An artist’s conception shows how the largest snake the world has ever known would have looked in its natural setting 60 million years ago. Partial skeletons of the giant, boa-constrictor-like snake named “Titanoboa” were found in Colombia. Jason Bourque / University of Florida

GAINESVILLE, Fla. — Tens of millions of years ago, snakes were as big as horses. Horses were almost as small as snakes. And in a warmer world, it could get that way again.

That's the implication of research showing that warmer temperatures generally favor smaller mammals and larger reptiles.

"You see the size of these animals dancing with the climate," said Jonathan Bloch, a paleontologist at the Florida Museum of Natural History.

Bloch delved into the connection between body size and global temperatures, particularly during a hot time known as the Paleocene-Eocene Thermal Maximum, on Monday during the ScienceWriters2013 conference here in Gainesville. Like so many facets of global change, the lessons from the distant past don't make the far future look all that sunny. Super-snakes, anyone?

Some like it hot

For years, Bloch and his colleagues have traced the ups and downs of the Paleocene Epoch, which lasted from the downfall of the dinosaurs 65 million years ago to the start of the Eocene about 56 million years ago. One of the key sites for fossils from that age is the Cerrejon Mine in Colombia, where the coal seams are so active they can spontaneously combust.



"It really is like hell, but it's heaven for fossils," Bloch said.

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That's where Bloch found evidence of 60 million-year-old turtles as big as breakfast tables, and a snake called Titanoboa that was as long as a bus. Pointing toward the entryway at the back of the hall, Bloch said, "Imagine that the snake would have to squeeze through the door, and come up to your waist."

Snakes, turtles and other reptiles tend to depend on the environment to regulate their heat — putting them in a category known as ectotherms. ("Cold-blooded" is a commonly used term, though it's a bit of a misnomer.) The only way ancient ectotherms could get as big as they did would be for them to live in a hot climate, and the world was indeed much hotter during the Paleocene-Eocene Thermal Maximum.

Experts estimate that global temperatures jumped somewhere in the range of 9 to 14 degrees Fahrenheit (5 to 8 degrees Celsius), due to a massive but mysterious release of greenhouse gases. Titanoboa, for example, thrived amid temperatures as hot as 93 degrees F (34 degrees C).

A modern-day horse looms like a monster over the earliest known horse, Sifrhippus (at right), in this illustration. Danielle Byerley / Florida Museum of Natural History

Mini-mammals

Paleontologists have also studied how mammals fared back in the Paleocene-Eocene. Last year, Bloch and other researchers said the ancestors of modern-day horses shrunk to the size of housecats when temperatures spiked 55 million years ago. This month, a different team reported that another episode of mammalian dwarfism occurred during a second warming event 2 million years later.



"The fact that it happened twice significantly increases our confidence that we're seeing cause and effect, that one interesting response to global warming in the past was a substantial decrease in body size in mammalian species," the University of Michigan's Philip Gingerich, one of the paleontologists behind the latest study, said in a news release.

Several factors have been proposed for the mammalian downsizing: Research suggests that when temperatures rise into the mid-90s (35 degrees C) for an extended period, mammals have a harder time regulating body heat, and less nutrition is available from plant sources. Under those conditions, smaller mammals would fare better than bigger ones.

What lies ahead?

Here's the scary part: If it's happened before, it could happen again — and perhaps sooner than we think. Bloch noted that atmospheric carbon dioxide levels are approaching what they were during the Paleocene-Eocene Thermal Maximum.



"You have to go back tens of millions of years before you get close to or higher than what we're talking about for the next couple of hundred years," Bloch said.

Bloch hinted that he and his colleagues may soon be filling out the picture for the rise of mammals with more fossil finds. In any case, learning more about the hot times of the ancient past — and how they cooled off — could provide the key for coping with future climate change.

Somehow, our planet found a way to reduce atmospheric carbon dioxide and its global warming effect. Perhaps humanity can take advantage of those same strategies — and if so, the record of the Paleocene-Eocene Thermal Maximum and its aftermath could serve as a "user's manual for Earth," Bloch said.

More tales of the first mammals:

Update for 9:40 a.m. ET Nov. 6: An earlier version of this story included an embedded YouTube version of the Smithsonian Channel documentary "Titanoboa: Monster Snake." However, that version of the video can only be played back from the original YouTube page. The episode is also available on the Smithsonian Channel website.

Alan Boyle is NBCNews.com's science editor as well as president of the Council for the Advancement of Science Writing. CASW and the National Association of Science Writers presented the ScienceWriters2013 conference in Gainesville, in cooperation with the University of Florida. Bloch's talk was part of CASW's New Horizons in Science briefings.



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