The Evolution of Cats:  4.  The First Cat

Last time, Silvester the African wildcat showed how our own housecat would live out in the wild . . . as long as the habitat was a place like the Kalahari plain in southern Africa.

Fluffy’s style would have to change if it lived in a swampy jungle or high up in the Andes.

Yes, there are small cats in the Andes. Andean mountain cats are rare and hard to study. (Sunquist and Sunquist)

Felids (members of the cat family Felidae) are up to the challenge. These very adaptable animals are found all over the world today.

They are also in the geologic record. For tens of millions of years cats have always:

  • Competed with each other in all these places
  • Hunted prey that was evolving rapidly and in diverse ways
  • Had sex and raised kittens
  • Coped as best they could with other community predators that ranged from prehistoric giant beardogs; through the very cat-like ancient nimravids and barbourofelids that we will meet next time; to today’s Homo sapiens.

That’s a lot of work. No wonder modern cats tend to be a little irritable!

This jaguar looks a little like the Joker!

Those interactions with the environment and with other living members of their communities have, of course, influenced the evolution of cats in many ways.

We need this broader post now to move beyond Silvester and take a very general look at how the first cats evolved.

Then, next time, we can meet some of them, starting with the sabertooths.

Now, ladies and gentleman, set aside some time for reading – this post does cover a central idea in my book, that the evolution of cats was an epic  – and step this way into the time machine . . .

(Mind the gorgon, the T. rex, and the angry quoll.)

Humble beginnings

Many of the complex feline features that we admire today actually evolved to meet a vital but simple need in the past.

Sometimes this need arose a very long time ago.

Believe it or not, a cat’s beautiful coat; its claws, whiskers, and teeth; and the glands it uses to advertise for a mate and to mark its territory all come from integument (Chuong and Harberger) – that is, from skin.

Cats are still recuperating from the evolutionary effort.

Integument first evolved in some of the oldest ancestors of mammals and reptiles.

Those former fish needed the outer covering to protect themselves from dehydration, now that they were spending more and more time out of the water.  (Alibardi)

Integument of various shapes and qualities also helped them slither around or pull themselves along as they colonized the land.  (Alibardi)

Today, molecular biologists say that things like claws and teeth come from the integument’s epithelial stem cells.

These cells can be organized in lots of different ways.  (Chuong and Harberger)

Exactly how this works isn’t well understood yet, but it has indeed led to the diversity of today’s animals. (Chuong and Harberger)

The development of integument is one of the points where the story of cat evolution (and much else) really begins.

We need to go back 350 million years for it (Benton and others) – a much longer span of time than the 65 million years or so that have passed since the K/T extinction of nonavian dinosaurs and some other forms of Cretaceous life.

A modern avian dinosaur poses with data from the geological record. Feathers developed from integument, too. (Chuong and Harberger)

We would have needed a time machine anyway.  Cats are mammals, after all, and the Linnean order Mammalia is very old.

You might have heard that the Age of Mammals began when nonavian dinosaurs went away.

Actually, that’s just the Cenozoic – sixty-five million years of what scientists who take the really long view call “recent life.”

Mammal beginnings are nowhere near as recent as that.

Mammals and dinosaurs

Two-thirds of the real Age of Mammals was already over (Kielan-Jaworowska) when a killer bolide suddenly appeared in Cretaceous skies.

Mammals first emerged on Earth along with the first dinosaurs over 220 million years ago, in the late Triassic geological period.  (Lillegraven;  Heske, Lecture 1)

If you have a geologic timescale handy, this world-changing event is near the Triassic-Jurassic boundary.

If all you have is a postcard, its rocky archives are currently washing off the top of North America’s Grand Canyon, where rocks from 500 to 250 million years ago (Ma) are exposed.  (Prothero, 2006)

Like this:

Once mammals and dinosaurs had appeared, the two groups then managed to live together for over 140 million years.

This was possible because each group had a very different ecological role.  (Lillegraven)

Dinosaurs, of course, became dominant and spectacular.

Early mammals kept a much lower profile. They were never much to look at.  Still, the little furballs would belong in a Jurassic Park.

They, too, were a part of the real Jurassic world.

Grass, on the other hand, probably wasn’t around back then (Sage; Strömberg), even though the movies show herds of veggie dinosaurs happily roaming meadows and fields.

What those veggie dinos, big (Sander and others) and small, are actually munching on is tree leaves and underbrush.  Check it out.

Leaves, brush, and seeds are also what many mammals lived on before, during, and after the K/T catastrophe 65 million years ago.

Of course, there were also carnivorous mammals around.

Repenomamus giganticus

A park visitor might pay to see woodchuck-sized Repenomamus, since one of this mammal’s fossils had a baby dinosaur in its tummy!

The world of mammals back then was secretive and small, but it contained lots of critters to look at (Kielan-Jaworowska), if you could tear yourself away from the nonavian dinosaurs for a moment.

Mammals and dinosaurs shared something else besides the Jurassic world (and the distant common ancestor that Benton and others have mentioned in a research paper).

Fluffy and Silvester still have this feature today, and so do you and I.

It’s the basic backbone-and-four-limbs body plan of a tetrapod.

The amazing tetrapod forelimb

What’s a tetrapod?

Cats and people are.  So are “fishy” mammals like whales and seals, some nonmammals like birds and lizards, and a host of other living and extinct creatures.

Backbone . . . check. Four limbs . . . uh . . . okay, it checks on x-ray.

The word “tetrapod” means “four-footed,” which is pretty self-explanatory, even if two of those “feet” in humans are called  hands.

“Forelimb” has a stiff sound to it, but this scientific term does make the thing easier to see than all the usual names like “flipper”, “arm”, “front leg”, or “wing”.

