The Evolution of Cats: 6: Nimravides and the Metailurini

It’s time to look at “the other sabertooths.”

Some of them are also called “false sabertooths.” (Antón)

Whether false or true, these now-extinct animals are giving scientists useful insights into the world of sabertoothed cats.

You probably haven’t heard of them.

Everybody knows Smilodon – the California state fossil and star of Ice Age. Quite a few laypeople recognize the name of Homotherium, too.

homotherium-profile
Homotherium’s teeth weren’t as long as Smilodon’s and it had a lighter build.

We met both of these and other popular sabertoothed cats last time.

But hardly anyone outside the Paleontology Department’s “cat” section has ever heard of Nimravides or the Metailurini.

As one noted geoscientist puts it:
“I am fond of saying that a geologist writes like a person overcoming very grave reservations. This is because no geologist can operate as an earth historian without continuously doubting such opinions—regardless of the accuracy of the observations upon which they are based. The realization of our uncertainty makes us uneasy, as does knowing that our explanations of the past are not more true, but only more plausible, than the stories told by creationists, extraterrestrialists, and other seers.” (Van Couvering)

When talking about fossil cats, paleontologists still have many “grave reservations” to overcome.

Very few of those issues came up last time, when we checked out the Homotheriini and the Smilodontini.

smilodon-cope
Smilodon, as the 19th century saw it.  This fossil sabertooth is still wildly popular in the 21st century.

Those sabertooths get a lot of attention from the public as well as researchers, and paleontologists have researched them in great detail. (Werdelin and others)
However, the two tribes weren’t the only toothy big cats around during the Miocene epoch and the Plio-Pleistocene ice ages.

No history of feline evolution is complete without mention of:

  • Nimravides. Long-legged and the size of a modern lion (Hunt, 2004), this was a mysterious North American saber-cat.
  • The Metailurini. Part sabertooth, part “normal” cat, these predators had a much wider range than Nimravides. Some of them were the most common Pliocene felids in Africa. (Werdelin and Dehghani)
632px-metailurus_metailurus_major_asenovgrad2
Metailurus is nowhere near as charismatic as Smilodon, but scientists are very curious about this cougar-like Miocene cat.  Was it a sabertooth or not?

 

Aside from the occasional complete skeleton, most of the other sabertooth fossils are fragments that can be interpreted in different ways. (Turner and Antón)

Of course this leads to lots of scientific controversy.

Yet the basic facts about Nimravides and the Metailurini are clear enough.

Both cat groups developed around the same time as the early members of the Homotheriini and Smilodontini. Then they coexisted with the two tribes for millions of years.

Nimravides didn’t make it out of the Miocene epoch (Werdelin and others), but the last member of the Metailurini died relatively recently, during the second half of the Pleistocene. (Werdelin and Dehghani)

All of them were very successful predators, and here is their story, as far as researchers have been able to outline it to date.

Nimravides

Probably the least controversial way for a layperson to introduce this sabertoothed cat is to say that it first appeared in North America 12 to 14 million years ago. (Werdelin and others)

We’re on sure ground with the word “appear” because Nimravides definitely shows up in the fossil record.

More specifically:
“…[A]t ~16.5-17 Ma, the first felids enter[ed] North America from Eurasia and persist[ed] as relatively small, lynx- to leopard-sized cats until the appearance of the large lion-sized felid Nimravides…” (Hunt, 2004)

It’s also clear that Nimravides species were indeed as big as a modern lion. They had a heftier build, though – like a tiger, with very long, muscular legs. (Antón, Figure 3.39; Hunt, 2004; Martin, 1980; Turner and Antón)

Nimravides also had saberteeth. These resembled those of Machairodus – the Old World sabertoothed cat that we met last time.
And this resemblance to Machairodus is where the scientific arguments begin.
Where did Nimravides come from?
Did it:

  • Emerge first in Eurasia as Machairodus and then travel across the Bering land bridge into North America? (Antón)
  • Evolve locally from those early lynx- and leopard-sized immigrant cats mentioned above by Hunt? (Werdelin and others)

No one knows. (Werdelin and others)

The earlier cats – some of the very first cats ever – were pseudaelurines, of course. They just weren’t the same Pseudaelurus species that had already developed in Europe. (Rothwell; Werdelin and others)

These pseudaelurines might have evolved in North America, but it’s also possible that they immigrated over the Bering land bridge from Asia, where very few Miocene cat fossils have been identified yet. (Rothwell; Werdelin and others)

Anyway, there was no slightly sabertooth-like Pseudaelurus quadridentatus – or P-Quad, as we called it last time – to provide an ancestor for a North America sabertoothed Nimravides.

That’s actually not a big problem for supporters of the “Nimravides was a home boy” hypothesis.

They point to local pseudaelurine species, like P. intrepidus or P. marshi, that Nimravides resembles. (Werdelin and others)

The resemblance could mean that one of these cats evolved into Nimravides.

