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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