People, cats, seals, birds, and other tetrapods all use their forelimb differently, so this structure has a variety of distinctive looks now.

That’s where all the common names come in.

But no matter what you call it,  anatomically The Amazing Tetrapod Forelimb always has the same bones – a radius and an ulna that are, almost without exception, connected to some carpal wrist, “hand,” and digit bones. (For the exception, see Pritchard and others)

Experts on ancient life say that the tetrapod forelimb is one of the most versatile structures that vertebrates – animals with backbones – have ever developed.  (Pritchard and others)

Those former fish that we mentioned above, the ones that came up with integument, were the first tetrapods.  (Benton and others)

They probably used their very primitive forelimb as a support strut in shallow water and mud.  (Pritchard and others)

And towering above them, as these newcomers struggled out of the sea 350 million years ago, was something just as amazing as the tetrapod forelimb – trees.  (Collinson and Scott)

A giant leap for plants, but you need a lot more than integument and a support strut to climb it!

These leafy ancestors of today’s classic feline refuge were well established by the time tetrapods arrived.

Trees had already been evolving on land for millions of years. (Collinson and Scott; Webb, 1987)

The latest news from the green world around 350 Ma (million years ago) included conifers and seeds (Collinson and Scott; Webb, 1987), which are still a very popular food group among mice and other modern small prey animals.

As tetrapod evolution progressed on land, a few of them probably explored trees a little or chased an insect up into one.

However, most tetrapods stayed close to the water.  (Webb, 1987)

Why bother climbing trees when you’re the top land predator in a fresh-water food chain, with lots of high-protein fish available? (Sahney and others; Webb, 1987)

The first carnivores

Back in those days, the land that now includes North America and Europe was part of a single equatorial continent.  It was covered with dense, swampy rainforest, the remains of which are still mined as coal. (Sahney and others; Webb, 1987)

After a little geologic time had gone by, tetrapod amphibians ruled the land, although they had to live near water.

But things were also going well for the other group of tetrapods – reptiles. These animals were a little more flexible than the amphibians.

Reptiles had turned their integument into protective scales that held in water better.  They also had hard-shelled eggs that could be laid on dry land.  (Sahney and others)

In fact, life was so good for reptiles that they now diversified into two groups (Benton and others; Heske, Simplified History of Mammals; McGhee and others):

  • True reptiles: These included, but weren’t limited to, the distant ancestors of dinosaurs and birds.
  • Synapsids.  We’re invested in this group – some of their descendants became mammals.

Both reptile groups had the Amazing Tetrapod Forelimb, of course.  But synapsids had another thing going for them.

What was it?  Here’s a hint.

Gorgonopsids like this were the first sabertooths.

Unlike true reptile teeth, synapsid teeth came in different shapes. Each shape had its own position on the jaw.  (Antón)

Why does this matter?  Technically, because variability is the raw material for evolution.  (Meiri and others)

In plain English, evolution is a lot easier when you have flexible genes.

Too, teeth are vitally important in a world where, as William Ralph Inge put it, “Nature” can be defined as a conjugation of the verb “to eat” in the active and passive voice.

Teeth, of course, are also very relevant to the evolution of cats.

Both saberteeth and the sharp little fangs that Fluffy shows when it’s time for a pill are the same basic upper canine teeth that all of us mammals inherited, along with molars and incisors, from the synapsids.

Like all tetrapods, synapsids already had very flexible integument epithelial cells. Now they also had teeth that could transform to take advantage of a variety of food sources (Antón; Heske), though of course they didn’t transform as quickly as in the cartoons.

These advantages held great promise for the future.  Which was lucky for synapsids, because at this point, the Coal Forests collapsed.

It sounds very dramatic, and it was, but it happened slowly, inexorably. (Sahney and others)

The ultimate cause of the Coal Forest collapse was global climate change and the Permo-Carboniferous ice ages. (Royer and others)

Those ice ages were much older than the ones we’re familiar with –  the very recent “Smilodon ice ages,” which is a name that I just invented for them.

Back in the days of the synapsids, great continental glaciers also began to inch down toward the Equator from the poles.  (Royer and others)

Soon, where rainforest had once stretched from Kansas to Kazhakstan (Smithsonian), seasons became a thing, and the world got drier.  (Sahney and others)

Tropical rainforests shrank into small patches, thriving only in a few areas where there was enough rainfall.  These “islands” of lush greenery were separated by widespread arid scrublands.  (Sahney and others)

These dry lands meant death to amphibians and they weren’t much fun for early reptiles either, at least not until they could get used to the change.

So at first all tetrapods stayed put in the rainforest refuges.  This strategy worked for amphibians and reptiles – the newest tetrapods – but it was too much for the older families. Those went extinct.  (Sahney and others)

Amphibians just managed to hang in there. (Sahney and others) This wasn’t the end of the Age of Amphibians – not yet.

However, reptiles could handle dry conditions better. They eventually began to diversify, especially the synapsids.  (Sahney and others)

Some scientists (Sahney and others) suspect that, around this time, isolated reptile communities survived by expanding their diet from fish and insects to other tetrapods.

Thus climate change hundreds of millions of years ago may have brought about the first carnivores – tetrapods eating other tetrapods.

The first cats, like the owner of this Pseudaelurus jaw, and all of their descendants are extreme carnivores.  They have lost the molars that we still possess for processing plants and other non-meat foods.  (Holliday and Steppan)

The path to mammals

As we noted, amphibians did manage to continue their global dominance after the world’s climate changed, but they had to stay close to water.

Reptiles, on the other hand, had their built-in flexibility.

