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Metre-Long Medusas And Molluscs Seize The Ocean

27.11.2002


The first outbreak of the evolution of multicellular organisms falls on the Wend, the last period of the Proterozoic (Precambrian), about 620-550 million years ago. At that time, climate of our planet was rather cold, and glaciers that covered the single supercontinent nearly reached the equator. The cold is beneficial for the evolution of sea creatures.



In modern seas, significant concentrations of dissolved oxygen, phosphates, and the organic matter provide for a high biological productivity and the appearance of very large animals. In ancient times, the situation was probably similar: first multicellular organisms lived in cold seawater.

As is known, there was a sharp increase in the fauna diversity in the Cambrian Period. However, in the preceding period, Wend, the fauna was rather rich too, as said Mikhail A. Fedonkin in his report of October 17 in the Vavilov Institute of General Genetics (Moscow). Fedonkin is the corresponding member of the Russian Academy of Sciences and the head of the Laboratory of Precambrian Organisms in the Paleontological Institute in Moscow.


Most exiting news has come from the White Sea area. Imprints of various creatures are found within deposits of the Wend Period (clay and sandstone strata with a total thickness of about one kilometre). Some of those creatures resemble modern sponges. Others are like polyps: actiniae or sea anemones plunged into the ground so that only the crown of tentacles is left above. Some animals were real giants: medusas that were attached to the sea bottom reached one meter in diameter!

However, German palaeontologist Adolf Seilacher recently stated that the fauna of the Wend had no relationship with any of modern types and classes of animals, but was a unique evolutionary experiment: there was the world of giant unicellular organisms. But studies of Russian scientists refuted this extraordinary hypothesis.

Specialists from the Paleontological Institute of the Russian Academy of Sciences proved using numerous facts that many Precambrian creatures had a three-dimensional shape (but not a two-dimensional plate-like shape, as Seilacher suggested). In addition, some of them were capable of moving: their pathways were found in fossils together with their body imprints. For example, one weird creature (Dikinsonia), which was one-meter-long, round-shaped, and covered with rough skinny shell, moved about with casual changes in direction and stops, like modern invertebrates do when look for food. A giant cell cannot be so bulky and crawl that way. So, the Seilacher’’’’s hypothesis is obviously false: the fauna of the Wend was multicellular.

Another curious animal (Kimberella) was reconstructed by Australian palaeontologists from imprints as a medusa and shown in that form in some books. However, that reconstruction was based on too many assumptions. Recently, Russian scientists have revealed that it was not a medusa, but a mollusc: a kind of a large snail that appeared from its shell as a fluffy frill resembling old-fashioned Spanish neckwear.

Such creatures populated warm water basins in the Cambrian Period, about 500 million years ago. An increased concentration of carbonates in warm water allowed the fauna to build a more complex skeleton. At that time, first attempts to conquer the land were done. Fedonkin and his colleague E. Echelson from the United States have made a detailed description of fossil imprints resembling those of automobile tires and left by large animals that lived within the tidal zone of the seacoast. Thus, one more chapter is added to the early history of the animal world of the Earth.

Mrs. Elena Kleschenko | alfa
Further information:
http://www.informnauka.ru/eng/2002/2002-11-27-02_261_e.htm

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