The Age of Supereruptions: White River Fauna

I first heard of the White River Chronofauna while researching nimravids—the “false cats”—for the upcoming eBook about cat evolution.

You’ll be glad to meet these White River animals, too, if you’ve ever worried about a big natural disaster destroying the world. White River fauna not only survived a multi-million-year swarm of nearby mega-eruptions, they thrived all the time that it happened.

The group is called a chronofauna because it “maintain[ed] direct continuity through time by persistence of [its] basic ecological structure.” (E. C. Colson, quoted in Webb, 1984)

In plain English, the White River fauna were very successful for a very long time (over 10 million years).

What’s amazing about these animals—to those of us who aren’t paleontologists, anyway—is that their heyday coincided with some 20 million years of hell on Earth called the Great Ignimbrite Flareup in the Southwest. It also included the eruption of one of the world’s largest explosive large igneous provinces (LIP) in northern Mexico.

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The largest known eruption ever—La Garita—happened during the Ignimbrite Flareup.  It was 9.2 on the normally 8-point VEI scale.  Today some of the debris from that cataclysm is called the Wheeler Geologic Area in Colorado.  (Image by G. Thomas, public domain)

That faunal stability in the face of what was basically an apocalypse clashes with our popular image of a nuclear-war-scale global disaster, with mass extinctions, resulting from a single supereruption at Yellowstone or somewhere else.

But it’s true.

While this stable collection of animals thrived and multiplied out on the Great Plains, almost half of 47 supereruptions that have been recognized in Earth’s geologic record happened in Texas, New Mexico, Colorado, and Arizona. One of them, La Garita, was a 9.2 on the normally eight-point VEI scale. (Mason and others)

As if all that wasn’t enough, gigantic ash flows repaved southern Nevada and Utah over and over again, while the explosive Sierra Madre LIP eruption buried western Mexico under hundreds of thousands of cubic miles of volcanic rock.

Even the very few details of the Great Flareup that a layperson can describe are a little overwhelming, so let’s first briefly meet the animals that considered such a world normal.

The White River Chronofauna

They weren’t superanimals. They were just the weird-looking prehistoric critters that you would expect to see in a museum diorama about North American wildlife during the Eocene and Oligocene geological epochs.

That’s about halfway through the Age of Mammals, some 33 million years after the K/T extinction and roughly 33 million years before cavemen and saber-toothed “tigers.”

White River combo
Lower right image: National Park Service; Upper right: Dinictis, a nimravid (Image: Daderot, CC0 1.0); Upper left: Mesohippus, a horse (Image: Heinrich Harder, public domain); Lower left: An aquatic rhino (Image: Charles R. Knight, public domain).

Museums love this group. The richest vertebrate fossil beds in the world are where they used to live along what is now the White River in South Dakota and northern Nebraska. (Webb, 1977)

So, yes, the long-lived and abundant White River fauna lived right next door to the apocalypse. Sometimes it dropped in for a visit—a few of those ash flows reached western Nebraska. (Best and others)

Besides the cat-like nimravids, there were some very primitive-looking carnivores and herbivores in the White River complex, as well as more familiar creatures like horses (with three toes), dogs, three kinds of ancestral rhino plus the first true North American rhinoceros, the first peccaries, camels (some with horns but none with humps), and rabbits. (Prothero, 2006; Webb, 1977)

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The saber-toothed “false cat” Dinictis, chasing a horned camel.  Seriously, that’s not a cat.  (Image: Charles R. Knight, public domain)

As the group came together in the late Eocene, many of the animals were new to the continent. They were from  Asia and had migrated across the Bering land bridge. (Webb, 1977)

The land bridge was open because sea level had recently dropped. (Haq and others)  Down in Antarctica, which had been quite warm and forested during the Age of Dinosaurs and the first half of the Age of Mammals, the first glaciers to exist there in hundreds of millions of years were forming.  (Francis and others)

Meanwhile, in North America, the Asian immigrants found plenty of ecological niches open because almost a quarter of its land animals had just disappeared. (Prothero, 1994)

We’ve mentioned that the world was cooling.  That chilly spell began in the middle Eocene (Lyle and others), and now North American forests were no longer tropical evergreens, like they had been for the dinosaurs and even fairly recently.

