Paleodictyon nodosum

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 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 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.Peter A. Rona, discoverer of the modern burrows, suggests that Paleodictyon nodosum may actually be a large protist. There are other known examples of protists reaching the sizes that Paleodictyon reaches, and they are known to be infaunal. Scientists ran various tests on the burrows of Paleodictyon and were unable to reach a single conclusion as to the form of Paleodictyon. 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.

They were originally photographed in 1976 on the Galapagos Rift between 2400-3700m depth. 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. 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.

The burrows of 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 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). 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. 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. 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. However the microbial counts didn't change from the inside of the burrows to the area surrounding it. The fossil record indicates that these burrows could be one of the earliest examples of complex structures being built by animals.

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