After adapting to drier conditions, both true reptiles and synapsids left the rainforests and began to explore the bush and its many new opportunities. (Sahney and others)

It worked out beautifully, so much so that more advanced synapsids made their first appearances in North America and Europe.  Called pelycosaurs, these included the famous sail-backed Dimetrodon. (Kemp)

Amphibians still outnumbered them almost three to one (Sahney and others), but for all that, pelycosaurs were still quite diverse. (Kemp)

Some were carnivorous, but there were lots of plant-eating pelycosaurs, and they were big.

The new dry habitats gave Earth’s large tetrapod herbivores their first chance to dominate open areas.(Webb, 1987)

Their dominance would continue down through geologic time – briefly interrupted now and then by a major mass extinction – until the end of the last Smilodon ice age roughly 10,000 years ago . . . and the end of Smilodon, other saber-cats, and almost all of the world’s megabeasts, as we will see next time.

Back in the day, this was all very new. Openings in the global forest and other changes – like the invention of seeds – really favored synapsids.  (Webb, 1987)

Soon synapsids outnumbered the true reptiles.  (Smithsonian)

Then, around 270 Ma (million years ago), some very advanced synapsid descendants arrived.  (Kemp)

Called therapsids, these mammal-like reptiles could be hefty, though in those days, anything over 200 pounds (100 kg) was a giant.  (Heske, Simplified History of Mammals; Kemp)

Even the largest therapsids were way more energetic and agile than the pelycosaurs.  They could breathe faster and eat faster, too.(Kemp)

Therapsids just couldn’t breathe and eat at the same time, like modern mammals.
In fact, some adorable modern mammals can eat, breathe, and purr at the same time!

Nobody is sure if therapsids were able to generate their own body heat instead of having to bask in the sun like the reptiles.  (Kemp)

The technical name for this is endothermy.  Of course, it’s something that we and all other mammals do today.  (Heske, Lecture 1)

Warm-blooded or cold, therapsids certainly seemed to be developing mammal characteristics, one by one.  (Kemp)

And then, in the closing years of the Age of Amphibians, therapsids turned into an even more advanced kind of synapsid – the cynodont. (Heske, Simplified History of Mammals)

Life was good for cynodonts, too.  These animals went on to develop even more mammal traits, sometimes over and over again.  (Rose)

We members of the order Mammalia have a lot of identifying characteristics, besides the ability to bear live young and the features described above.

However, most of these things don’t fossilize well.

There is no way this will show up as fossils a hundred million years from now.  (By the way, meerkats are feliforms, but they belong in the mongoose family, not Felidae.)

This is why it’s not easy for paleontologists to say exactly when the first true mammals appeared.

Some classic innovations the experts look for in the fossil records are features any of us today can recognize, even if we’ve never thought about it before.

These include, but aren’t limited to (Heske, Lecture 1):

  • No ribs between roughly the base of our chest and our hips, so all mammals can bend more easily than, say, Triceratops or T. rex.
  • Vertebral changes to strengthen the backbone and make it more flexible.  Cats have carried this to extremes.  (Kitchener and others)
  • Inner ear bones, so we can hear better.
  • A stronger jaw, compared to reptiles, and special muscles to better control jaw movements.

Back in the day, of course, cynodonts didn’t have all that advanced stuff.

Their mammalian adventures more often involved dental changes (Carroll), so they could process food better.  (Heske, Lecture 1)

Eating was crucial and not merely for day-to-day survival. Mammals need about ten times more energy than a reptile does. (Heske, Lecture 1)

In order to become mammals, our ancestors had to evolves way to solve this energy crisis.

Such changes aren’t likely to appear overnight in any reptile, even a synapsid. But the big move up the ladder of life did seem to be on the horizon for cynodonts.

That horizon was a broad one, too.

The supercontinent Pangea was almost completely assembled now.

Scientific diagrams are more accurate, but let’s face it – this is how most of us picture Pangea.

All that land was available for mammal-like reptiles to explore and evolve in.  (Smithsonian; see Kemp for a detailed description of  therapsids and Pangea biozones)

Too, the Permo-Carboniferous ice ages were over and Earth was warming up again (Royer and others) – good news for all reptiles, not just the mammal-like ones.

In fact, the South Pole was now completely ice-free (Royer and others) and it would stay that way for hundreds of millions of years. (Smithsonian)

Yes, life in the late Permian epoch was good for cynodonts, and the odds were high that mammals would soon evolve, if they hadn’t already.

What could possibly go wrong?

The worst mass extinction in history

Before much more geologic time had passed, Earth was a very different place.

Vegetation had died everywhere, leaving behind a rotting mess that rains washed away over denuded landscapes.  Storms also caused heavy erosion.  (McGhee and others)

Hills, valleys, canyons, and plains on every continent were bare and silent, except for the sound of running water, although there was still a little life out there, and a little life in the seas, too.

But very little in either place.

Death counts aren’t precise yet, but it looks as though 75% to 96% of all known Permian species vanished forever.  (Keller)

And it only took 60,000 years . . . or less.  (Burgess and others; Keller)

That’s right.  Almost all land and marine life on the planet was wiped out over roughly the same, geologically short amount of time that has passed since Homo sapiens walked out of Africa. (Coolidge and Wynn)

Nobody can ever really know what happened to close down the Permian period so horrifically.

Some conceivable causes – like a deadly worldwide mutant virus – don’t show up in the fossil record. The presence of such inconspicuous mass murderers in the distant past can’t be proven or disproven.  (Prothero, 2006)

However, many experts suspect that volcanism played a role in the end-Permian catastrophe.  (See, for instance, Burgess and others; Keller)

Enormous floods of basalt lava – the Siberian Traps – were surging out of the ground and covering much of what is now northern Russia right around then.

