Mammals of unusual size

When we've gone to the Museum of Natural History in Washington, D.C., I always gravitate toward the prehistoric animals.

I guess that's understandable enough, given that I made my career as a biology teacher.  Judging by the crowds, I'm not alone.  However, unlike most folks -- who seem especially taken by dinosaurs like T. rex and triceratops -- I always head toward the prehistoric mammals.

I love to picture what "endless forms most beautiful and most wonderful" (to pilfer a phrase from Darwin's Origin of Species) crawled, ran, jumped, scampered, and thundered across the planet long before we ever showed up on the scene.  Mammals have been around for a long time, a lot longer than you might think if you learned that "mammals arose once the dinosaurs were extinct" in grade school.  The first certain mammal fossils date from the late Triassic, about 225 million years ago, so at that point the non-avian dinosaurs still had around 160 million years to enjoy their hegemony before the double-whammy of the Chicxulub Meteorite Impact and the eruption of the Deccan Traps in India wiped them out.

The mammals were small for a while, of course.  Prior to the Cretaceous extinction, most of them fell into one of three groups; multituberculates (which looked superficially like rodents, but were only distantly related), eutriconodonts (a bit weasel-like, but again, not related), and spalacotheriids (something like a modern mole, but once again...).  None left any living descendants, and the biggest ones were the size of a small dog.

Understandable that they did what they could not to be noticed when there were loads of hungry dinosaurs around.

It's true that once the non-avian dinosaurs were wiped out, there was significant evolutionary pressure to diversify and get larger, to take advantage of the niches emptied by the mass extinction.  And one of the groups that got big fast were the brontotheres -- Greek for "thunder beasts."

They, like other mammal groups, started small.  They're perissodactyls -- the "odd-toed ungulates," a group that contains modern horses, rhinos, and tapirs.  And although they looked superficially like rhinos, their teeth show a closer relationship to horses.  One of the classic brontotheres is the slingshot-horned Megacerops (formerly named Brontops):

[Image licensed under the Creative Commons Creator:Dmitry Bogdanov, Megacerops-coloradensis, CC BY 3.0]

The reason this comes up is a paper last week in Science, which I found about from my author friend (and frequent contributor to Skeptophilia) Andrew Butters, in which a team from the University of Alcalá in Madrid used patterns of evolution in brontotheres to investigate Cope's rule -- that in the absence of other factors, larger individuals have a higher survival rate, and species evolve to get larger over time.

The results certainly seem to hold here.  The survival rate of brontothere species during the Eocene Epoch, from 55 to 34 million years ago -- their heyday -- is directly proportional to their size.  However, one corollary to Cope's rule is that when conditions suddenly change, large species are less able to respond flexibly, and are more prone to extinction.  Which is exactly what happened at the end of the Eocene; by the beginning of the next epoch, the Oligocene, the brontotheres were gone.

It was hardly the end of the large mammals, however.  Another perissodactyl group, the rhinos and their relatives, stepped in to fill the empty niches, and this led to the largest terrestrial land mammal known, Paraceratherium (formerly called Baluchitherium and Indricotherium).

[Image licensed under the Creative Commons Dmitry Bogdanov creator QS:P170,Q39957193, Indricotherium11, CC BY 3.0]

Standing next to Paraceratherium, you'd have come up to his kneecap.

If that's not scary enough, the Oligocene also saw mammals like the enormous Daenodon -- the name means "terrible teeth" -- which looked a bit like a pig on stilts:

[Image licensed under the Creative Commons Max Bellomio, Daeodon shoshonensis , CC BY-SA 4.0]

Oh, and there were also phorusrhacids stomping around the place.  Colloquially known as "terror birds."  Think of an enormous carnivorous ostrich on steroids, and you have the idea.

So yeah.  Even though I love hanging around in the prehistoric mammal part of the Museum of Natural History, it would be another thing entirely to go back there and actually try to survive.  An Eocene Park or Oligocene Park would be just as terrifying as a Jurassic Park.

Nature is red in tooth and claw, and all that sort of stuff.  Guess it always has been.

In any case, it does make me glad that the scariest thing I have to deal with around here are squirrels, raccoons, and the occasional coyote.  I'll take those over "thunder beasts," "terrible teeth," and "terror birds" any day.

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Analysis of a triple-whammy

I always find it wryly amusing when I hear someone talk about the Strong Anthropic Principle -- the idea that the universe was fine-tuned by a creator to be hospitable to life, particularly human life -- because it doesn't take much of a close look to become aware of exactly how inhospitable the universe actually is.

Even here on Earth, the situation is kind of dicey.  Our planet's history is really one cataclysm after another; that our own particular lineage survived is probably more a matter of luck than anything else.  In fact, only 74,000 years ago, climate havoc caused by the eruption of the Indonesian volcano Toba is thought by some scientists to have nearly wiped out our species -- they estimate the "Toba bottleneck event" to have reduced the entire population of Homo sapiens to under ten thousand individuals.  (For reference, that means that the human population of the whole planet was about as many people as currently live in Sitka, Alaska.)  Besides major volcanic eruptions, add in earthquakes and tsunamis, global marine anoxia events, and meteor strikes, and you can see that where we live isn't nearly as hospitable as it might look at first.

