The fossil site of Messel, near Darmstadt (central Germany) is a world heritage site; it is famous throughout the world for the fossils of animals and plants from a tropical landscape 47 million years ago, all of them excellently preserved. Nowhere else have so many bats and birds been found in lake deposits. Among the mammals even the contents of the stomach are usually preserved. But how did these animals die? The well-filled stomachs are not exactly an indicator of disease or fatal debility. Until recently the cause of death was assumed to be, inter alia, gases of volcanic origin which may have collected over the lake. This might explain why the animals suffocated. But such clouds of gas – if they indeed existed – must have dispersed rapidly, given the size of the lake. It is still a moot point whether, after hundreds of thousands of years, gas was still escaping from the volcanic subsoil which formed the extinct volcanic crater lake of Messel.
The University of Bonn palaeontologists on Professor Wighart von Koenigswald's team have proposed a new theory in the latest issue of the Paläontologische Zeitschrift which sheds light on the possible cause of death. While examining the fossils the researchers became aware that the deaths must have occurred at the same time of year in different years. The five pregnant mares which were found at completely different levels in the oil shale at Messel all died at the same time of year, as the foetuses were at the same stage of development. Among the tortoises there were also five pairs which died during copulation, i.e. during the breeding season.
One more piece of the puzzle was provided when the Bonn lecturer Dr. Andreas Braun noticed that there are lime deposits in the sedimentary structures of Messel. A very similar structure occurs in lake deposits which Professor von Koenigswald's doctoral student Thekla Pfeiffer discovered in Neumark-Nord. In deposits which were about 200,000 years old she was able to detect traces of the highly toxic microcystine, a poison which is produced by cyanobacteria. The researchers assume that the sedimentary structures in Messel are also due to these microbes, also known as 'blue-green algae'. The animals may therefore have died from microcystine poisoning due to the seasonal algal bloom caused by deadly cyanobacteria.
From Canada we know that during algal bloom cyanobacteria cause toxic foam to collect in the surface water. Anything that drinks this water collapses almost immediately. This is true of both land animals and birds. Observations have shown that even the tiny quantities of water drunk by bats when flying low over the water can be fatal. Many aspects of the fossil finds of Messel which were not previously understood can be explained by this theory of a seasonal growth of highly toxic cyanobacteria which was repeated year after year. The theory still awaits further confirmation. One difficulty, however, is already apparent: it will be very difficult to provide direct evidence of toxic agents after 47 million years.
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