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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