After all, both Intrepidus and Marshi were big, like Europe’s probable sabertooth ancestor – P-Quad. (Antón)

They were also very common, accounting for three-quarters of the North American Pseudaelurus fossils that have been found thus far. (Rothwell)

Both species have been found in Texas. Nebraska, and Colorado. In addition, Intrepidus also inhabited Colorado and Nevada. (Rothwell)

That was a good place to be back in the day.

At around 17.5 Ma, there were major changes in the movement of the Pacific and North American tectonic plates, and as a result, the Basin and Range province of western North America began to open up. (Chapin and others)

This region, including much of present-day Nevada and Utah, had been a high plain. Now it began to slowly stretch out into the modern landscape, making a lot of new habitats.

This may have helped drive the increase in wildlife diversity that happened in North America around this time. (Kohn and Fremd)

The pseudaelurines were a part of all that, and so was Nimravides, when it appeared around 12 Ma. (Werdelin and others)

Indeed, the Basin and Range region is still expanding today!

Our next question is how Nimravides was related to the Machairodus species that showed up in North America around 7 Ma. (Hunt, 2004)

According to the “home boy” view, Machairodus arrived from Eurasia, and Megantereon soon followed. (Hunt, 2004)

Then, from this point until around 4.5 Ma – when Nimravides finally went extinct without leaving any descendants (Werdelin and others) – sabertooths, smaller “normal” cats like the lynx, and Nimravides coexisted in North America. (Hunt, 2004)

But some scientists say that there are too many resemblances between Nimravides and Machairodus for this to be a coincidence. (Antón and others)

What if P-Quad originated in Asia, back in the late Miocene, not in Europe where all of its fossils have been found so far? (Antón)

Then some individuals could have headed west into Europe and evolved into Machairodus. Others – the ancestors of Nimravides – might have traveled eastward over the Bering land bridge into North America. (Antón)

This alternative version works so well with one Nimravides species that experts have changed its name from Nimravides catocopis to Machairodus catocopis. (Antón and others; Fossilworks)

But for the other four recognized Nimravides species (Werdelin and others) . . . not so much.

Perhaps those species did descend from the early North American pseudaelurines. (Antón and others)

The scientific controversies over Nimravides will probably continue until paleontologists can establish just how the Pseudaelurus complex of cats got started, and what relationships there were among pseudaelurines in Europe, Asia, and North America. (Antón; Werdelin and others)

The Metailurini

This group of prehistoric cats roamed Africa and the nothern continents for some nine million years (Antón), but it is very hard for a layperson to describe them.

Paleontologists have many disagreements over the Metailurini.

First, there aren’t very many fossils available. Different experts often interpret what fragmentary evidence there is differently. (Antón)

Everybody does accept at least two basic Metailurini subgroups:

  • Metailurus. Ranging in size from a modern snow leopard to a large cougar, this Miocene cat apparently evolved in Eurasia. (Werdelin and others) It had moderately long, flat upper canines and was slightly more advanced than P-Quad, which it resembled. (Antón)
  • Dinofelis. This cat probably first showed up in Africa (Werdelin and Lewis) and then traveled to Eurasia and North America. (Turner and Antón) It was generally bigger than Metailurus, up to small-lion in size. (Antón) More muscular than a modern leopard, Dinofelis had the same overall proportions. Some species had moderately flat but not very long saberteeth; others had almost “normal” teeth, like today’s big cats. (Werdelin and Lewis)
mountain-lion-1055259_640
Metailurus is often compared to a cougar.  Experts say you can’t get an accurate reconstruction by simply adding saberteeth to a modern animal, but it’s temping.

Just for the record, other groups associated with the Metailurini are Adelphailurus, Stenailurus, Pontosmilus, and Fortunictis. (Antón; Berta and Galiano; Werdelin and others)

Things quickly get very technical when you start looking into those names, so I am just going to move on to the second big Metailurini controversy, which is how to classify the tribe.

First, where did they come from?

Even if we just count Metailurus and Dinofelis, nobody knows how they evolved or whether they were even closely related to each other. (Werdelin and others)

Both started out in the Miocene, but their evolution was centered on different continents.

Too, Dinofelis was most successful during the Pliocene and Pleistocene epochs. Metailurus had its heyday in the Miocene, millions of years earlier. (Werdelin and others)

Some paleontologists suspect Metailurus may have given rise to Dinofelis. (Beaumont, quoted by Werdelin and Lewis)

Others only put these two cat groups in the same Metailurini tribe because they were clearly different from modern cats but didn’t have enough sabertooth features to be classed in either of the two major saber-cat tribes. (Werdelin and others)

Were they “Knife-Tooths” (members of the Machairodontinae, or sabertoothed cats) with only slight to moderate saberteeth, or were they “normal” cats (belonging to the Felinae group of modern cats) with a tendency toward a few sabertooth characteristics? (Antón; Werdelin and others)

No one yet knows.

Only one thing is clear – when it came to saberteeth, Dinofelis just couldn’t make up its mind.

