggesting that the burrows were engineered to catch food as prey.[1] However the microbial counts didn’t change from the inside of the burrows to the area surrounding it.[1] The fossil record indicates that these burrows could be one of the earliest examples of complex structures being built by animals.[1]
The theory that Seilacher supports, that the burrows we find are trace fossils of a worm like animal, is supported by several features of the burrows, and is also not an unheard of concept in the animal kingdom. The shape of the burrows is consistent with other graphoglyptids. The burrows also had several exits, which is inconsistent with the idea of a megalith foraminifera. Another animal that creates burrows and cultivates food within its burrows is the leafcutter ant.[3] This suggests that the idea of an animal cultivating its own food is not unreasonable, and the fact that the environment is so low in nutrition it seems like a logical evolutionary step to have been taken.
The second hypothesis, supported by Rona, is that a sponge or megalith foraminifera such as a xenophyophore left the structures as a cast of its body. Through testing of collected burrow samples, this theory gained a lot of evidence both for and against it. One test that was run was watching how water flowed over the burrows. The forced water flow exhibited by these specimen was similar to the forced water flow in several sponges.[4] Another reason that this hypothesis seems likely is that best estimates of the size of the worm suggest that it would have to travel unreasonably long distances (compared to its body length) to fully navigate its burrows (10^3-10^4). However there are several problems with this hypothesis as well. If this was the remains of the body of a creature, you would expect to find organic matter from that creature throughout the burrow. However when the burrows were tested for DNA, scientists found DNA from different types of large protists between different burrows. This suggests that the DNA found is just there because it was transported there by currents. However one encouraging fact is that one of the types of DNA found in a burrow was of the Vanhoeffenella, which creates hexagonal burrows similar to those of the Paleodictyon nodosum.[1] The barium content in the sediment making up the burrows had no significant difference from the barium content in the surrounding sediment. This is inconsistent with the burrows of other xenophyophores. Also the complexity and evenness of the burrows is not consistent with the forms that xenophyophores generally create.
The fossils of the Paleodictyon nodosum were first found in the cliffs of Spain in the 1950’s.[2] Since then, they have been discovered all over Europe and in Wales. They are generally found in flysch deposits from the Eocene period, which began with the arrival of mammals.[1] Over time the fossils gain a much more uniform shape. The oldest fossils show much less uniformly hexagonal burrows, but as time goes on their burrows become much more consistent and precise.[2]
The IMAX film Volcanoes of the Deep Sea[2] describes the search for the Paleodictyon nodosum, using the deep-water submersible DSV Alvin near volcanic vents that lie 3500 meters (12,000 feet) underwater in the Mid-Atlantic Ridge. Samples were taken from several honeycomb burrows, however no creatures were found in any of them. They theorized that the burrows were being used for bacterial farming by whichever creature created them.[2]
George Wald presented the idea that the 4 necessities of life were:[5]
Many of these ideas are fairly vague, and while the general principles hold true, how we define each part of this set of rules has been challenged by the Paleodictyon nodosum.
It was once assumed that the only source of energy that supported life was the sun. However the discovery of chemosynthetic bacteria in giant tube worms introduced a new source of energy, chemicals from within the Earth.[6] However this discovery led people to believe that creatures could only survive in these environments when the vents were active. However the Paleodictyon nodosum can survive even after the extinction of these vents. This challenges our ideas of what a suitable environment truly means. The fact that a creature can survive in an area where there is extreme pressure and almost no biomass gives us a new appreciation of the adaptations that animals have developed to survive in even the most inhospitable locations on the planet.