Around 28 million years ago, in what’s now Oregon, a small carnivorous mammal died, was buried, and some parts of it were fossilized.

Which, yes, is how most of our stories start - we are a paleontology channel, after all.

But it didn’t have to happen this way.

Most of the things that have lived on our planet didn’t become fossils.

This little animal died in just the right place at just the right time to get buried and preserved.

Its skull and lower jaw were eventually discovered, and it was given the name Corumictis wolsani.

And it’s one of the earliest known mustelids - the family that includes things like weasels, otters, and honey badgers, to name a few.

But since we only found its skull, we don’t know for sure if its body was shaped exactly like a living weasel.

Now, in the world of mustelids, there are two main body shapes: long and slinky or big and beefy.

Some researchers call them skinnymorphs and musclimorphs -- seriously.

Skinnymorphs are things like ferrets and minks.

Their long, narrow bodies are specialized for chasing small prey like rodents and rabbits through their burrows.

Musclimorphs are things like wolverines and badgers.

They ambush prey out in the open or dig it up, instead.

And there aren’t a ton of skinnymorph mammals out there, because it’s an extreme body plan that may push the boundaries of what’s metabolically possible.

So when and how did this happen?

Why'd the weasels get so skinny?

Well, it looks like the roots of that weird slinky body plan lie in the expansion of grasslands.

Based on fossil evidence, the earliest mustelids lived in the Northern Hemisphere, but whether they originated in North America or Eurasia is still debated.

They seem to have diversified first in Eurasia, but some of the oldest mustelid fossils are from North America.

These early mustelids - sometimes called paleomustelids - include Plesictis from Europe, dated to between about 34 and 23 million years ago, which looks to have been a tree-climbing hunter, not a burrower.

And they also include Oaxacagale from southern Mexico, dated to at least the Early Oligocene Epoch about 30 million years ago, and they may go as far back as 40 million years ago.

Now, this fossil was published while we were writing this episode and actually threw a bit of a wrench into the story, because it’s the earliest fossil mustelid that looks like it might’ve been a tunnel-hunter, like living weasels.

And the researchers who worked on it don’t know what it means yet for the debate about where mustelids got their start and when they got skinny.

So, what we can say is that, for now, the fossil record suggests that skinnymorphs wouldn’t really take off until millions of years later.

But I’m getting ahead of myself.

Regardless of where they lived, the early paleomustelids were about to witness a big global change that took place in the Oligocene.

In the early Eocene Epoch, probably before mustelids appeared, our planet had a warm climate.

But as Antarctica drifted towards the South Pole and the levels of CO2 in the atmosphere decreased, global climate shifted from greenhouse to icehouse conditions.

The cooling climate triggered a change from ecosystems dominated by forests to ones filled with dry deciduous woodlands.

And, during the early Oligocene, wooded grasslands that thrived in lower CO2 levels and a cooler climate also expanded.

This transition from the Eocene to the Oligocene caused the extinction of 60% of terrestrial mammals around the world, but by the middle Oligocene a new biome emerged that really worked for mustelids - grasslands.

And mustelids took full advantage; they crossed the Bering Land Bridge between North America and Eurasia in both directions, moving into new habitats on either side.

The outcome of all this interchange was a large diversification of mustelids between 24 and 20 million years ago as they encountered the emerging grasslands and exploited new prey.

Small mammals like rodents and rabbits, which are especially sensitive to environmental change and have short generation times, evolved rapidly as adaptations to run and burrow became more common.

As their prey adapted to life in the new grasslands of the Oligocene, the mustelids did too.

And this is where we see those two major mustelid body plans start to take shape.

One earlier wave of diversification gave us the musclimorphs - those bigger, badger-like forms that used their claws to rip open rodent burrows.

These were species like Pseudobassaris, a ground-dwelling badger-like creature whose fossils have been found in Oligocene deposits in France.

And a later wave of diversification gave us the ‘skinnymorphs’ - long, thin animals with small front legs that could chase rodents into their burrows.

Now, there are a lot of reptiles and fish that are shaped like this, but not many mammals.

And it might be because the high surface-to-volume ratio of this body shape means they lose heat faster, making it especially inefficient in the winter.

And here’s where we come back to that ancient paleomustelid we talked about earlier: Corumictis.

Dating to between 29 and 26 million years old, Corumictis might’ve been part of the transition towards skinnymorphs.

Its skull was definitely small, like a living weasel, but without the rest of its skeleton we can’t know for sure.

But we do know from paleoclimate clues that where Corumictis lived had undergone a drastic decrease in rainfall leading to a dryer climate and more open habitats.

And while the first burrowing rodents in Oregon show up around this time, features of its skull, like its frontal sinuses and simple nasal bones, suggest that Corumictis didn’t have a very refined sense of smell - so it probably wasn’t chasing those rodents into their burrows.

But it wouldn’t be long before we’d finally have solid evidence of a skinny, burrowing mustelid!

Enter Zodiolestes.

This close-to-25-million-year-old mustelid was found in Nebraska, curled up in the burrow of a beaver.

Its presence there suggests that it likely preyed on beavers in their burrows.

So, it looks like the ecosystem transition towards the grasslands of the Oligocene drove the first wave of burrowing skinnymorph mustelid diversification - but it certainly wasn’t the last.

The climate cooled repeatedly into and throughout the Miocene and Pliocene Epochs, driving other periods of selection for weasel-like body plans.

For example, the ultimate group of skinnymorphs - the weasels and martens - arose during a cool period of the Pliocene, around 5 million years ago.

And as weasels evolved into even longer, thinner body plans, their jaws got stronger, which may have helped them compensate for their lighter-weight bodies with a more powerful bite.

But how does a body plan get skinnier?

Well, in the case of weasels, their vertebrae and heads got longer, and an extra lumbar vertebra was added to their spine.

That extra vertebra made them more flexible for squeezing through burrows, and also helped them wrap around their prey to get better leverage for biting.

If you’ve seen a ferret in action, you know its long body and short limbs allow it to compress its back and hips to fit through very tight spaces.

As the climate continued to cool and dry throughout most of the Pliocene, weasels caught their stride in open grassland, steppe, and taiga habitats.

These skinny, maneuverable predators were able to specialize on grassland rodents, ground squirrels, and pocket gophers.

And this suite of adaptations to grassland life wasn’t limited to North America.

The skinnymorph body plan also shows up in other places, like one species in Hungary that weighed less than 100 g and could chase rodents through narrow burrows under the snow.

Mustelid migrations across the Bering land bridge continued until around 5 million years ago, when the Bering Strait severed that route.

By then though, skinnymorph mustelids had also dispersed into Africa, colonizing savannah habitats.

Some of their descendents today include the African striped weasel, which preys on rats and raises its young in burrows it excavates or steals.

Now, as for the musclimorphs, well, they would go on to include some of the biggest fossil mustelids of all time.

And we didn’t even get into the so-called aquamorphs, like the otters.

But those are stories for another day.

And there you have it - how the weasels got skinny.

From their early, possibly-tree-climbing origins in the Late Eocene or Early Oligocene, they diversified and expanded with the spread of grasslands in the Miocene, taking advantage of new predatory niches.

They reached their skinnymorph peak in the Pliocene with the rise of true weasels, and