The only catch with this explanation is that the die-off happened so quickly.  Only a single pulse of magma (Burgess and others) during that 20- to 25-million-year eruption (Ivanov)  could have done it.

It’s possible. There was a pulse in the basalt floods about then. It lasted for a million years or so.  (Burgess and others; Ivanov)

Chemicals in volcanic gases can break down the planetary ozone layer that protects us from deadly levels of solar radiation.  (Oppenheimer)

Perhaps this happened during the Siberian Traps eruption. (Sobolev and others)

Or perhaps it was just the worldwide consequences of the Siberian eruption. Such large eruptions can cause major changes in the chemistry of the air and the oceans. (Saunders)

Perhaps Permian life everywhere, at different times and in different places, experienced a deadly cascade of effects. (Burgess and others)

It wouldn’t take much of a push to start a major mass extinction.

If, each year, only 1% of a species dies because those individuals can’t take the environmental stress any more, the whole species will be extinct in a century.  (Oppenheimer)

And if this happened here and there, over and over again, maybe most life on Earth could disappear in a few tens of thousands of years.

Scary, isn’t it.

amurtiger-1990571_640
The Siberian Traps are too big to think about.  Here is a Siberian tiger.

Whatever may have caused the Permian extinction, synapsids still had their built-in advantages.

The Permian mass extinction came in two waves.  The first, around 267 Ma, happened at about the same time as a huge diversification of therapsids in the tropics and elsewhere.  (Keller; Kemp)

So that first lethal wave clearly didn’t bother synapsids much.

The Big One – literally the biggest mass extinction in over 500 million years – hit around 251.941 Ma. (Burgess and others – zircon dating LINK is amazing!)

It knocked the synapsids down, but it couldn’t take them out.

Little Lystrosaurus, a dicynodont, survived.  And then something incredible happened.

Almost immediately after the Permian extinction, Lystrosaurus and other synapsids took over the world, starting the Age of Reptiles.  (Antón; Heske, Simplified HIstory of Mammals)

Granted there wasn’t much around at first to stop them, but that’s still quite a comeback.

Obviously the dinosaur and bird ancestors were also among the living, but during the Triassic period that followed this mass extinction, true reptiles had to keep their heads down because of all the synapsids.

Nevertheless, the first recognized dinosaurs appeared at almost the same time that mammals evolved from synapsids.

This was late in the Triassic, about 25 million years after the Permian epoch had ended so tragically. (Heske, Lecture 1; Lillegraven)

But living on Earth is never easy or very secure.

Another major mass extinction – not as overall devastating as the Permian disaster but bad enough (McGhee and others) – shut down the Triassic geologic period.

Both mammals and dinosaurs survived into the Jurassic, but that end-Triassic extinction had changed the balance between them.

Now dinosaurs became dominant.  (Olsen and others) It was the synapsid mammals’ turn to keep a low profile.

Then, after roughly 140 million years of coexisence, the end-Cretaceous extinction took out nonavian dinos and made room for mammals and birds.

Life had diversified after each of the previous major mass extinctions.  (Benton; Krug and Jablonski)

Now, after the K/T extinction, mammals almost literally exploded out of their hiding places and took over the world.

Where had they all come from?

The K/T extinction and modern mammals

Let’s pick up their story on the supercontinent Pangea, shortly after mammals and dinosaurs evolved in the late Triassic.

Back then, mammals were spread all over the world. Their fossils from what are now China, Europe, and southern Africa are very similar.  (Lillegraven and others)

But today, cats and most other placental mammals are found on the northern continents and Africa, while marsupials (and a rare group of mammals called monotremes) are more common in South America, and especially in Australia.

Mammals were apparently carried off in these different directions when Pangea began to split apart at what is now the northern Atlantic Ocean. (Lillegraven and others)

The supercontinent ultimately broke into two pieces (Lillegraven and others):

  • Gondwana:  Basically Antarctica, South America, Africa, India, and some smaller pieces, all stuck together.
  • Laurasia:  North America and vast Eurasia; these two continents were often connected by land bridges even after Laurasia disintegrated.

Plate tectonics then subdivided Gondwana and Laurasia, over hundreds of millions of years.  It shaped the continents into the forms we’re familiar with and moved them into their present positions.

While all this was going on, newly isolated groups of mammals (and other forms of life) went down a variety of different evolutionary paths.

The only relevant paleogeography for us to keep in mind is that North and South America didn’t hook up until very recently, in geologic terms.

Also, ever since Gondwana disintegrated, Australia has been an island continents.

Why does that matter?

Because wildlife on those continents, particularly in South America and Australia, had no contact with the rest of the world as it evolved.  And . . .

wp-1480184558653.jpeg

G’day, mate.  I’m a marsupial mammal – that’s ‘quoll’ to you.  I once had the nicest crib an Aussie small predator could ask for.  Then back in the 19th-C, you people brought cats to Australia that had never seen anything like them before.  None of us was built to compete with cats or to defend ourselves from them, so there’s quite a mess here now.  Ta, I DON’T think.”

Very little is known about mammals in the southern hemisphere during the Jurassic and Cretaceous periods, while dinosaurs ruled the Earth and continents did their lumbering dance.

In the north, on the former lands of Laurasia, early mammals probably lived as night hunters and browsers, since coldblooded dinosaurs wouldn’t be very active in the dark.  Perhaps this is how mammals developed acute senses and the ability to smell pheromone signals.  (Lillegraven)

Yes, today many animals besides cats can “read” odor markers.

wp-1480184091342.jpeg
This “flehmen response” looks a little silly to those of us who can’t smell through our mouths.

