Extinções em massa: fósseis 'gigantes' estão revolucionando o pensamento atual

Extinções em massa: fósseis 'gigantes' estão revolucionando o pensamento atual

Quinta-feira, Fevereiro 11, 2010

Mass Extinctions: 'Giant' Fossils Are Revolutionizing Current Thinking

ScienceDaily (Feb. 11, 2010) — Large-sized gastropods (1) (up to 7 cm) dating from only 1 million years after the greatest mass extinction of all time, the Permian-Triassic extinction (2), have been discovered by an international team including a French researcher from the Laboratoire Biogéosciences (CNRS/Université de Bourgogne), working with German, American and Swiss colleagues. These specimens call into question the existence of a "Lilliput effect," the reduction in the size of organisms inhabiting postcrisis biota, normally spanning several million years.

Large-sized gastropods found in marine sediments in Utah dating from only ~1 million years after the P-T mass extinction. The scale bar represents 1 cm. (Credit: Copyright A. Brayard/J. Thomas)

The team's results, published in the February 2010 issue of the journalGeology, have drastically changed paleontologists' current thinking regarding evolutionary dynamics and the way the biosphere functions in the aftermath of a mass extinction event.

The history of life on Earth has been punctuated by numerous mass extinctions, brief periods during which biodiversity is considerably reduced, followed by phases of re-conquest of the biosphere, corresponding to the diversification of those species that survived. Over the last 540 million years, around twenty mass extinctions, of greater or lesser intensity, have succeeded one another. The most devastating of these, the Permian-Triassic (P-T) mass extinction, which decimated more than 90% of the marine species existing at the time, occurred 252.6 million years ago with a violence that is still unequaled today.

In the aftermath of such events, environmental conditions are severely disrupted: the oceans become less oxygenated, water becomes poisonous, there is increased competition, collapse of food chains, etc. Until now, it has generally been accepted that certain marine organisms, such as gastropods or bivalves, were affected by a drastic reduction in size in response to major disruptions of this nature, both during and after the event. It took several million years for such organisms to return to sizes comparable to those that existed prior to the crisis. This is what scientists call the "Lilliput effect," in reference to the travels of Gulliver (3) who was shipwrecked on the island of the same name, inhabited by very small Lilliputians.

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Gastropod evidence against the Early Triassic Lilliput effect

Arnaud Brayard1,*, Alexander Nützel2, Daniel A. Stephen3, Kevin G. Bylund4, Jim Jenks5 and
Hugo Bucher6,7

-Author Affiliations

1UMR 5561 CNRS Biogéosciences, Université de Bourgogne, 21000 Dijon, France

2Bayerische Staatssammlung für Paläontologie und Geologie, Ludwig-Maximilians-University Munich, Department für Geo- und Umweltwissenschaften, Sektion für Paläontologie, 80333 München, Germany

3Department of Earth Science, Utah Valley University, Orem, Utah 84058, USA

4140 South 700 East, Spanish Fork, Utah 84660, USA

51134 Johnson Ridge Lane, West Jordan, Utah 84084, USA

6Paläontologisches Institut und Museum, Universität Zürich, 8006 Zürich, Switzerland

7Department of Earth Sciences, ETH Zürich, Switzerland

*E-mail: arnaud.brayard@u-bourgogne.fr.

Abstract

Size reduction in the aftermath of the Permian-Triassic mass extinction event has repeatedly been described for various marine organisms, including gastropods (the Lilliput effect). A Smithian gastropod assemblage from Utah, USA, reveals numerous large-sized specimens of different genera as high as 70 mm, the largest ever reported from the Early Triassic. Other gastropods reported from Serbia and Italy are also as large as 35 mm. Size frequency distributions of the studied assemblages indicate that they were not unusually small when compared with later Mesozoic and modern faunas. The occurrence of large-sized gastropods less than 2 Ma after the Permian-Triassic mass extinction refutes the Lilliput hypothesis in this clade, at least for the last ∼75% of the Early Triassic.

Received 22 July 2009.
Revision received 7 September 2009.
Accepted 10 September 2009.
© 2010 Geological Society of America