2019-01-22 10:15:06
A few years ago we talked about the Cambrian explosion and the enigmatic Ediacaran organisms that preceded it. Among these we cited Dickinsonia, with a flat and oval body, marked with radial striations that divide it into segments, and which lived between 567 and 550 million years ago.
Dickinsonia fossils are casts in sandstone beds. They measure between a few millimeters and almost a meter and a half in length, with a thickness between a fraction of a millimeter and a few millimeters. Its oval body, which lacks a mouth and anus, has one end wider than the other. At the broad end is a triangular region without segments, called the deltoid region; and the segments defined by the radial striae are also broadest near this region. Along the midline of the Dickinsonia body a narrow ridge or groove divides the segments into two halves. These halves are not perfectly aligned, but instead the center of the segments on one side coincide with the serrations that separate the segments on the other. In biology, segments with this type of sliding reflection symmetry are called isomers. The German paleontologist Adolf Seilacher proposed that Dickinsonia isomers are pressurized fluid-filled chambers, inflated like an air mattress.
Other organisms more or less contemporary with Dickinsonia also have their bodies divided into isomers, such as Spriggina, which is more elongated, Yorgia, in which one end, perhaps the head, lacks isomers, and Rutgersella, in which the midline is not. extends to the ends of the body. The isomers are not unique to Ediacaran organisms; They are also present in some living animals, such as cephalochordate or amphioxus larvae and some sea pens.
The first Dickinsonia fossils were discovered in the Flinders Mountains in South Australia in 1947. Australian geologist Reginald Sprigg named them Dickinsonia after his boss, Ben Dickinson, the director of mines for South Australia. Later, other fossils have been found in the Ukraine, in the Dniester basin, and in Russia, in the Archangel Region near the White Sea and in the central Urals. Nine Dickinsonia species have been described since their discovery in 1947, although only four or five are currently accepted.
Some trace fossils have also been attributed to Dickinsonia. These are rounded impressions with less detail than fossils of the organism. They may be the traces left by Dickinsonia on the seabed, perhaps by secreting some type of slime to isolate itself from the microbial mat that covered the seabed, or perhaps to dissolve and devour it by external digestion on its lower surface, like current placozoans, with whom you might be related. They have also been interpreted as the tracks left by organisms rolling along the bottom dragged by the currents.
Dickinsonia spends most of its time anchored to the seafloor, although it seems that it could move from one place to another. Two overlapping fossils have never been found; when they are adjacent, they are deformed to avoid contact, as if they compete with each other or secrete some kind of repulsive substance. The way in which the fossils are preserved indicates that the hard parts of the organism are not mineralized, but made of a resistant biopolymer, such as keratin.
Regarding its relationship with other living beings, Dickinsonia has been interpreted as a jellyfish, a coral, a polychaete worm, a planaria, an anemone, a chordate or even a fungus, a lichen or a giant unicellular organism. Two recent discoveries point to Dickinsonia being an animal. The first is the growth pattern of the species. Since we have many Dickinsonia fossils of very different sizes, it has been possible to determine how it developed. Dickinsonia grew throughout its life, which is why some individuals reach such large sizes. The smallest specimens, we assume the youngest, had a proportionally larger deltoid region and much fewer segments than the larger specimens. Some specimens have been found in which new segments appear partially formed next to the deltoid region; this implies that it is in this area where the growth of the animal occurs. These new segments, which are larger and larger, inflate until, by the appearance of more segments behind them, they have traveled a third of the length of the body, where the animal reaches its maximum width. The segments located at the opposite end to the deltoid region are the smallest and the oldest; Due to the push for the generation of new segments, they are also the narrowest. Growth from a region near an end, but not from the end itself, also occurs in some animals, but not in other living things. In those animals, the growth region is usually at the rear of the animal, so we can assume that the deltoid region is the Dickinsonia tail.
The second discovery has been possible thanks to the extraction of eight small fossils in an extraordinary state of preservation from the cliffs near Lyamtsa, on the Russian coast of the White Sea. To do this, Ilya Bobrovskiy, a doctoral student at the Australian National University, had to travel in a Russian military helicopter to the top of the cliffs and rappel down from there. The fossils, excavated from the cliff face and immediately wrapped in sterile foil to prevent any contamination, were transported to a clean laboratory in Australia; there the remains of organic matter present in the fossils and in the surrounding rock matrix were extracted. The analysis of these remains in search of sterols, compounds that are part of the cell membrane of living beings, found a significant difference between the sterols present in the fossil and those in the matrix. The sterols in the fossil are almost exclusively cholesterol, as is typical of animal cell membranes, while stigmasteroids, characteristic of algae, abounded around them. A similar analysis carried out by the same team on fossils of another species from the same time, Beltanelliformis, which is disc-shaped, has shown that it is not an animal, but colonies of bacteria.
While some paleontologists have drawn attention to the presence of cholesterol in certain fungi, which, together with stigmasteroids on surrounding algae, might not rule out identification of Dickinsonia with a lichen, hints that this was a motile organism eliminate that possibility. Although we still do not know which groups it was most closely related to, or whether it is a side branch extinct before the Cambrian Explosion or, on the contrary, it is the ancestor of some later group, what we can be pretty sure of is that Dickinsonia is the oldest animal we know of. For now.
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