Histórias da Geologia

Este blog pretende ser um espaço de reflexão sobre a evolução do conhecimento geológico ao longo dos tempos, principalmente a partir do século XVII, não esquecendo porém as pequenas histórias que fazem a grande história

segunda-feira, abril 10, 2006

Eozoon canadense - os problemas das classificações

When William Dawson established Eozoon canadense, he hailed this fossil as "one of the brightest gems in the scientific crown of the Geological Survey of Canada".

To many mid-Victorian geologists and paleontologists these laminated green and grey rock specimens from altered limestones of the Canadian Shield of Ontario and Quebec were the most important fossils ever found because they constituted evidence of the existence of complex life forms deep in the Precambrian. J. William Dawson, the Principal of McGill University and one of the foremost geologists in Canada, named the fossil Eozoon canadense -- the Canadian dawn animal. In his presidential address to the British Association for the Advancement of Science in 1864, Sir Charles Lyell singled out this fossil as "one of the greatest geological discoveries of his time". Charles Darwin, in the fourth edition of Origin of Species in 1866, was relieved to be able to cite the first fossil evidence that the succession of life on earth proceeded from simple unicellular organisms to complex multicellular animals and plants.

Dawson concluded that Eozoon was the shell of a foraminiferan, a single-celled protistan complete with chambers and canal systems, but one hundreds of times larger than any of the living forms which are all microscopic. This view was almost immediately challenged by a pair of Irish geologists who claimed that Eozoon was not organic, but a metamorphic feature formed from the segregation of minerals in marble through the influence of great heat and pressure. For the next ten years, increasingly acrimonious debates on the nature of Eozoon appeared in British and American journals. An exhaustive analysis by a German professor of zoology, Karl Möbius in 1879 demonstrated that Eozoon displayed not a single characteristic trait of foraminiferans. This settled the matter for virtually all geologists and paleontologists, but not Dawson.

Compelling evidence came from an unlikely source in 1894 when two geologists reported Eozoon in limestone blocks ejected from the volcano Mount Vesuvius. Precambrian fossils shot out of an Italian volcano was something few paleontologists were prepared to accept. Undeterred, but virtually alone in the geological community, Sir William continued to defend the organic nature of Eozoon. When he died in 1899 he was working on yet another Eozoon paper.

(informação recolhida no site do Geological Survey of Canada)

O naturalista do Museu Britânico Randolph Kirkpatrick (1863-1950), que visitou a Madeira e Porto Santo, afirmou ter encontrado Eozoon canadense nesta última ilha.

A propósito do problema das classificações no domínio da paleontologia: 'The history of paleontology shows what difficult it has had to face and which continue to lie ahead. Late into the 18th century it was disputed wheter fossils were representations of organisms or products of a vis plastica, 'tricks of nature', which only accidentally resembled the shapes of plants and animals. Even today 'pseudofossils' are known, structures of inorganic origin which resemble organic remains. 'Problematica' are structures and shapes in sedimentary or methamorphic rocks whose organic origin is dubious. Up till certain structures in Precambrian strata constituted such problematica. These were called 'Eozoon bavaricum' in the Bavarian Forest, and 'Eozoon canadense' in Canada, and are now attributed to marine algae.' (Engelhardt e Zimmermann, Theory of Earth Science, p. 119)