The oldest and youngest Dinofelis species were definitely sabertooths (Werdelin and others), but some species in between had upper canines that weren’t much different from those that Leo the Lion sports today. (Werdelin and Lewis)

In fact, one species –Dinofelis cristata – may be the only known case where a sabertoothed cat switched over to “normal” upper canine function! (Werdelin and Lewis)

This flexibility – going from sabertooth to “normal” back to sabertooth again – leads some paleontologists to suspect that Knife-Tooths and “normal” cats competed much more directly than previously thought. (Turner and others)

We don’t know exactly how they hunted, but the sabertooths probably went after big prey (Antón; Turner and others), leaving smaller prey to the “normal” cats that were also evolving back then. (Werdelin 1989)

Theoretically, the two groups of cats could have easily shared resources because their hunting techniques were different.

But the variability in Dinofelis teeth may show that cats had more hunting options than just “sabertooth” and “normal.” (Turner and others)

This in turn could mean that “normal” cats interacted with the sabertooths in many ways, and vice versa. (Turner and others; Werdelin and others)

Only time and more research can tell how much each group of cats – Knife-Tooths and felines – affected the other group’s evolution.

Another way to look at sabertoothed cats

When you get right down to it, besides being cats, Metailurus and Dinofelis had only one physical detail in common: their lower canine teeth were unusually small. (Turner and others)

And this obscure dental detail may someday prove beyond question that the Metailurini were Knife-Tooths.

Most of us go by the sabers when defining a sabertoothed cat.

However, as we saw last time, there is also a “sabertooth complex” of physical features that include a more powerful body and, of crucial importance to us here, a arch of very large incisors. (Antón)

Paleontologists know that, as the upper canines turn into saberteeth and the incisors get bigger, lower canine fangs usually get smaller to match the incisors.

wp-1485383666152.jpeg
This reconstruction of “Barbourofelis loveorum” suggests how arcs of seriously-bite-sized incisors/lower fangs could be useful to a sabertooth.

A researcher now says that the size of lower canine teeth may be more important than sabers when it comes to sorting out sabertoothed cats.

He ran an advanced computer analysis on selected sabertooth fossil features to see what patterns showed up among the different sabertoothed cats. (Christiansen)

Surprisingly, the conventional groups of Homotheriini, Smilodontini, and Metailurini didn’t show up. (Christiansen)

Instead, Metailurus and Dinofelis, as well as Nimravides – and even Machairodus and Promegantereon! – were all linked together, mostly because their lower canines were all reduced in size. (Christiansen)

Dr. Christiansen calls these cats the Machairodontinae (Knife-Tooths). He uses the name Eumachairodontinae (True Knife-tooths) for the later saber-cats, including Megantereon, Smilodon, Amphimachairodus, Homotherium, and Xenosmilus, who sported “true” saberteeth.

amphimachairodus
Amphimachairodus giganteus.

This view is not widely accepted yet, and there are some criticisms. (See Spassov and Geraads, “Cladistic Analysis”)

We’ll have to see how this idea turns out over time.

But the study is a good example of how paleontologists are using some very advanced tools to learn more about the evolution of cats.

When we start to look at the evolution of modern cats next time, we’ll have to lean on that technology more than we have done thus far.

That’s because there is a seven-million-year-long gap in the geological record between the pseudaelurine ancestor of all modern cats and the oldest known modern cat fossils. (Werdelin and others)

fossil-studier-478x346
“Here, kitty, kitty . . . “

A lot must have happened in those seven million years, and paleontologists have no direct evidence of it whatsoever.

Fortunately, the living descendants of this ancestral feline are all around us today. Experts can therefore collect indirect molecular evidence to fill in the gap.

They have found eight major lineages of modern cats and a history of intercontinental migration over at least the last ten million years. (Nyakatura and Bininda-Emonds; O’Brien and Johnson)

We will look at all that In the next post and see how lions, tigers, and the other wild cats evolved. Then, separately, we will meet the first Felis and watch one of its descendants – Felis catus – take over the world with human assistance. (Driscoll and others)


IMAGES:
Featured image: Felis catus and Amphimachairodus giganteus, to scale. Coluberssymbol.

Homotherium. Sergiodlarosa. CC BY-SA 3.0.

Smilodon populator. Cope, E. D. 1880. On the Extinct Cats of America. American Naturalist. xiv (12):833-857, Figure 12.

Metailurus major. Izvora. Asenovgrad Paleontological Museum. CC BY-SA 3.0.

Puma. Skeeze. http://pixabay.com/en/mountain-lion-puma-cougar-wildlife-1055259.  Public domain.
Barbourofelis loveorum, Florida Museum of National History Fossil Hall at University of Florida: Dallas Krentzel. CC Y 2.0.

Amphimachairodus. Ghedoghedo. See page for licensing.

Fossil studier. John Day Fossil Beds National Monument staff (National Park Service). Public domain.


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