Paris, July 25, 2008
Conodont. SEM image showing the mouthpart of this primitive eel-shaped marine vertebrate.
© Arlette Armand – Scanning Electron Microscope image from Laboratoire PaléoEnvironnements et PaléobioSphère
The researchers found that marine water at the beginning of the Ordovician (480 million years ago) was very warm (around 45°C), too warm for complex living organisms to develop. The temperature measurements were obtained from fossils of primitive eels called conodonts, whose geologic age was known by the researchers. They analyzed a mineral found in these eels' skeleton for changes in the ratio of two oxygen isotopes, which is dependent on the temperature of the ocean water in which the animals lived. The early Ordovician was a time when our planet's atmosphere was still very rich in CO2, causing a strong greenhouse effect and therefore very high ocean temperatures.
The progressive ocean cooling coincided with an explosion in marine biomass and biodiversity (the number of genera and families jumped by a factor of three to four). This event took place during the Upper Ordovician, around 460 million years ago, when ocean temperatures became comparable to those of present day equatorial waters. Not only did marine animals diversify, but their range also spread to the seafloor, and the first coral reefs appeared.
The cooling of the oceans was coupled with atmospheric cooling, indicating that a global change in climate took place. This could have played a major role in the unprecedented increase in biodiversity seen in the Ordovician, which opened the modern era of diversity and complexity.
(1) INSU-CNRS Laboratoire PaléoEnvironnements et PaléobioSphère (CNRS, Université Claude Bernard Lyon 1).
(2) Australian National University.
(3) Geologic period extending from 490 to 440 million years ago.
Did cooling oceans trigger Ordovician biodiversification: evidence from conodont thermometry, Julie A. Trotter, Ian S. Williams, Christopher R. Barnes, Christophe Lécuyer & Robert S.Nicoll, Science, July 25, 2008.
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