The team, led by Julie Trotter of the Australian National University, used a new, more accurate technique for analyzing oxygen isotopes in the fossilized teeth of an ancient eel-like animal called a conodont.
About the size of an adult human finger, conodonts once swam in the tropical shallow seas that covered North America and Australia. The fossils were collected throughout sites in modern-day Canada and Australia.
Like today's sharks, the animals' teeth are the only hard parts in their bodies that are preserved as fossils.
(Related: "Tiny Fossils Reveal Warm Antarctic Past" [July 25, 2008].)
The composition of the teeth—which are made of calcium phosphate—hasn't significantly changed with time or temperature, Barnes said.
This makes the teeth better research subjects than shells made of calcium carbonate, which recrystallize over time and lose their isotopic value, he added.
Based on ratios of the isotopes in conodont teeth, the scientists established a steady decline in sea-surface temperatures from 488 to 444 million years ago.
The research was published recently in the journal Science.
Clouded by Assumptions
The authors have produced a very nice data set, Richard Norris, a paleooceanagrapher at the Scripps Institution of Oceanography in San Diego, said in an email. The interpretation of that data, however, is clouded by assumptions, he said.
One assumption, for instance, is that there was no ice until late in the end of the Ordovician period.
Ice would have affected the ocean's isotope composition and could have lowered the temperature by several degrees. While he agrees that the study used a novel technique, Norris expressed skepticism about absolute temperature values derived from it.
He pointed out that no adjustment was made for an isotope measurement that varies due to evaporation or rainfall.
"It is also possible that the long-term temperature trend that is reported in the paper is an artifact of changes in oceanography between the different sites used in the study," Norris said.
For example, changes in the water depth, geographic position of the sites relative to areas of high rainfall or high evaporation, or changes in ice volume on the Poles could influence the results.
Norris added that he thought a link between temperature and diversification was not conclusive.
"Lots of things are correlated in nature without being strict causal relationships."
But lead author Trotter said that her team assumed there was no ice until the end of the Ordovician era based on the established geological record.
In any case, the large decrease must mainly result from temperature, not ice volume, and the known glacial event is clearly reflected in our [findings], she said.
The team also chose conodont samples from shallow water depths that weren't completely drained by tides to avoid temperature changes, she added.
Our paper does indeed acknowledge many other potential regional and local effects on the biosphere, but clearly shifts of this magnitude in global temperature will be an important higher level controlling factor," Trotter said, "as recognized by many studies and most of the scientific community that today worries about the effects of future global warming."
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