The scenario "is very speculative but plausible," says Donald Jackson, an animal physiologist at Brown University. It's possible that the ancient turtles, like some modern species, were relatively oblivious to their surroundings when they were copulating and, therefore, unable to escape the deadly depths. Also, he notes, if the waters were warm, the turtles' revved-up metabolisms and increased activity would likely have consumed the oxygen in their bloodstream at an exceedingly high rate. "This could have produced mortality very fast."
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This is adapted from ScienceNOW, the online daily news service of the journal Science. http://news.sciencemag.org