When a researcher stumbled upon the fossil  of a small mammal called a multituberculate   in the late 1960s in the foothills of  southwestern Montana, it came as a shock.

You see, at that point, fossil collecting   around the Rocky Mountains had been  going on for just over a century,   so paleontologists had a pretty good idea  of what animals they should expect where.

And multituberculates – a group that  first appeared in the fossil record   about 165 million years ago – were extremely  common until about 56 million years ago,   after modern groups of mammals evolved  and started to outcompete them.

But these Montana rocks dated to 17 million  years after most of them went extinct.

And these fossils came from  the foothills, instead of the   low-lying basins where paleontologists  typically find multituberculate fossils.

This discovery pointed paleontologists toward  the mountains, which would eventually reveal   something even stranger about a group of  mammals even closer to home: primates.

Because, it turns out, mountains have a  unique effect on diversity, messing with   our understanding of animals through time,  and pretty much just making evolution weird.

At the beginning of the Eocene  epoch, 56 million years ago,   the area that’s now Wyoming was as  warm and wet as the modern tropics.

Here, trees related to swamp-dwelling bald  cypress, and fruiting and flowering elms   supported a novel group - our  own relatives - the primates.

And these primates were thriving, diversifying  into a range of species, their grasping   hands and feet allowing them to navigate the  canopy more easily than any mammals before.

After many decades of collecting  fossils in the American west,   scientists had a pretty good sense of their  evolutionary story – or so we thought.

They seemed to fall into two groups: the larger,   lemur-like adapoids and the  small, tarsier-like omomyoids The adapoids, which were fruit- and leaf-eaters,   were represented by only a  few species at any one time.

While, in contrast, the omomyoids were extremely  diverse, evolving multiple species that were   highly specialized for eating insects or small  vertebrates, as well as fruits and seeds.

In North America alone there  were over almost forty genera   of omomyoids between 55 and 36 million  years ago, which themselves fell into two groups.

The first was the anaptomorphine omomyoids,   which appeared about 55 million years  ago and dominated North America.

These were extremely small  critters.

One of the smallest,   Trogolemur, weighed only about 50  grams, about as much as a lemon.

And these little anaptomorphines could  be identified by the sharp cusps on   their teeth and their especially  prominent fourth premolars.

But within the next five million years, in  the middle Eocene, they began to decline,   leaving the other group, the omomyine  omomyoids, more prevalent on the landscape.

These omomyines were slightly bigger, up  to 3 kilograms in one genus, Macrotarsius.

They had shorter cusps on their teeth and   didn’t have the prominent fourth  premolar seen in anaptomorphines.

Now, because there are Eocene-aged  deposits all over North America – from   Mexico to Mississippi to Canada –  we could see that the omomyines were   outcompeting the anaptomorphines all  over…or so paleontologists thought.

Because… here’s the thing - those  fossils had all been collected in   relatively low elevation sites where  sediments accumulate in basins.

That began to change in the 1970s, when a group   of scientists inspired by the discovery  of those multituberculates from Montana   started looking for primate fossils  in the foothills of central Wyoming.

And despite being from the middle Eocene,  the foothills, sitting about 1,980 meters   in elevation, revealed anaptomorphine  primates that were not only surviving,   but diversifying and evolving into new species.

Seeking answers, researchers  moved further up in elevation.

And at a pass to the south at around  2,200 meters high, researchers working   in the 1990s collected a huge amount of fossils -  including more than a dozen species of primates.

They were tiny and tarsier-like  in their anatomy...  …With subtle differences in their teeth  marking all these species as distinct   from one another…from Artimonius with its  massive fourth lower premolars to that   lemon-sized Trogolemur with its small  fourth premolar and diminished cusps.

So this new site had substantially more species  than any of the lower elevation sites dating to   this time – despite those having been explored for  nearly a century, with many specimens collected.

These new finds included multiple genera that  are usually extremely rare during this period,   like Artimonius, which was among  the most abundant primates here.

The relative abundance we’d come to  expect was basically flipped on its   head.

Even Omomys carteri, an  omomyine that is often one of   the most abundant species of that  time period, was barely present.

And data from the 2010s from an even higher  elevation site near Yellowstone National Park,   at just over 3,100 meters in elevation,  showed that same pattern with Omomys carteri.

