The White River Fauna and an Ignimbrite Flare-up

Came across this 2016 post from my old Robin Huntingdon blog while working up Chapter 4 of the cat-evolution series and contemplating using nimravids (to keep the “cat” focus) as well as the striking color bands of the badlands in it. That’s still under consideration, but in the meantime, hey! We haven’t had a Friday Feline post in a while. Granted, nimravids were not cats, but they sure looked like sabercats. Anyway, I’ve cleaned up the old post a bit, added embedded videos, and here you go!

The Oligocene critters we’re about to meet lived in these badlands before they were bad — back in the day when what are currently reddish bands of sedimentary rock in these stark hills were floodplains through which broad rivers meandered, loaded with mud and sand from the Black Hills; also a time, tens of millions of years ago when massive eruptions from nearby volcanoes sometimes buried everything in thick ash that, nowadays, is just a pretty white rock layer in between the colorful bands.

Life found a way to survive those catastrophes and thrive here, but how?

White River Chronofauna

I first heard of the White River Chronofauna while researching nimravids — the so-called “false cats” — during initial work looking up papers on cat evolution.

You’ll be glad to meet nimravids and other 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).

One of the largest known eruptions — 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 via Wikimedia, public domain)

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.

It happened in the US Southwest and also included the eruption of one of the world’s largest explosive large igneous provinces (LIP) in Mexico.

That White River fauna’s stability in the face of what was basically an apocalypse clashes with our popular image of the sort of nuclear-war-scale global disaster, with mass extinctions, we laypeople expect from a single supereruption at Yellowstone.

But it’s true.

This stable collection of animals thrived and multiplied out on the North American Great Plains, while almost half of the total of 47 supereruptions that have been recognized in Earth’s geologic record went off in Texas, New Mexico, Colorado, and Arizona. (Mason et al.)

And gigantic ash flows repaved southern Nevada and Utah over and over again, including the incredibly violent Wahwah Springs supereruption. (Best et al.)

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

The nimravid Dinictis pursuing a protocerid. (Image: Charles Knight via Wikimedia, public domain)

They weren’t superanimals.

They were just the weird-looking prehistoric beasts 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.”

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)

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

That would have sterilized everything, so our animal group did occasionally luck out.

As the group came together in the late Eocene, many of the animals were new to North America, having migrated across the Bering land bridge from Asia. (Webb, 1977)

Nimravids lived in Asia and may have been among the new arrivals. Per Kitchener et al., though, no one yet knows how that cat-like shape evolved before nimravids dropped it into the fossil record around this time.

That’s right. With nimravids here and in northwestern North Amerca, we’re looking at the first-ever appearance of a cat-like predator in Earth’s rocky archives!

Exploiting climate change

You didn’t want to turn your back on Hoplophoneus, either. (Image: ghedoghedo via Wikimedia, CC BY- SA 3.0)

The Asian immigrants found plenty of ecological niches open because almost a quarter of North America’s land animals had just disappeared. (Prothero, 1994)

This wasn’t a mass extinction so much as inability to handle climate change.

Global temperatures began to drop in the middle Eocene, for complex reasons. The Oligocene world was a lot cooler than in previous epochs. (Lyle et al)

This affected rainfall patterns, and plants had to cope with drier conditions.

As the Eocene wound down, North American forests were no longer tropical evergreens, as they had been for the dinosaurs and earlier mammals after the K/T event.

Forests had become more like the ones we see today in New England — 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 couldn’t cope and died out. (Prothero, 1994; Rose)

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

Even more change in the habitat was on the way.  After eastern Antarctica froze over at the start of the Oligocene (Zachos et al.), 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)

All those prehistoric herbivores literally ate it up. They and the carnivores that lived off them — including rather wolf-like creodonts (specifically, hyaenodonts), as well as the first true canids and, of course, nimravids (Prothero, 2006) — settled down even more comfortably into their 10-million-year-long White River Chronofauna roles.

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.

The Great Ignimbrite Flare-up

Geologists say that the more dramatic the scenery, the more violent its geologic history. This is one small part of the Sierra Madre large igneous province. (Image: Andrea Correon S/Shutterstock)

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 et al.; 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 et al.)

Four million years is a short span of geologic time for calderas to blast out more than 186,000 cubic miles (300,000 cubic kilometers) of tephra. (Bryan and Ferrari)

That’s something bigger than a supereruption — a large igneous province.

Of course, the Sierra Madre LIP is a long way from what are now Nebraska and the Dakotas, 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.

As far as I know (which isn’t too far), there’s no scientific consensus yet on climate effects of any of these huge eruptions.

They certainly didn’t seem to bother the White River Chronofauna much.

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.

And after all, this group had already survived a major shift in global climate. Maybe to them, such catastrophes were just part of their world.

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 et al.; McDowell and McIntosh) That’s around the same time that nimravids — the only 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 also still nimravids in Eurasia, far from North America’s geological turmoil. And they only lasted a few million years longer than the North American nimravids.

Maybe it was just time for nimravids to go.

The “cat gap”

In any event, after the last White River nimravid passed away, North America would see no cat-like predator for six and a half million years.

That’s when a member of the Pseudaelurus complex of true cats found its way across the Bering land bridge and set up shop.

We don’t know which pseudaelurine it was (Werdelin et al.), but its descendants are still flourishing in the Americas today.

Here is one person’s video review of them all, which I haven’t checked for accuracy but which does show their amazing diversity.

Edited July 24, 2021.

Featured image: The nimravid Hoplophoneus, by rama via Wikimedia, CC BY- SA 2.0.

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

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