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2008-10-29 – La fossilizzazione delle argilliti di Burgess (Burgess Shale’s fossil diagenesis)

Nuove indiazioni sul processo di fossilizzazione che ha portato alla formazione dei fossili dei Burgess Shale’s (argilliti di Burgess, Canada – Cambriano).

Lo studio pubblicato sul numero di Novembre di Geology sostiene che i fossili si sono formati in una fase diagenetica precoce (metamorfismo di basso grado) con la migrazione di fillosilicati che si sono sostituiti al materiale originale delle diverse specie e dei diversi tessutti di esse gradualmente e in maniera differenziale a seconda del loro grado di decomposizione.

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Riddle of Burgess Shale’s fossil-rich deposits solved: Scientists

Area of B.C.’s Yoho a treasure trove for fossilized prehistoric soft tissue

Randy Boswell ,  Canwest News Service

Published: Tuesday, October 28, 2008

Researchers have unravelled how one of Canada’s greatest gifts to science – the Burgess Shale fossil site in British Columbia – survived a subterranean superheating a half-billion years ago to preserve hundreds of “exquisite” images of slithering creatures, including a primeval human ancestor, from the “dawn of animal life.”

The existence of the Burgess Shale, a UNESCO World Heritage Site found on a Rocky Mountain cliff in Yoho National Park, is “a paradox,” says a British team that has published a study in the November issue of the journal Geology.

“The fossils have been buried deep in Earth’s crust and heated to over 300 degrees C before being thrust up by tectonic forces to form a mountainous ridge in the Rocky Mountains,” said a statement announcing the study. “Usually, such extreme conditions are thought to destroy fossils. But, in the Burgess Shale, the most exquisite detail of soft tissues has been preserved.”

 'Usually, such extreme conditions are thought to destroy fossils. But, in the Burgess Shale, the most exquisite detail of soft tissues has been preserved,' an article in the November issue of Geology states.

 

‘Usually, such extreme conditions are thought to destroy fossils. But, in the Burgess Shale, the most exquisite detail of soft tissues has been preserved,’ an article in the November issue of Geology states. J.B. Caron

The team, claiming to have “solved this riddle,” concludes that the “beautiful, silvery fossils” survived because the animals’ tissues were replaced during heating in the underground crucible by minerals able to withstand the high temperatures and capture “intricate details such as gills, guts, and even eyes.”

The special geological conditions in which shale formed, helped to “accentuate fossil outlines” and contributed to their “exceptional preservation,” the scientists note in the journal article, written by University of Cambridge geologist Alex Page and three co-authors.

The latest study follows the publication of a paper earlier this year that reconstructed how the sudden burial of an entire seabed ecosystem, following a catastrophic underwater landslide, led to the formation of the Burgess Shale some 530 million years ago.

That study described how an avalanche of “mud-rich slurry” killed tens of thousands of marine animals representing hundreds of species, then sealed them instantly – and enduringly – in a deep-sea tomb.

Interest in solving the mysteries surrounding the formation of the Canadian fossil treasure is further testament to its international importance for scientists.

The site, close to the B.C.-Alberta border, is considered crucial to understanding the so-called Cambrian “explosion” of life – a time when the future Canadian land mass was drifting in tropical climes close to the Earth’s equator.

U.S. paleontologist Charles Walcott, following reports of fabulous fossil finds by railway workers laying tracks across the Rockies in the late 19th century, is said to have tripped over a block of shale that revealed the area’s remarkable supply of fossils.

Scientists have gathered tens of thousands of specimens from the site, capturing in remarkable detail the rich diversity of organisms that suddenly filled the world’s oceans a half-billion years ago.

Among the imprints of animal remains excavated from the Burgess Shale is one called pikaia, an eel-like creature that has been classified as the earliest known, identifiable ancestor of modern vertebrates – including humans.

© Canwest News Service 2008

source: http://www.canada.com/topics/news/national/story.html?id=235d4299-5943-48bb-9aaf-aaffd5305b8d

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Original article:

Geology – Volume 36, Issue 11 (November 2008) pag.855

Ubiquitous Burgess Shale–style “clay templates” in low-grade metamorphic mudrocks

Alex Page, Sarah E. Gabbott, Philip R. Wilby, and Jan A. Zalasiewicz

Abstract . Full Text . PDF (473K)

Abstract:

Despite the Burgess Shale’s (British Columbia, Canada) paleobiological importance, there is little consensus regarding its taphonomy. Its organic fossils are preserved as compressions associated with phyllosilicate films (“clay templates”). Debate focuses on whether these templates were fundamental in exceptional preservation or if they formed in metamorphism, meaning that it is important to establish the timing of their formation relative to decay. An early diagenetic origin has been proposed based on anatomy-specific variations in their composition, purportedly reflecting contrasts in decay. However, we demonstrate that these films bear a remarkable similarity to those that occur on organic fossils in graptolitic mudrocks and form as a normal product of low-grade metamorphism. Such phyllosilicates may also occur within voids created by volume loss in maturation, a process that may have aided their formation. In bedding-plane assemblages from graptolitic mudrocks, different taxa are associated with distinct phyllosilicates. This likely reflects stepwise maturation of their constituent kerogens in an evolving hydrothermal fluid, with different phyllosilicates forming as each taxon progressively underwent maturation. These observations provide an analogue for the distribution and composition of phyllosilicates on Burgess Shale fossils, which we interpret as reflecting variations in the maturation of their constituent tissues. Thus, their clay templates seem unremarkable, forming too late to account for exceptional preservation.

Received: March 25, 2008; Revised: July 9, 2008; Accepted: July 22, 2008

DOI: 10.1130/G24991A.1

 *E-mail: aap30@esc.cam.ac.uk.

ottobre 29, 2008 - Posted by | America Northern, Articolo sc. di riferimento, G - Diagenesi, Italiano (riassunto), P - Preservazione eccezionale, Paleontology / Paleontologia | , , , , , , , , , ,

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