Whether the dinosaurs that dominated the earth for 125 million years were warm-blooded or cold-blooded is a continuing source of debate, as is the question of whether their descendants live among us today as birds. Also hotly debated is the cause of their sudden and mysterious disappearance at the end of the Cretaceous period, 65 million years ago.

Blond and boyish, Markwick is a native of Worthing, on the south coast of England. He earned his undergraduate degree at Oxford University, spending his summers doing research for British Petroleum, which is what got him interested in paleoclimatology. After finishing at Oxford, he worked for BP for two more years. “I was looking for a PhD, basically because the only way to advance in the company was to have a piece of paper,” he says. So he started a program in the Department of Geophysical Sciences at U. of C. But he says he likes teaching and would like to stay in academia.

SBM: Nice work if you can get it.

PM: Since the earth’s magnetic field is assumed not to have changed through time, iron minerals preserve the magnetic field at the place where they were formed. By reconstructing the magnetic field preserved in the rock we can reconstruct the latitude at which it was formed.

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What I found was that modern crocodilians began to diversify sometime in the Cretaceous and just continued to diversify all the way into the Paleocene–and it seemed to cross the Cretaceous-Tertiary boundary without any sign of a major extinction. You get to the Oligocene–about 30 or 35 million years ago, when we know that there is antarctic ice forming, we know the climate is changing from other evidence–and we suddenly see the diversity of crocodilians drop. Then they recover a little bit, until you get to about three million years ago, when we start getting the formation of more arctic ice. The climate cools, and again the diversity drops.

The crocodiles are clearly following some sort of climate signal, but there’s no major extinction, no major change in diversity at the Cretaceous-Tertiary boundary. This suggests that whatever caused the KT extinctions, it wasn’t climate–or at least climate wasn’t a major player.

Some crocodilians actually do go extinct at that point. What’s odd is that they all seem to be marine crocodiles.