And he's up!!
Here's a pic to show what happened, and serve as warning to other people with (especially large) dogs.
Cosmos was bounding back into the house for his I'm-A Good-Dog-For-Peeing-Outside treat, just like he's done a bazillion times, but this time he stepped on the hose. The hose rolled under his front foot, so that his leg went under the bottom step, while the rest of him kept goind forward. You can see on the Xrays how the bones broke at the top and bottom edges of the bottom step. This could have been avoided if we'd put a 2"x4" strip of wood as a "header" beneath the bottom step. If your steps look like this, please consider modifying them.