Now, as the Eocene epoch wound down, North American forests looked more like the ones we see today in New England. There were conifers and broad-leaved trees that shed their leaves for part of the year. Such a change can be deadly when your life depends, directly or indirectly, on year-round tropical plants—many of the North America’s early primates and other primitive animals did go extinct. (Prothero, 1994; Rose)

The animals that had just migrated into North America could handle it.  Asia had subtropical forests, too, of course, but they had also known open shrublands and vegetation that was adapted to a dry climate. (Strömberg)

More change in the habitat was on the way.  After eastern Antarctica froze over at the start of the Oligocene (Zachos and others), woodland savannas and thorny forests and scrub lands appeared on North America’s Great Plains for the first time in more than 65 million years. (Webb, 1977)

And all those prehistoric herbivores and carnivores just settled down even more comfortably into their White River Chronofauna roles.

At the top of the food chain were rather wolf-like creodonts (specifically, hyaenodonts), as well as the first true dogs and some beautiful animals that you would swear were sabertoothed cats but weren’t really: these were the “false cats,” or nimravids. (Prothero, 2006)

Felids – the true cats – were strictly Old World when they first arrived. While there are some intriguing hints of early cats in North America (Hunt; Werdelin and others), none has been identified there until some six million years after the last nimravid disappeared at the end of the Oligocene. (Those feline-free years are the famous North American “cat gap.”)

So life went on for the diverse and abundant White River fauna in South Dakota and northern Nebraska. They lived there from roughly 37 million to around 21-23 million years ago. (Janis and others; Martin, 1980)

There must have been some spectacular sunsets back then, if the animals could appreciate such things as sunsets . . . or if they could even see the sky through all the volcanic haze.

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Even more spectacular than Frederic Edwin Church’s “Cotopaxi”!

Dates vary for the Great Ignimbrite Flareup, but the most intense pulses of ignimbrite eruptions in New Mexico, Colorado, and the southern Rockies—the regions closest to the White River Chronofauna—happened between 36-37 million and 21 million years ago. (Chapin and others; McDowell and McIntosh, Table 4)

In addition, the first and biggest of the ignimbrite pulses that built northern Mexico’s Sierra Madre Occidental happened from about 32 million to 28 million years ago. (Ferrari and others) In that short span of geologic time, more than 186,000 cubic miles (300,000 cubic kilometers) of tephra was erupted. (Bryan and Ferrari)

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The Sierra Madre Occidental is debris left over from gigantic explosive eruptions. (Image by Cataclasite, CC BY-SA 3.0)

Of course, Mexico is a long way from South Dakota and northern Nebraska, but you have to wonder what sort of effect the sudden addition (geologically speaking) of all that volcanic rock and gas had on global climate, on top of everything else.

But before talking about climate, we need to look at why this was happening.

The short answer is, “Plate tectonics.” It usually is, when the planet’s surface is involved.

Some of the best minds in geoscience today are still working on the longer, more detailed explanation.

The Great Ignimbrite Flareup

Okay, we have to get into plate tectonics a little bit. This is awkward, because we laypeople only understand plate tectonics intellectually.

We know, because geologists keep telling us, that Earth’s crust is broken into large plates, and that these plates jostle around. Life on the edge can be exciting, when two plate edges collide and build a mountain range, or when one plate edge slips under the other plate to form a subduction zone and then lots of volcanism happens.

But our hearts aren’t in it.

Any time of the day or night, we can look out the window at mountains and plains that appear to be fixed in time and space. We quite reasonably feel that something as solid as all that can’t move. The land is the steady foundation upon which we build our homes, our cities, and our lives.

Then an earthquake happens. It shakes up our souls as well as our physical world because we haven’t developed the sort of feeling that experts have for the very slow but inexorable movements of the Earth’s surface over geologic time.

To them, an epoch is a book page, a million years is a sentence fragment, and a hundred thousand years is merely a phrase. Stratigraphy, physics, chemistry, fossils, and advanced mathematics provide both punctuation and alphabet.

And the planet’s surface is a multidimensional Autobahn.

Well, something as vast and majestic as the Rocky Mountains can be considered eternal, given the short span of a human lifetime. But when we do that, we miss something really cool about the Rockies that can only be seen with plate tectonics.