Some Jurassic and Cretaceous therian mammals – the ancestors of, among other creatures, quolls, cats, and us – also used their versatile synapsid teeth to invent a very handy tool: molars that could both slice and crush food.  (Kielan-Jaworowska; Heske, Simplified History)

That simple development opened up a new world of possible lifestyles.

Today, thanks to this therian invention:

  • Cats have turned their molars into meat slicers
  • Dogs and bears – which will eat almost anything – have molars designed for either grinding stuff or processing meat
  • Hyenas have developed molars strong enough to crush bone
  • The molars of cows and horses have all the dental features needed to chew up plants.

Opossums give the original molar five stars and haven’t changed it much at all. (Heske, Simplified History)

Mammal evolution progressed, and by the late Cretaceous period, early members of at least four modern mammal groups could be found on the northern continents (Rose):

There probably were other groups that would eventually help drive the post-K/T expansion of mammals, but those fossil fragments are so primitive that paleontologists haven’t been able to classify them into orders yet. (Rose)

Hide! Here comes T. rex.

Remember, at this point in geologic time mammals may have been successful but they were still very small and vulnerable.

One of the unclassified little furballs cowering in the bushes was Cimolestes. It had very sharp cheek teeth that it probably used to devour insects.  (Rose)

Perhaps that’s why scientists have given it a name that means “Bug Thief.”

Along with other mammals, as well as tyrannosaurs, Triceratops, and other dinosaurs, the Bug Thief lived in a closed-canopy rainforest on swampy lowlands. The area was near a great inland sea that covered central North America in those days, and it would eventually become the US state Montana.  (Prothero, 2006; Rose; Sewall and others; Stoffer; Sweet)

This shallow sea was connected to the Gulf of Mexico (Stoffer and others) where, almost 2,000 miles (3,000 kilometers) south of Cimolestes, the Yucatan platform sat – a big, inviting space target.  (From Sweet, Figure 2)

Marsupials and a highly successful rodent-like group called multituberculates dominated the Bug Thief’s community, but hedgehog-sized Cimolestes did well enough to show up in today’s fossil record. (Clemens and others; Prothero, 2006; Rose)

This swampy land sat along the western shore of the great sea. Hundreds of miles to the east, on the sea floor, was the site that would one day host the White River Chronofauna and in modern times turn into what is now Badlands National Park.  (Stoffer and others)

The Bug Thief’s tropical home was rich in “low-energy sedimetary environments” (Sweet), but one day something shook it up badly.

Hundreds of square miles of semisolid mud on the sea floor chaotically mixed together or curled up into rolls and curvy folds.  The direction of this movement was always to the south.  (Stoffer and others)

dz
Today the deformed seabed is frozen in place as a layer in rock outcrops.  Geologists say that the movement here was toward the right (south).

Sediment structures like sand boils and other features that only appear during earthquakes also formed.  (Stoffer and others)

And after that there are no more Cretaceous bone fragments or mollusk fossils, despite an intensive five-year field search for some.  (Stoffer and others)

Researchers suspect that the cause of all those changes was the bolide impact that excavated the Chicxulub crater around 65 Ma. (Stoffer and others)

A 6- to 9-mile-wide (10-14 kilometers) (Kring) space rock hit Earth there, on the Yucatan peninsula, causing magnitude 11 earthquakes and collapsing the Yucatan platform.  (Schulte and others)

Yes, that kind of shaking could have rolled up the seabed near the Bug Thief thousands of miles away, though it’s not certain that impact-generated tsunamis could have traveled so far north.  (Stoffer and others)

The details of what happened on Earth after this impact are controversial.  (Oppenheimer)

The only unquestioned evidence has already been mentioned: Something big hit Earth around K/T boundary time.

Beyond that basic fact, every discussion about impacts and the K/T extinction, as far as I can tell, is hypothetical only.

And this means there are some wild but still plausible ideas about the K/T extinction.

Because of my forestry background, I like the following description, based on plant evidence, of how the impact affected western North America where Cimolestes lived.

Per the paper by Sweet in the source list at the bottom of this post:

  • The Chicxulub impact melted tons of rock and blasted it all sky-high.
  • Some of the rock turned into molten glass that then rained down on the Bug Thief’s world, igniting the forest canopy.
  • Crown fires raced northward, spreading on winds that reached hundreds of miles an hour (Kring), but the understory of ferns and flowering plants was generally untouched.
  • Dust from the impact rained out of the darkened skies  for months until, finally, the Sun broke through again.
  • Understory plants, released by the death of the former canopy forest, now thrived.
  • Pioneer plants came in. Then slowly, over hundreds to thousands of years, the green world restored itself.  Eventually the same dense rainforest once again shaded the swamps.  Only a few exotic flowering plants had vanished from it.

But some other living beings were missing, too.

During this extended catastrophe, dominant life forms, including but not limited to Earth’s land and marine nonavian dinosaurs, disappeared.

So did most mammals, including enough marsupials to give the surviving placental mammals dominance in North America.  (Prothero, 2006; Rose)

Even with that edge, it wasn’t easy for them. Terrestrial ecosystems collapsed as completely as they had in the Permian extinction, although the global environment fared better this time around.  (McGhee and others)

But Cimolestes and some other mammals somehow made it through and started the new age.  (Rose)

Three cheers for the Bug Thief!  Its survival is very important to our story of where cats come from.

Cimolestes’ sharp cheek teeth are somewhat like those of all modern carnivorans, including cats.

The common ancestor of cats and all other members of the order Carnivora is currently a mystery, but this dental resemblance means that the Bug Thief or one of its relatives may be in the family tree somewhere close to the trunk. (Rose)

However, for that to be true, I think that the Bug Thief and other cimolestids had to be placental mammals, like the carnivorans. Nobody seems to know yet if this is the case.

