Snakes the size of school buses? Horses the size of house cats? It happened before and could happen again

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Digital reconstruction of a Titanoboa skeleton (Smithsonian Channel)

Sleeping well, lately? This will cure that: Imagine a snake the length of a school bus, with a mouth that opens wider than you can spread your arms. Not only could such a snake exist, it did exist ... and may exist again in the future as temperatures warm.
That's among the findings reported this week at the ScienceWriters2013 conference in Gainesville, Florida. During a period of time known as the Paleocene-Eocene Thermal Maximum, high temperatures favored larger reptiles ... and, in turn, smaller mammals. Imagine, for instance, a horse the size of a house cat.
The Paleocene Epoch lasted about 9 million years, starting with the fall of the dinosaurs 65 million years ago. Jonathan Bloch, a paleontologist at the Florida Museum of Natural History, discovered gargantuan turtle fossils and a snake known as Titanoboa from that period.
That's right. Titanoboa. Beat that, Sharknado.
On the other end of the spectrum, mammals were much smaller in warmer climes. Consider the horse, which, during certain periods in earlier epochs, looked as if it would make a perfect little pet.

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Comparisons of horses across eras. (Danielle Byerley / Florida Museum of Natural History)

When temperatures spiked twice at about 55 million and 53 million years ago, mammals shrunk in size, according to a new study published by the University of Michigan. The changes come about when temperatures rise into the mid-90s for a sustained period of time; the thought is that larger mammals have a harder time regulating body temperatures and finding nutrition.
"Developing a better understanding of the relationship between mammalian body size change and greenhouse gas-induced global warming during the geological past may help us predict ecological changes that may occur in response to current changes in Earth's climate," said Will Clyde of the University of New Hampshire.
As terrifying as the thought of a gigantic snake might be, or as precious as the thought of a football-size horse might be, don't go looking for them any time soon. These changes take centuries, not years. But you might want to leave your great-great-grandchildren a heads-up note, just in case.