New Scientist - USA (2022-01-15)

15 January 2022 | New Scientist | 45

“It has a distinct bend, which you find in other

animals that are adapted for running at high

speeds, because it puts the eyes in a place

where you more easily see what’s quickly

coming towards you,” he says. Dinaelurus also

has bigger sinus cavities than other nimravids,

allowing it to take in more oxygen as it ran.

But even this evolution wasn’t enough.

By 23 million years ago, nimravids were gone

from North America. The continent was free

of cat-like creatures. Then, around 6.5 million

years later, the cooling climate that paved the

way for the nimravid extinction gave their

feline successors access to the continent.

Sea levels dropped as glaciers grew, exposing

the Bering land bridge that connected Siberia

to Alaska. Across it came Pseudaelurus, a

lynx-sized cat that was an agile tree-climber. It

flourished in the expanding conifer forests in

North America, which were also made possible

by plant migrations over the bridge. Another

group of cat-like animals called barbourofelids

also arrived, and new analysis suggests that

they were nimravids originating in Africa.

These felines and feliforms finally brought

an end to the Cat Gap. Eventually, around

5 million years ago, the barbourofelids died

out. However, Pseudaelurus persisted and

is thought to be the common ancestor of

everything from North America’s mountain

lions to bobcats and even the fluffballs currently

occupying the best spots on many sofas.  ❚

was meat – with blade-like teeth towards the

front of their mouths used for stabbing, and

jaws that allowed them to open their mouths

to 90 degrees to better pierce prey. Behind the

stabbing canines sat pairs of carnassials: sharp,

triangular teeth that fit together like puzzle

pieces. “They’re like horrible scissors,” says

Poust. As they slide past one another, the

bottom teeth grind against the top and hone

them to a point. “From the moment they stop

drinking milk to the moment they die, they

need to use that tool,” he says.

The perils of hypercarnivory

Such specialisation often leads to an

evolutionary dead end, and if hypercarnivory

caused the extinction of nimravids, they

wouldn’t have been the only ancient animals

to succumb to an over-reliance on meat-eating.

It also played a part in the demise of several

species of wild dogs in North America around

11,000 years ago. Even today, a set of more

general-purpose teeth has been key to the

survival of various large predators. “If you’re

a black bear, you can eat almost anything. You

can eat garbage. That’s part of why they do a

better job of dealing with living near big cities

and today’s tigers often don’t,” says Poust.

There is some evidence that late nimravids

had started to adapt to the changing

environment. As dense forests gave way to

grasslands, they would have needed to run

faster and over longer distances to catch

their prey. “One of the last nimravids of the

Oligocene, Dinaelurus, seems to have a similar

morphology to what we see in cheetahs today,”

says Barrett. Its skull is tilted in a similar way.

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Chelsea Whyte

is a news editor

at New Scientist

height of their existence. From about 50 million
to 25 million years ago, an ancient tectonic plate
called the Farallon plate was spreading under
North America. When it met the Pacific plate at
the western edge of the continent, the result was
explosive. Across what is now Colorado, Utah,
Nevada and further south into Mexico, there
were dozens of volcanic eruptions. The largest
of these occurred around 28 million years ago,
creating the La Garita Caldera in Colorado,
which is 75 kilometres across at its widest
point. Volcanic ash spewed out and blanketed
the region with 5000 cubic kilometres of
material. For comparison, the 1980 eruption
of Mount St Helens in Washington expelled
2.5 cubic kilometres of debris.
The energy of the La Garita eruption may
have been partly due to its silica-rich magma.
“The higher the silica, the more explosive an
eruption can be,” says Peter Lipman at the US
Geological Survey, who discovered and studied
the caldera. More silica gives the liquid rock
higher viscosity, which can help trap more
sulphur and carbon dioxide. Then, when the
molten material rises and decompresses, the
gases are released and create bubbles that
explode. “Everything would have been killed by
the heat of the ash alone for at least
150 kilometres beyond La Garita,” he says.
“Beyond that, ash that went up higher in the
atmosphere would certainly reduce sunlight
and temperatures for a year or two.”
“It would have been devastating for the flora
and fauna,” says Barrett. And nimravids were
no exception: some of the best-preserved
specimens come from sites rich in ash layers.
“Some did bite it. But the nimravids seemed
to persist through these cataclysmic events,”
he says. Poust also thinks that although the
10-million-year flare-up of volcanic activity
may not have been easy for individual
nimravids, it doesn’t explain why they
went extinct altogether.
If volcanism didn’t finish them off, what did?
Beginning around 23 million years ago, there
was a period of massive cooling and drying.
Forests gave way to grasslands, which would
have affected the animals that nimravids
hunted. “Prey species at the time were going
extinct, so that is probably related to why the
predators followed soon after,” says Barrett.
Nimravids were at a disadvantage when
attempting to adapt. They had evolved to be
hypercarnivorous – meaning most of their diet

A 37 million-year-old

nimravid from the

genus Hoplophoneus,

which means “armed

murderer”

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