Molecular biologists do say that the roots of this order laid down very soon after the K/T extinction.  (Benton and others; Nyakatura and Bininda-Emonds)

The new age begins

Survivors of the K/T extinction, along with the first mammal species to evolve in this new age, may have been resetting evolutionary speed limits for our time (Krug and Jablonski), but they certainly didn’t look like trendsetters.

“. . . [C]ompared with our present world, and in contrast to the succeeding epochs, the Paleocene [the first 10 million years after the end-Cretaceous extinction] appears to us as a strange time, in which the present orders of mammals were absent or can hardly be distinguished: no rodents, no [hoofed animals], bizarre noncarnivorous carnivorans.  In other words, although the Paleocene was mammalian in character, we do not recognize it as a clear part of our own world; it looks more like an impoverished extension of the late Cretaceous world than the seed of the present Age of Mammals.  But the seeds were there.”  (Agustí and Antón)

Those “seeds” included The Amazing Tetrapod Forelimb and its remarkable epithelial stem cells.

Carnivorans weren’t the only mammals putting down roots.

Soon after the K/T extinction, mammals had established 44 new families.  They added another 41 by the end of the Paleocene, and 61 more in the early Eocene – the epoch following the Paleocene.  (Rose)

By then, all modern mammal orders existed, “from shrews and rodents to giant whales and flying bats.”  (Prothero, 2006, including quote)

That’s extremely fast evolution.  (Agustí and Antón; Prothero, 2006; Rose)

And its distinguishing mark was adaptive modification of the distal limb.  (Hamrick)

The first new mammals quickly evolved (Hamrick):

  • Hoofs: Short, thick, and strong material for those who walked around on land or other relatively flat surfaces
  • Claws:  Narrow,  sharp, and usually short structures that tree-dwelling animals could lock into bark or use for digging or to hang from branches – after all, much of the world was covered by dense forests. (Agustí and Antón; Prothero, 2006)
  • Fingernails:  Thin plates, of the same material as hoofs and claws, for mammals that needed to some sensitivity in their digits as they grasped branches or poked around in mud or crevices for food.

Hoofers became the earliest dominant mammals of our Cenozoic time, at least in North America where the Paleocene fossil record is clearest.  Most of them ate plants, but some had a taste for meat. (Prothero, 2006)

wp-1480184369546.png
Mesonyx is the namesake of a group of carnivorous mammals called mesonychids.  Look at its toes – those are hoofs, not claws!

Mesonychids may have been the first mammals to go for the large-carnivore niche (Werdelin, 1989), but they faced some stiff competition.

Technically speaking, dinosaurs still ruled North America and Europe after the K/T extinction – avian dinosaurs, sometimes called “terror cranes.”  These flightless birds had big sharp beaks and long, heavily-clawed legs that propelled them through the forests.  (Prothero, 2006)

At a height of almost 7 feet (2 meters), Diatryma and Gastornis were the biggest land predators through the middle Eocene and could handle just about any mammal of their day.  (Prothero, 2006)

And that wasn’t all.  Lurking near the water’s edge were crocodiles and champosaurs, some of them bigger than the now-extinct small dinosaurs.  (Prothero, 2006)

So early mammals did not lead a care-free existence, even though T. rex was no longer around.  (Agustí and Antón)

But they survived and prospered.

Caniforms and feliforms

What about the “noncarnivorous carnivorans” mentioned in the quote up above?

This particular group of early mammals probably lived on insects (Flynn and Galiano), not other tetrapods – yet.

They are nevertheless called carnivorans because paleontologists and molecular biologists consider them to be the ancestors of cats and all other members of the modern order Carnivora.

Somehow.

Apart from the mystery about its origins, different experts have been classifying this group in different ways for a long time – almost since paleontology first became a field of science. (Flynn and Galiano)

That’s why I’m not labelling the first Paleocene and Eocene carnivorans here.  There is just no broad consensus on what to call them because their fossils can be interpreted in so many ways.

See Flynn and Galiano, pages 3-16, if you are curious about the historical details of this classification problem, but be warned – that’s a mindblowing read for a layperson.

Basically, biologists divide today’s Carnivora order into two groups:

  • Caniforms, including but not limited to dogs.
  • Feliforms, including cats and other families.

That’s right.  It’s not just a cultural thing we have about dogs and cats.  There really is a basic separation there.

And apparently it has existed all the way back to the Paleocene – or even earlier, if their last common ancestor was Cimolestes back in the day. (See different viewpoints presented in Werdelin, 1989)

Nimravids and creodonts

As we’ve seen, mammals exploded over the post-K/T world and quickly developed into all the modern orders

By the second half of the Eocene, some 30 million years after the K/T extinction, they looked much less primitive.  Still, we wouldn’t recognize any of them (Prothero, 2006) . . . except the nimravids.

Any time-traveling human back then would take one look at a nimravid and call it a sabertoothed cat.

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Our friend Hoplophoneus is a nimravid.  It likes to dress up on Halloween.

Despite their appearance, nimravids weren’t actually cats, according to the latest research (Bryant; Flynn and Galiano), but they definitely had saberteeth.

Nimravids might have originated in Asia. (Averianov and others) They arrived in North America at a time when mesonychids were scarce and the “terror cranes” had just disappeared.  (Prothero, and Heaton; Prothero, 2006)

The White River Chronofauna was taking shape, and all predatory mammals were starting to get big.  (Hunt, 2004)

Nimravids became apex predators in the long-lived White River group, along with amphicyonids (a caniform group of carnivorans often called “beardogs”) and creodonts.  (Hunt, 2004)

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Amphicyonids enjoy Halloween, too.