Rockies
The Front Range, near Denver. (Image: Adam Ginsburg, CC BY 2.5)

Those mountains are sitting in the middle of the continent, nowhere near a plate edge or subduction zone. (Laramide) They didn’t just suddenly pop out of the Great Plains when a comet passed by. How did they form?

As I understand the highly-cited reports of several geologists who have investigated this, the story goes back to the early Cretaceous epoch, some 140 million years ago.  The western coast of North America looked very different. For one thing, it was one long subduction zone, including the part in California that is now the San Andreas fault.

An ocean plate was subducting under leading edge of the North American continental plate, much as one is doing in the Pacific Northwest today. As is typical in subduction zones, there was an arc of volcanoes on the overriding plate (North America, in our example). They were located about 90-125 miles (150-200 kilometers) away from the trench. (Laramide, slide 16; Zandt, slide 8D)

Volcanic_Arc_System
Image: United States Geological Survey. The USGS home page is http://www.usgs.gov.

A volcanic front like this sits above the point where the subducting slab of rock gets down far enough to melt. You get volcanoes, of course, because some of that melt reaches the surface. They’re arranged in an arc because of Earth’s curvature.

So, there was our Cretaceous subduction zone, with a volcanic arc sitting on what is now southern California and northern Mexico, carrying on normally for tens of millions of years while dinosaurs ruled the Earth and mammals kept a low but active profile in the background. There was no White River Chronofauna yet and the Great Plains weren’t a wooded savanna; where that land wasn’t drowned by an inland sea, it was covered with dense tropical rainforest. (Rose; Smithsonian, Cretaceous)

Then the volcanic arc seemed to develop a mind of its own about 100 million years ago. (Chapin and others)

It began to sweep northeastward. This unusual behavior for a line of volcanoes probably happened because the slab of subducted rock between the arc and the trench started to flatten out, moving the melting edge away from the trench. Experts in Earth science are still debating what caused that flattening. (Chapin and others; Ferrari and others)

Whatever the reason for it, the slab kept flattening as it subducted instead of dropping down at the typical angle like it had done before. As time passed, this flat section got longer and longer. The melting edge, and therefore the arc of volcanoes above it, moved farther and farther inland, away from the trench, and eventually reached the general area of modern Colorado. (Laramide, slide 16)

That volcanic arc didn’t exactly melt its way from California to Colorado; it massively deformed the rocks as it passed through. This certainly helped push up the Rocky Mountains (Zandt, slide 8E), but they also rose because, deep underground, the flat part of the slab was sort of stuck to the bottom of North America. Subduction was still going on, so the sticky slab did move, dragging against the North American plate and compressing it. (Chapin and others) There was nowhere for the overlying part of the continent to go in response to this compression but sideways and up.

Then, after it had reached Colorado, the volcanic arc headed back toward the coast. This was more unusual behavior—and it was bad news for the Southwest and northern Mexico.

Many experts think that this change of direction happened because the ongoing plate collision that had caused the west coast subduction zone in the first place slowed down. As a result, the flattened part of the subducted slab between California and Colorado started to break up and sink down into the mantle, piece by piece. The melting edge of the slab rolled back toward the trench. Besides moving the arc trench-ward, this roll-back also reduced compression on the North American plate and shut down the Laramide orogeny. (Chapin and others)

It also allowed lots of hot asthenosphere material to ooze up closer to the surface where the slab had once sat. (Chapin and others; Zandt, slide 34)

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The asthenosphere (the orange layer) doesn’t usually get close to continental crust. (Image by By NealeyS at English Wikipedia, CC BY-SA 3.0)

Sure; the extra heat from the asthenosphere melted the crust . . . somehow.  The details of exactly how that led to the production of hundreds of thousands of cubic miles of silica-rich explosive magma are controversial.

Suffice to say that there was lots of molten rock ready to go when the regional stress field completely switched over from compression to stretching as the slab continued to break and roll back. (Chapin and others)

The part of North America that had been compressed now cracked open as it was stretched. Through those weak zones, tremendous ignimbrite eruptions then surged out across the Southwest.

