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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A bone to pick

Dear Skeptophiles,

This is just to let you know that I'll be going on a wee hiatus to attend the annual Writers' Retreat held by my publisher, Oghma Creative Media, in the lovely Ozark Mountains.  So I'll be away for two weeks, and will be back in the saddle on Monday, August 12.  Please keep sending me ideas and links, making comments on posts, and so on -- I always love hearing from my readers.

Until then, hoist high the banner of skepticism!

cheers,

Gordon

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The lovely thing about science is that you never have a reason to stop learning.

I just retired after teaching biology for 32 years, and the area of biology I studied the most (and enjoyed teaching the most) was evolution and phylogeny.  I'm not a researcher, and nowhere near a specialist (I've been called a "dabbler" and a "dilettante," and I don't think they were meant as compliments), but I think that about those topics I'm at least Better Than the Average Bear.

So I was a little surprised yesterday to run into a group of ancient mammals I had honestly never heard of.  They're called docodonts, and technically I misspeak by calling them "mammals;" they're mammaliforms, which sounds like a species of alien on Doctor Who but isn't.  The docodonts and other mammaliforms are cousins of modern mammals, seem to have left no living descendants, and are more like mammals than they are like any other extant group.  Take, for example, this docodont, Castorocauda (the name means "beaver-tail"):

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

Of course, like the proto-bird-with-teeth we met earlier this week, the reconstruction is done to accentuate mammal-like characteristics; there's no guarantee that the sleek-pelted otterish look is accurate.

The reason this comes up is the discovery of a mid-Jurassic docodont whose skeleton shows some remarkably mammal-like features.  This little guy, called Microdocodon (evidently named by someone who believes in keeping things simple and obvious) was around 165 million years ago, which (for reference) is a good hundred million years before the non-avian dinosaurs became history.

Well, prehistory.

What's interesting about Microdocodon is that it had a mammalian hyoid bone -- unique in our skeleton as the only bone that does not articulate with another bone.  It's a horseshoe-shaped bone that connects to the tongue, epiglottis, larynx, and muscles that support the neck, and gives us our ability to chew, swallow, keep an open airway while we're asleep -- and talk.

In non-human mammals, it's all about the first three, and it's thought that the evolution of the hyoid bone was instrumental in improving the range of food mammals could eat, since the ability to chew meant they weren't confined to swallowing big chunks of food at once.

"It is a pristine, beautiful fossil. I was amazed by the exquisite preservation of this tiny fossil at the first sight," said Zhe-Xi Luo, a professor of biology at the University of Chicago and lead author of the study, which appeared in Science last week.  "We got a sense that it was unusual, but we were puzzled about what was unusual about it  After taking detailed photographs and examining the fossil under a microscope, it dawned on us that this Jurassic animal has tiny hyoid bones much like those of modern mammals...  Now we are able for the first time to address how the crucial function for swallowing evolved among early mammals from the fossil record.  The tiny hyoids of Microdocodon are a big milestone for interpreting the evolution of mammalian feeding function."

The most amazing thing about all this is that Microdocodon catches evolutionary remodeling of a pre-existing skeleton right in midstream.  "Hyoids and ear bones are all derivatives of the primordial vertebrate mouth and gill skeleton, with which our earliest fishlike ancestors fed and respired," said Bhart-Anjan Bhullar, postdoctoral scholar at Yale University and co-author of the paper.  "The jointed, mobile hyoid of Microdocodon coexists with an archaic middle ear -- still attached to the lower jaw.  Therefore, the building of the modern mammal entailed serial repurposing of a truly ancient system."

So that's our lens into the past for today, and a look at a group of mammal relatives that until I read this paper, I didn't even know existed.  All of this making me question how anyone can think science is boring.  If after studying and/or teaching science over the past forty years I can still find something new and astonishing, you have to appreciate science's capacity for inducing awe -- and wonder what new discoveries lie ahead.

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The subject of Monday's blog post gave me the idea that this week's Skeptophilia book recommendation should be a classic -- Konrad Lorenz's Man Meets Dog.  This book, written back in 1949, is an analysis of the history and biology of the human/canine relationship, and is a must-read for anyone who owns, or has ever owned, a doggy companion.

Given that it's seventy years old, some of the factual information in Man Meets Dog has been superseded by new research -- especially about the genetic relationships between various dog breeds, and between domestic dogs and other canid species in the wild.  But his behavioral analysis is impeccable, and is written in his typical lucid, humorous style, with plenty of anecdotes that other dog lovers will no doubt relate to.  It's a delightful read!

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