This week, the auction house Sotheby's will sell off the bones of "Gus," a Tyrannosaurus rex fossil unearthed in the South Dakota Badlands.

With a starting bid of $19 million, Gus is expected to become the most expensive dinosaur in history, a price tag that says as much about the collectors in the room as it does about the fossil on the block.

But strip away the dollar signs, and Gus is something far more valuable than a trophy. It's a window into a version of Earth almost unrecognizable from our own.

A planet without ice

Gus lived roughly 67 million years ago, near the end of the Cretaceous period. It was a hothouse world, warmer and wetter than today, with little to no permanent ice at the poles. Sea levels were higher. Forests grew in places we now think of as frozen and barren.

There was a familiar culprit behind all that warmth: carbon dioxide. Intense volcanic activity had pumped enormous amounts of the heat-trapping gas into the atmosphere over millions of years, warming the globe all the way to the poles.

Nowhere illustrates that better than Antarctica. Today it's a continent buried under ice sheets miles thick. But in the late Cretaceous, it was green, covered in lush, temperate rainforests and home to a surprising variety of dinosaurs.

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We know this in part thanks to a discovery that has been hiding in plain sight for four decades.

The fossil that spent 40 years in a drawer

In December 1985, a geologist named Mike Thomson collected a fossil during an expedition to James Ross Island, off the Antarctic Peninsula. His team, part of the British Antarctic Survey (BAS), was mapping rock layers and gathering marine reptile fossils to help date them. One mysterious vertebra got logged in Thomson's field notebook as a "vertebra of large reptile," boxed up, shipped back to the United Kingdom and shelved in the BAS collections.

There it sat, unstudied, for 40 years.

Its true identity came to light when Mark Evans, a paleontologist and collections manager at BAS, was sorting through the archive and suspected the uniquely shaped tailbone belonged to a dinosaur. He contacted Paul Barrett, a paleontologist at the Natural History Museum in London. This year, the team confirmed in the journal Acta Palaeontologica Polonica that the specimen was no marine reptile at all. It was an 82-million-year-old tail vertebra from a titanosaur, a member of the group that includes the largest land animals ever to walk the Earth.

More striking still: It was the very first dinosaur bone ever collected from Antarctica, misidentified from the start under, as the researchers note, harsh field conditions.

Small bone, big implications

The vertebra itself is modest, a single tailbone about 10 centimeters long. But paleontologists can read a surprising amount from a single bone. A telltale hollow on one end and a rounded bump on the other revealed how it once fit into a line of ball-and-socket joints, a combination of features Barrett says is unique to titanosaurs. Its closest match was a species previously found in Argentina.

The animal was small for a titanosaur, an estimated 23 feet long, though the team can't yet say whether that means it was a juvenile or simply a smaller-bodied species. Because the fossil is so fragmentary, they were careful not to over-claim, stopping short of naming a new species.

What excites scientists isn't the size, though. It's the geography.

A dinosaur highway across the bottom of the world

Titanosaur fossils have turned up in South America and New Zealand, but curiously, not in Australia. That's a puzzle, until you remember that the continents weren't arranged the way they are today.

In the late Cretaceous, the southern land masses were still knitting apart from the ancient supercontinent Gondwana. New Zealand sat far closer to South America and the Antarctic Peninsula than it did to Australia. That put the tip of South America, the Antarctic Peninsula, and the ancient continent of Zealandia all in roughly the same neighborhood.

Confirming that titanosaurs lived in Antarctica helps fill in a missing link. It suggests these giants could have migrated from South America into New Zealand by way of the Antarctic Peninsula, effectively using a warm, forested Antarctica as a land bridge, while bypassing Australia entirely.

In other words, a single overlooked bone helps redraw the map of how some of the planet's largest animals moved across a warmer world.

Why any of this matters to your forecast

So what does a T. rex on the auction block, or a titanosaur tailbone from a museum drawer, have to do with the weather?

Quite a lot, as it turns out. That ancient greenhouse world is one of the best reference points we have for understanding where a warming climate can lead. By studying fossils and the rock they're buried in, scientists can reconstruct how life responded the last time the planet was as warm as Gus's world: which forests spread, which species thrived, which vanished. It's some of the only real-world data we have for a hotter Earth.

And remember what drove that ancient warmth: carbon dioxide. Back then it came from volcanoes, building up slowly over millions of years. Today we're adding it ourselves, in a matter of decades.

That difference in speed is everything. The climate shifts of the Cretaceous unfolded over hundreds of thousands of years, giving ecosystems time to adapt and migrate, like those titanosaurs ambling across Antarctica. Today's warming is happening in a geological blink. And Antarctica, the same continent that once hosted rainforests and giant dinosaurs, is now making headlines for shattering heat records.

Which brings us back to Gus. Whoever wins today's auction, the real value of these old bones isn't the price tag. It's the story they tell about a warmer world, and the warning they carry into ours.

Lead editor Chris DeWeese edits Morning Brief, The Weather Channel’s newsletter.