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Shiprock, in New Mexico, formed as a volcanic pipe when some of the rising magma came into contact with water. (Image: By Bowie Snodgrass, CC BY 2.0)

In Texas, New Mexico, Colorado, and Arizona, there were three great eruption pulses, with gaps of roughly 2 million years in between each one. An example of just part of one pulse was the eruption of almost 4,000 cubic miles (6,000 kilometers) of volcanic debris in New Mexico. At roughly the same time, at least 5,000 to 6,200 cubic miles (8,000 to 10,000 cubic kilometers) of ash and tephra blasted out of volcanic centers in Colorado. (Chapin and others)

Meanwhile, thick ash flows repeatedly swept over southern Utah and Nevada from the Sierra Nevada eastward to western Colorado. (Best and others) Think about those the next time you’re traveling through the area and see mesas and hills that resemble layer cakes. You’re really looking at stacked ignimbrite flows in cross-section. Each layer is a single eruption unit that cooled in place.

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The Bullfrog Hills in southwestern Nevada.  Names of the individual cooling units aren’t available.  (Image By Finetooth – Own work, CC BY-SA 3.0)

Some experts (Christiansen, 1979, 2001, quoted in Bryan and others) believe that each unit surged into place within just a few hours or days. That’s awesome, given the size of those layers.

But wait . . . there’s more!

As massive ash flows raged through the Southwest, one of the world’s biggest explosive LIPs also erupted in Mexico. At least a quarter million cubic miles (400,000 cubic kilometers) of volcanic material poured out of vents, fissures, or calderas to form the Sierra Madre Occidental mountains. That took more than a few hours or days, but most of it probably happened during a short span of geologic time. (Bryan and Ferrari; Ferrari and others)

And all this time the very diverse White River Chronofauna was stable in South Dakota and northern Nebraska.

Their complex group was not stagnant—the animals evolved new forms. Nimravids, for example, came up with the cheetah-like Dinaelurus crassus. This was the first known cat-like animal with “normal” teeth, i.e., relatively short, conical upper canines like the ones that all modern cats have. (Martin)

How could the White River fauna do so well during an age of supereruptions? Why didn’t those ignimbrite and LIP eruptions devastate the planet?

Surviving the apocalypse

One thing in the animals’ favor is that the ignimbrite eruptions came in pulses (McDowell and McIntosh). At least a million years of quiet passed between each pulse. Perhaps that allowed just enough time for some kind of a return to normalcy.

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OK, this is a modern leopard, but the coat pattern is primitive and some nimravids probably did shelter in trees. Imagine the saberteeth.  (Image by U.S. Fish and Wildlife Service)

But I have seen no mention that the White River animals were stressed.

They are actually the epitome of a healthy chronofauna. White River animals had no need to return to normalcy because, to them, the age of supereruptions apparently wasn’t extremely stressful.

As mentioned above, I haven’t found any expert opinions discussing this group in the context of the Great Ignimbrite Flareup. Take this for what a layperson’s opinion is worth, but I think the volcanism-dominated times were normal to this group of animals because the end dates coincide fairly closely.

The flareup quieted down some 20-24 million years ago. (Chapin and others;McDowell and McIntosh) That’s around the same time that nimravids, which are the White River animals I know some details about, went extinct in North America.

I’m so tempted to say that they were adjusted to nearby supereruptions and couldn’t handle a world without some. But I have to admit that a direct effect like that is probably not the case.

There were still nimravids in Eurasia (if you’re familiar with these gorgeous animals, I’m just talking about the first wave of nimravines). And they only lasted a few million years longer than the North American nimravids.

There must be some other reason, probably a global one, for their extinction.

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Solar radiation reduction due to relatively small volcanic eruptions. What would VEI 8 and 9 eruptions do to the planet’s climate? (Image source)

Climate is global.

And we have just seen hundreds of millions of cubic miles of rock and who knows how many gigatonnes of water vapor and gases blown up into the stratosphere, over and over again, for 20 million years.

That must have made a difference.

How did it affect Earth’s climate?

And why did animals living so close to it thrive?

Next time we’ll take a look at how Earth’s climate changed around that time and the role(s) that supereruptions might have played in those changes.

(to be continued . . .)


SOURCES:
Featured image: Prehistoric camel Poebrotherium labratum by Robert Bruce Horsfall, public domain.

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