So about five hundred million years ago, give or take, there was this little creature called Plectronoceras.  It was about 2 cm long — just under an inch — and it had a conical shell with a bunch of tentacles sticking out.  It was a cephalopod, an early member of the group that includes octopuses and squid.  And it was an /armored/ cephalopod, with most of its soft body protected by that hard little shell.

Let’s pause here and rewind:  this was five hundred million years ago.  That’s the late Cambrian, if you’re a geology nerd.  It’s before the dinosaurs.  It’s before sharks or cockroaches or ferns.  This is *old*.  Complex life had barely gotten started.  Life in general was pretty much confined to the oceans.  But there were no fish yet — just invertebrates.   Half a billion years, yeah?  Long, long time.

And a lot of the stuff swimming around was weirdly alien.  Again, if you’re a geology nerd, you know about stuff like Opabinia, Anomalocaris, or Hallucigenia.  If you don’t, then let’s just say that you wouldn’t have recognized much from those ancient seas.  Not just “no fish”.  There were no clams or lobsters, no starfish or barnacles or crabs or anemones, no coral or kelp.  The world was new.  Those things hadn’t evolved yet.

But almost from the beginning, there was this thing: shell, plus tentacles.

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