So the truth lies closer to the Weak Anthropic Principle -- of course the universe, and the Earth in particular, have the conditions necessary for life to exist, because if they didn't, we wouldn't be here to consider the question.

The reason all this catastrophic stuff comes up is a paper that came out a couple of weeks ago in Nature Communications Biology.  Entitled, "Widespread Loss of Mammalian Lineage and Dietary Diversity in the Early Oligocene of Afro-Arabia," by Dorien de Vries (of the University of Salford), Steven Heritage (of Stony Brook University), and Matthew Borths, Hesham Sallam, and Erik Seiffert (of the Duke University Museum of Natural History), it looks at a little-known extinction event that was apparently the result of a one-two-three punch that nearly halted the mammalian species in Africa in their tracks.

Which, of course, also includes our own ancestors.

Some of the Eocene animal groups affected by the extinction -- the left two are primates, the upper right a hyaenodont, and the lower right a prehistoric rodent.  [Photo by Matthew Borths]

The researchers had their first clues about the magnitude of the event, which occurred on the order of thirty-three million years ago, from looking at fossilized mammal teeth from East Africa, and found that right on the boundary of the Eocene and Oligocene, the number of mammal species dropped drastically.

"In our anthropoid ancestors, diversity bottoms out to almost nothing around thirty million years ago, leaving them with a single tooth type,” said Erik Seiffert, senior co-author of the paper, in an interview with EurekAlert.  "That ancestral tooth shape determined what was possible in terms of later dietary diversification...  There's an interesting story about the role of that bottleneck in our own early evolutionary history.  We came pretty close to never existing, if our monkey-like ancestors had gone extinct thirty million years ago.  Luckily they didn’t."

What seems to have driven the extinction was a triple-whammy -- rapid cooling of the climate after the equally catastrophic Paleocene-Eocene Thermal Maximum (during which the global average temperature was eight degrees warmer than it is now), followed by the huge Chesapeake Bay meteorite collision and a near-simultaneous massive eruption of flood basalts in Ethiopia.

Diversity plummeted.  More interestingly, the extinction preferentially spared generalists -- mammals which, from their tooth morphology, apparently could eat a variety of different kinds of food.  Specialists got hit the hardest, once again reinforcing the general evolutionary concept that if the ecosystem is stable, specialization is a good thing, but during periods of rapid change, it's the generalists who come out on top.

This -- as Seiffert says, luckily for us -- left our own distant ancestors to continue loping about on the African savanna, and also knocked out a lot of the competition.  The shape and structure of our own teeth reflect that event.  "We lost a lot of diversity at the Eocene-Oligocene boundary," said study co-author Matthew Borths.  "But the species that survived apparently had enough of a toolkit to persist through this fluctuating climate."

This should be yet another cautionary note about the fragility of the Earth's suitability for humanity, not to mention our current heedless messing-about with the planetary climate and ecosystems.  It's unlikely to, of course.  We don't have a very good record of listening to scientists, even when our own survival is at stake.  Part of this is that in recent memory we haven't had to deal with planet-wide natural disasters -- it's made us cocky.

"We've always been fine before," is the general attitude.

All it takes is reading the de Vries et al. paper to realize that "always" is singularly inapt.

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My master's degree is in historical linguistics, with a focus on Scandinavia and Great Britain (and the interactions between them) -- so it was with great interest that I read Cat Jarman's book River Kings: A New History of Vikings from Scandinavia to the Silk Road.

Jarman, who is an archaeologist working for the University of Bristol and the Scandinavian Museum of Cultural History of the University of Oslo, is one of the world's experts on the Viking Age.  She does a great job of de-mythologizing these wide-traveling raiders, explorers, and merchants, taking them out of the caricature depictions of guys with blond braids and horned helmets into the reality of a complex, dynamic culture that impacted lands and people from Labrador to China.

River Kings is a brilliantly-written analysis of an often-misunderstood group -- beginning with the fact that "Viking" isn't an ethnic designation, but an occupation -- and tracing artifacts they left behind traveling between their homeland in Sweden, Norway, and Denmark to Iceland, the Hebrides, Normandy, the Silk Road, and Russia.  (In fact, the Rus -- the people who founded, and gave their name to, Russia -- were Scandinavian explorers who settled in what is now the Ukraine and western Russia, intermarrying with the Slavic population there and eventually forming a unique melded culture.)

If you are interested in the Vikings or in European history in general, you should put Jarman's book in your to-read list.  It goes a long way toward replacing the legendary status of these fierce, sea-going people with a historically-accurate reality that is just as fascinating.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]