Very like a mammal

"Prior to the End-Cretaceous Extinction, mammals were all small and shrew-like, restricted to skulking, scurrying forms because of competition from, and predation by, dinosaurs.  Once the dinosaurs were out of the way, the mammals were free to diversify and to grow larger."

How many times have we all heard this?  And it certainly sounds plausible; being large and obvious when there were hungry carnivores like Velociraptor around seems like a good way to be turned into dinner.

But the fossil record shows that the truth is more complicated -- and far more interesting.

Take, for example, Castorocauda lutrasimilis.  This animal was around fifty centimeters long and weighed in at around three-quarters of a kilogram.  It was sleek, streamlined, with a bullet-shaped head, a fine pelt of soft fur, and a flat, paddle-like tail.  Here's an artist's reconstruction:

[Image licensed under the Creative Commons Nobu Tamura (http://spinops.blogspot.com), Castorocauda BW, CC BY 3.0]

If you're reminded of something like a beaver or an otter, you're not alone; the scientific name means "beaver's tail and looks like an otter."  Surprisingly, it was closely related to neither one; in fact, it's not even a true mammal, but a docodont, which split off from other mammal-like forms (including our own ancestors) way back in the early Jurassic period -- while there were plenty of dinosaurs lumbering around the place.

The docodonts, and a handful of other groups of Mesozoic cousins to mammals, are mostly known from the exceptional fossil beds of the Tiaojishan Formation in northern China, where paleontologists have found a wealth of mid- to late-Jurassic fossils of mammaliaformes -- as they call Mesozoic mammals and their near relatives.  And amongst those fossils they not only find otter-like aquatic species, but ones that have adaptations an awful lot like moles, squirrels, and possums.

This adds another cluster to the list of cool examples of convergent evolution, where two only distantly-related species evolve to resemble each other superficially because of similar selective pressures.  (A famous modern pair is the North American flying squirrel and the Australian sugar glider; at a quick glance these two look very much alike, but a closer examination would show that they're not even in the same order.  The flying squirrel is a rodent, and the sugar glider a marsupial.)

The docodonts and other side branches of the mammaliaformes all disappeared by the middle of the Cretaceous Period, replaced by true mammals including multituberculates, monotremes, marsupials, and placentals.  Why this happened isn't certain; given that we know the non-mammal mammaliaformes from only a few isolated geological strata, our information on them is limited.  We do know, however, that the mammals who survived were mostly "small and shrew-like," so there's a grain of truth to the old model.

What's most fascinating is that after the End-Cretaceous Extinction, these survivors re-diversified, and "re-invented" a bunch of the adaptations the docodonts had a hundred million years earlier.  This has interesting implications, not only for the evolution of life on Earth but for the kinds of living things we might expect to find on other planets.  It's long been a fascinating question to me to what extent evolution is constrained -- what limitations there are on natural selection that might result in its generating the same patterns over and over because those are the features that work best in pretty much any environment.  There are a few that seem likely, such as having the main sensory organs near the mouth and at the anterior of the body; I'd expect those to be frequent no matter where you go.

But what Castorocauda and the other docodonts show is that other sorts of traits can repeat, too.  After all, there are only so many ways you can move around, find food, find shelter, avoid being eaten, and regulate your own body temperature.  It might be surprising at first that the otter-like Castorocauda (and the possum-like Borealestes and the squirrel-like Shenshou) "re-evolved" (as it were) over a hundred million years later, but it suggests that making a living requires the same toolkit pretty much regardless.  

So maybe when we find life on another planet, it'll be far more familiar than we expect -- and that "life as we know it, Jim" might be there to greet us when we arrive.

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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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Mammals down under

Sometimes all it takes is one new discovery to send scientists back to the drawing board.

Of course, as astrophysicist Neil deGrasse Tyson correctly points out, scientists are always at the drawing board, or should be.  "If you're not at the drawing board," he says, "you're not doing science."  But still, it does seem sometimes like things are pretty well figured out, and then...

... boom.

There was a "boom" moment in the field of mammalian evolution this week, delivered by a paper in the journal Alcheringa: The Australasian Journal of Palaeontology.  The authors -- led by the brilliant paleontologist and polymath Timothy Flannery, of the University of Melbourne -- describe a fossil find that would seemingly be of interest only to people fascinated by minutiae of paleontology; a jawbone of a tribosphene, a proto-mammal with distinctive triangular, three-pointed molars, from the early Jurassic Period in Australia.

The problem is, it kind of shouldn't have been there.  Tribosphenes, which are in a group that is ancestral to both marsupial and placental mammals, were thought to originate in Laurasia, the northern half of the (at that point, split) supercontinent Pangaea.  (Laurasia comprised land that is now found in North America, Europe, and Asia.)  Australia, on the other hand was part of the southern half of Pangaea, called Gondwana, along with Africa, Antarctica, and South America.

This origin for the tribosphenes was considered so certain that they used to be called boreosphenes -- from the Greek word Βορέας, which was the name of the god of the north wind.

Guess it's a good thing they changed the name.

Eomaia, an early tribosphene mammal from China [Image licensed under the Creative Commons Nobu Tamura (http://spinops.blogspot.com), Eomaia NT, CC BY-SA 3.0]

There's no doubt that there were tribosphenes in Laurasia, too; one of the earliest, Tribactonodon, can be found in the Lower Cretaceous Durlston Formation in England.  (Others have been found in Mongolia and in Portugal.)  The idea was that they started in Laurasia and only later spread southward to Gondwana -- so Australia's iconic marsupials originally started out much farther north.

The discovery of a tribosphene in Australia sixty million years earlier than that indicates that some rethinking may be in order.

"I was re-analyzing these fossils that turned up in Victoria from the age of dinosaurs," Flannery said, in an interview with Australian Geographic.  "And then I started looking more widely for similar sorts of fossils found elsewhere and it turned out all of them were in the southern hemisphere and all are Jurassic or Cretaceous in age [from 199–66 million years ago]...  And we realized the thing that unites all these Southern Hemisphere fossils is they have these very strange, complicated molars that let the animals puncture shear and crush, all at the same time, what they were eating.  I resisted the conclusion as long as I could, but the evidence is compelling.  These shrew-like animals from Australian are actually the ancestors of both the earliest placentals and the earliest marsupials."

"We’ve been able to show that the relevant fossils that look like they are anatomically likely to be close to the common ancestor of marsupials and placentals are found exclusively in the southern continents and are from an older time period than the oldest mammal similar fossils seen the north," said Kristofer Helgen, who co-authored the paper.  "And that indicates these groups of mammals had their ancestry in the southern continents at an earlier time period and then later colonized the northern continents.  It absolutely turns our previous understanding on its head."

Which is tremendously exciting.  Far from being frustrated by stuff like this, these are the moments scientists live for -- when they find out that our previous understanding is incomplete, skewed, or flat wrong.  That's when the real process of discovery happens, and often when we gain a lens on a bit of the universe we weren't seeing clearly.

It's why I get so profoundly frustrated with the ridiculous attitude, "why study science?  It could all be proven wrong tomorrow."  To me, that's a completely backwards way of looking at it.  The truth is that science, unlike just about every other path to knowledge humans have ever utilized, has the ability to self correct.  When scientists find out a bit of our understanding is wrong, they neither throw their hands up in despair, nor do they double down on the error; they take steps to fix it.

And isn't that better than remaining in a state of error, incomprehension, or ignorance?

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