Time travel is hard, which makes learning about the earth’s past climate difficult. For example, what happened to the earth’s temperature at the end of the Cretaceous, when dinosaurs went extinct?

To find out, we have to use proxies – ancient objects whose form or composition indicate the climate at the time they were made. Last week I traveled to the Denver Museum of Nature and Science to study fossil leaves, which will hopefully be useful proxies for the late Cretaceous climate.

The museum has vast store-rooms of all sorts of specimens collected from around the world. Walking through these spaces is like a treasure hunt – one never knows if a sloth or stegosaurus might be hiding in the next cabinet over!

I was working with leaf fossils collected from what is now North Dakota. This paleo-flora comprises hundreds of species, nearly all of which are now extinct. Leaves may be useful climate proxies because their form indicates their function, and their function is tightly linked to optimal resource acquisition in different climates (which is why, for example, large and thin water-hungry leaves are never found in the desert).

Kirk Johnson and others have done the hard work of collecting, identifying, and cataloging thousands of leaf fossils from this flora. They are now stored, orderly-like, in hundreds of drawers in dozens of cabinets – just waiting for someone to look at them.

I’m especially interested in the venation network of each leaf, because veins may be a very good proxy for temperature via their role in constraining leaf water usage. Fortunately many fossils have exquisite preservation of veins!

I made micrographs of thousands of specimens last week. Here’s an example image – amazing that this detail is preserved in an 66 million-year-old object!

Currently I don’t know how good a proxy these fossil leaves are – but after a few weeks of work I will, and hopefully we will discover something new about the end of the Cretaceous!