Creodonts were very common predators back then.  (Hunt, 2004; Prothero, 2006)

They weren’t carnivorans, since they used different teeth for carnassials. (Rose)

Because of those carnassial cheek teeth, though, it’s possible that creodonts may have descended from some of the Bug Thief’s relatives. (Rose)

All things come to an end at last. The last North American nimravids disappeared around 24 Ma.  (Hunt, 2004)

The result was a seven-million-year “cat gap.” (Rothwell; Van Valkenburgh, 1999)

As we will see in the next few posts, this gap would eventually be filled, around 17.5 Ma, by a member of the Pseudaelurus complex of early cats.  (Rothwell; Werdelin and others)

However, the fossil history of the first cats is clearest in the Old World, so we are going to pause here in the North American part of the story.

It’s time to go to Europe and meet the Dawn Cat.

And Europe looked very different back then.

The first cats in Europe

From the late Triassic all the way through the Cretaceous and into the Paleocene and Eocene, sea level was usually higher than it is today. Europe was basically an archipelago of large islands.  (Lillegraven and others)

These lush green islands sat amid the glistening equatorial waters of the Tethys Sea, an east-west marine belt that circled the world and separated North and South America, as well as Eurasia and Africa (and for a little while, India and Eurasia).  (Agustí and Antón)

Don’t look for this sea on a modern map.

Over geologic time, plate tectonics has destroyed Tethys by smashing first India and then Africa into Eurasia.

Land bridges, as well as great mountain chains that stretch from the Himalayas to the Alps and the Pyrenees, now sit where tropical waters once flowed.

All that’s left of Tethys now is the much smaller Mediterranean Sea.  (Agustí and Antón)

Until about 33 Ma, another huge sea to the east of Europe stretched between Tethys and the Arctic Ocean. This isolated Europe from the rest of Asia.  (Agustí and Antón; Akhmetiev and Beniamovski)

Of course, those were greenhouse times.  Earth had no polar ice caps, and not much of a temperature difference between the Equator and the poles. (Smithsonian)

Then came the K/T extinction.

Two-thirds of the world’s known Cretaceous mammal families disappeared, though Eurasia wasn’t hit as hard by the disaster as western North America was.  (Rose)

Europe’s survivors were generally the same mammal groups that had outlived the dinosaurs in western North America, although the communities had different proportions of species and other local variations.  (Agustí and Antón; Prothero, 2006)

Hoofed meat-eating mesonychids roamed the islands of Europe, along with creodonts and “terror cranes,” but unlike in North America, there were no European carnivorans yet.  (Prothero, 2006)

Then, early in the Eocene epoch, a host of North American animals, including early carnivorans and creodonts, traveled into Europe across a North Atlantic land bridge that included Greenland and parts of Scandinavia.  (Agustí and Antón)

The creodonts did quite well, spreading across all of Europe’s islands, but carnivorans there never really diversified much.  (Agustí and Antón)

That didn’t matter, because those carnivorans weren’t going to have much of a future in Europe, either.

Around 33 Ma, Greenhouse Earth turned into an icehouse. (Lyle and others; Prothero, 2006; Zachos and others)

As ice fields expanded over much of Antarctica for the first time since the Permo-Carboniferous ice ages (Smithsonian), global sea level dropped almost 100 feet (30 meters). (Agustí and Antón)

The inland waterway between Europe and Asia disappeared. So did the Tethyean waterways that had separated the islands of Europe from the continent.  (Agustí and Antón)

Huge migration corrdors over dry land were now open. Thanks to the arrival of Asian animals, as well as cooler climate conditions that opened up the former rainforest, over half of Europe’s native mammals from the Eocene died off. (Agustí and Antón; Prothero, 2006)

Among the new species were feliforms – sometimes called aeluroids – that had evolved from early Asian carnivorans. (Agustí and Antón)

They resembled modern civets.

Brown palm civet | Paradoxurus jerdoni
A brown civet.

As time passed new aeluroid forms evolved.  (Agustí and Antón)

One lineage included Stenogale, possibly Haplogale, and a likely relative of Stenogale called Proailurus lemansis. (Hunt, 1998; Werdelin and others)

Proailurus appeared around 24 Ma (Rothwell) and is sometimes called the Dawn Cat, because it was probably the first true cat.

We can only say “probably” because Proailurus lacked a few of the diagnostic but very subtle details that modern felids have. (Hunt, 1998)

However, at a glance, the Dawn Cat was everything a layperson calls a cat. It had a short face, retractile claws, and a rather hypercarnivorous set of teeth. (Agustí and Antón; Cope)

Proailurus was the size of an ocelot and may have had a spotted coat.  (Agustí and Antón; Antón; Werdelin and others)

wp-1480184398123.jpeg
A modern ocelot

It was a little flat-footed, compared to modern cats, but its short, powerful legs were perfect for scampering along branches and up and down tree trunks.  (Agustí and Antón)

The Dawn Cat’s fossils have only been identified in Europe, but there might also have been a proailurine in North America.(Hunt, 1998; Werdelin and others)

Proailurus was rare in the European oak/laurel forests, and it had vanished by the time the next batch of cats showed up in the early Miocene, around 20 Ma.  (Agustí and Antón; Rothwell)

These members of the Pseudaelurus group of true cats were more successful than Proailurus, and they clearly belonged in Felidae – the cat family.  (Rothwell; Van Valkenburgh, 1999; Werdelin and others)

Pseudaelurines lived on all northern continents. As we saw up above, the ones in North America filled in the “cat gap” there.

Paleontologists still have many questions about the Pseudaelurus complex, but they are fairly sure that this group included the ancestors of both modern cats and, as we will see next time, the sabertooths.