It was clear that these mountainous  regions were different.

But why?

What   could be going on to create such a different  evolutionary pattern from the basins?

There almost seemed to be a mismatch between the  ages of the rocks and the animals found in them.

…Which sometimes happens with a process called  time-averaging, where instead of representing   one discrete chunk of time, sediments  of different ages are mixed together.

This can happen when different layers of  rock or sediment are exposed along a hill,   and rain erodes out the layers,  which then accumulate at the bottom…  And that accumulation turns into a new  fossil deposit that actually represents   all of the time periods exposed  along the hill, not just one.

So you end up with a jumble of  things that don’t seem like they   belong together – because they  actually don’t belong together.

But, at these sites, there were well-dated  layers above and below the sediments,   so that couldn’t really explain the pattern.

So the scientists started to wonder:  what if it was the mountains themselves   and the environments they create  that were causing these patterns?

Elevation can play an important  role in supporting diversity,   in part because mountains often have greater  environmental variation than low-lying areas.

There are a few reasons for this.

The changes in  atmospheric conditions as you move up and down the   mountains create variation – from temperature  shifts to air density to UV radiation.

And in mountains, there’s  greater topographic complexity.

Not just in the sense that mountains are  tall, but also because they get more rain,   leading to more active erosion dividing the  mountain into many different small river systems,   each carving up the landscape in different ways.

Which means that within a single square mile,   there are more topographic changes in  mountains than in low elevation areas.

Not to mention the cliffs, crests, and  further divisions to the environment   that can happen during the tectonic processes  that form mountain ranges like the Rockies.

These varied environments mean more  environmental niches supporting more species.

And more habitat variation means it’s increasingly  likely that a trait might offer an animal some   advantage and be selected for, which means there’s  a better chance that new species will evolve.

So when we see higher diversity,   we may be seeing the preserved evidence of  this speciation caused by habitat variation.

This is probably why we get higher primate  diversity at that medium elevation site.

Additionally, more niches means less competition  for the same niche, so animals that may be   out-competed in low-lying areas might be able  to persist longer in mountainous regions.

These are called refugia, where a  species survives in a small region   that forms a subset of a once much-larger range.

So those anaptomorphines persisting longer in the   foothills without getting outcompeted by  the omomyines are evidence of refugia.

This combination of speciation potential  and refugia may lead mountains to act   almost like islands, supporting isolated  pockets of diversity in a similar way.

And this island-like pattern  might explain why there are   primate species seen only at  these high elevation sites.

Also, some animals just prefer to  live in mountains So when we see things like Artimonius, which  is typically rare in lowlands, maybe we’re not   seeing an uncommon animal like scientists once  thought, we’re just seeing habitat preference.

But there’s one final piece to consider about  the world in which these Eocene creatures lived:   the mountain-building events themselves.

In fits and bursts, ranges like  the Rocky Mountains appeared on   the landscape through faulting and folding  that lifted and shaped the earth’s crust.

And these events were often  accompanied by episodes of   volcanic activity that deposited  thick layers of igneous rock.

While there’s debate about the exact  timing, much of the Rockies were the   product of a mountain-building episode that  unfolded between 80 to 40 million years ago.

So during the time the anaptomorphines and  omomyines were evolving and competing for   resources, mountains were actively  forming and dividing the landscape.

And part of this activity included nearly 15  million years of periodic eruptions in Wyoming   itself, that cast ash and volcanic material  for hundreds of miles in all directions.

This single volcanic group alone is responsible  for depositing about 1.5 kilometers of sediment   in parts of Wyoming - only increasing the effects  of the mountains on the animals living in them.

As this complex landscape comes into focus,   it becomes clear that there’s  still a lot to be learned.

And this is tricky in mountains,  which have steeper gradients,   big erosion potential, and  overall poor preservation.

But what we have learned from the fossil  record of these unique environments,   from multituberculates to Eocene primates,   is that extinction isn’t always uniform  – it isn’t always an on-off switch.

It wasn’t until scientists found the  anaptomorphines surviving and speciating in the   mountains of the Middle Eocene that we realized  they hadn’t gone extinct, they’d just adapted!

So, while the original story of  anaptomorphines being replaced   by omomyines was far more complex than it  had seemed, it was also more interesting.