The evolution of cats

Many features that define a cat – things like whiskers, claws, teeth, and glands – started out as the integument that life on Earth developed when it climbed out of the sea and colonized the land.

After those pioneering tetrapods, amphibians and reptiles evolved in the swampy global rainforest.

Then major climate change – the Permo-Carboniferous ice ages (as opposed to the recent Smilodon Ice Ages that we’re all familiar with) – led to the collapse of the Coal Forests and the development of carnivory: tetrapod eating tetrapod.

Synapsids, with their very adaptable teeth, were quite good at both carnivory and herbivory. Once forests opened up and more browse was available, they grew bigger and dominated the open landscapes.

The first sabertooths were synapsids – the gorgonopsids. Interestingly, no sabertooth has appeared outside the synapsid line of descent. It probably isn’t impossible, but it hasn’t happened yet, as far as we know.  (Antón)

The synapsids turned into therapsids and cynodonts, some of which became more and more mammal-like.  Luckily for us, it was one of these mammal-like lines that survived the devastating Permian extinction and made a fast comeback afterwards.

The first mammals and dinosaurs appeared at around the same time, late in the Triassic period that followed the disastrous end-Permian extinction.

Another major mass extinction at the end of the Triassic gave dinosaurs an edge, but mammals hung in there for the next 140 million years or so, until the K/T extinction took out all dominant species on land and in the sea.

Marsupials had dominated at least the North American mammal scene – the fossil record is clearest there – but they were hit hard by the end-Cretaceous mass extinction, and placentals moved in there and in Eurasia.

In some other places, like Australia, marsupials continued to evolve until humans brought in cats, which small predators like the quoll were unable to handle.

No one knows how the order Carnivora, the one that includes cats, got started, but it might have descended from a K/T extinction survivor called Cimolestes – the Bug Thief – or one of its cimolestid relatives.

Anyway, while carnivorans were working out the whole feliform/caniform thing, the first cat-like creatures suddenly appeared in North America in the late Eocene epoch.

Called nimravids, they had saberteeth and certainly resembled cats, but paleontologists believe that they weren’t true cats.

The origin of nimravids isn’t well known, either, but it may have been in Asia.

Nimravids and other animals formed the White River Chronofauna, a stable, long-lived collection of animals on North America’s High Plains.

For ten million years, the White River Chronofauna withstood major climate change, asteroid impacts, and a next-door supervolcanic apocalypse, and then slowly they faded away.

The disappearance of nimravids left a seven-million-year-long “cat gap” in North America.

Things were different in Europe.  It had been an island archipelago in the tropical Tethys Sea for tens of millions of years, but took on its present continental configuration about 33 Ma, when Antarctica began to freeze over, drawing down worldwide sea levels.

Some 60% of Europe’s formerly isolated native species disappeared when watery barriers to migration came down.

They were replaced by Asian species that included more modern carnivorans in both the feliform and caniform lines.

One of the new European carnivorans was the feliform Stenogale.  It may have been the ancestor of Proailurus.

And Proailurus was probably the first true cat .

After the Dawn Cat came the Pseudaelurus cat complex, from which both modern and sabertoothed cats eventually developed.

Next time, we will meet two major tribes of sabertoothed cats – the Homotheriini and the Smilodontini.

We will also explore possible reasons why these beautiful animals went extinct.

Post last revised: February 11, 2017.


IMAGES:

Featured image: Proailurus images, page 837, in  Cope, E. D.  1880.  On the Extinct Cats of America.  American Naturalist.  xiv (12):833-857.

Andean cat (Leopardus jacobita). Sanderson, J. CC BY-SA 3.0.

Jaguar. US Fish and Wildlife Service.  Jaguar.  Public domain.

“Roswell’s big feet.” Neil. CC BY 2.0.

Bird and rock cores.  Blastcube.  Gnu Free Documentation license

Grand Canyon rain video:  jwstang3119

Repenomamus giganticus skull fossil displayed in Hong Kong Science Museum. Laikayiu. CC BY-SA 2.0.

Crabeater Seal in Pléneau Bay, Antarctica. Quinn, L.  CC BY-SA 2.0.

Skeleton of a Hoplophoneus primevus.  On display at Zurich natural history museum.  Rama. CC BY-SA 2.0 FR.  Used multiple times, sometimes edited or labeled by RH.

“Sequoia sempervirens.” brewbooks. CC BY-SA 2.0.

Gorgonopsid. Дибгд.  “Арктопс Ватсона”  From ru.wikipedia.

Fossil of Pseudaelurus. Ghedoghedo (photo)/Macesito (cropped).   CC BY-SA 3.0

Purring kitten video: jonwooz

Meerkat Family Breakfast. Blaikie, D.    CC BY 2.0.

Projection of the political map of 2012 on Pangea. Pietrobon, M.    CC-BY-3.0.  http://capitan-mas-ideas.blogspot.com.br/2012/08/pangea-politica.html.

Siberian tiger.  Pixel-Mixer.  Public domain.

Spotted-tail quoll. Bennett, S. J.    CC BY-2.0

Arisha displaying Flehmen response. Tambako.   CC BY 2.0

Closeup of sediment deformation in end-Cretaceous Disturbed Zone, Badlands National Park. Stoffer, P. W. 2003. Geology of Badlands National Park: A preliminary report. United States Geological Survey Open-file Report 03-35, Figure 58.

Mesonyx. 1896.  Knight, C. R. Public domain.

Amphicyon ingens (bear-dog). v. Vogelsang, C.  CC BY 2.0.

Brown palm civet. Anil G aka Neelakandan Madavana CC BY-SA 3.0.

Ocelot. Hale, M. CC BY 2.0


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